International Energy Agency building simulations 108 7.1 Impact of different weather years 108 7.2...

International Energy Agency

Energy Conservation in Buildings & Community Systems Programme (ECBCS)

IEA Annex 41 – MoistEng

Subtask 4: Moisture-Engineering Application

by: Andreas Holm With contributions from: Domique Derome Arnol Jansen Matrin Krus Ralf Kilian Kristin Lengsfeld Masaru Abuku Staff Roels Hans Janssen Targo Kalamees …… Draft – 16 October 2007

IEA ECBCS Annex 41 Subtask 4 report 16.10.2007

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1 Introduction 4

2 Different climate zone, different problems 6 2.1.1 Example: Influence of the Outdoor Climate on

the Necessary Air-Conditioning of Interiors 9

3 Acceptable Moisture Levels (Performance Indicators) 12 3.1 Human health and comfort 12

3.1.1 Investigation on the Importance of Humidity for Thermal Comfort, Health and Performance 16

3.1.2 Indirect Effects of Humidity 22 3.2 Deterioration of a building’s content 25

3.2.1 Preservation of cultural heritage 26 3.2.2 Causes of damages and risks 27 3.2.3 Museum environments 28 3.2.4 Indoor environment conditions requested for

loan exhibitions 31 3.2.5 Recommendations concerning the preservation

of historical buildings and monuments 31 3.2.6 Example: Indoor environment of an unheated

church in Southern Bavaria 31 3.2.7 Concepts for preventive conservation 34

3.3 safeguard the building’s structure 35 3.3.1 Mould 36 3.3.2 Wood deterioation 47 3.3.3 Corrosion 52 3.3.4 Algae growth on façades 52

3.4 Minimize the energy consumption 62 3.4.1 Moisture and thermal conductivity 62 3.4.2 Infuence of build in moisture on the energy

performance 67

4 Indoor climate control 70 4.1 Active Strategies 70

4.1.1 Humidity controlled ventilation strategies 70 4.1.2 Minimum ventilation requirements for buildings

from the mould point of view 76 4.1.3 Influence of different ventilation strategies on

the indoor climate 80 4.1.4 Application in residential buildings 83 4.1.5 Application in schools 83

4.2 Passive Strategies 86

5 Moisture Safty Aspects 90 5.1.1 Recommendations for sloped roof design based

on HAM-modelling 90 5.1.2 Controlled Ventillation of cold attics 102

6 Influence of Moisture Buffering Materials on the Energy Performance of a Building 103

6.1 Impact of moisture buffering on energy performance of cooling ceilings 103


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6.2 Indirect Evaporative Cooling: Interaction between thermal performance and Room Moisture Balance 103

6.3 Advantages of moisture buffering materials in hot and humid climate 103

7 Impact of the boundary conditions on the results of whole building simulations 108

7.1 Impact of different weather years 108 7.2 Impact of wind-driven rain on historical buildings with

brick walls on indoor climate, mould growth and energy consumption 112

7.3 The influence of internal boundary conditions on the hygrothermal performance of construction assemblies - Simplified approaches versus hygrothermal building simulations 118 7.3.1 Simplified approaches 118 7.3.2 Hygrothermal Performance of a whole Building 121 7.3.3 Comparison of the resulting indoor conditions 124 7.3.4 Influence on the hygrothermal performance 125

8 Benefits 128

9 Conclusion 129

10 Outlook 130

11 Unclear Papers 131


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1 Introduction

Typological analysis of moisture problems in roofs.

by: Arnold Janssens (Department of Architecture and Urban Planning, Ghent)

As an illustration of the problems this paragraph discusses some results of an analysis of individual moisture problem cases in roofs. The database consists of 72 cases in the period 1975-2001 for which the K.U.Leuven Laboratory of Building Physics was asked to give advice. The analysis is not necessarily statistically representative, but merely gives an indication of common causes for moisture problems in roofs.

Figure 1 lists the different types of moisture problems both for flat membrane roofs and sloped roofs. There is also a division between the cases reported before and after 1990. In this year technical guidelines were published in Belgium on ‘roofing membranes’ and ‘flat roofs’, in which rules for a physically correct flat roof design were given. The typology of the cold ventilated roof was abandoned in favour of the warm ‘built-up’ roof. At the same time a ‘thermal insulation law’ became in force, through which the application of thermal insulation, also in roofs, was encouraged.

The figures show that the occurrence of moisture problems has shifted in the last decade from flat roofs to sloped roofs. Before 1990 the majority of moisture problems occurred in flat membrane roofs (80% of total). Afterwards problems in flat roofs were less common (35% of total). This shift is a.o. related to the development of more durable membrane technology in the 1980’s (polymeric bitumina), and with the warning against the cold roof type in the technical guidelines. Possibly the improved insulation quality has also contributed to an increased sensitivity of sloped roofs for moisture problems. Apparently the existing guidelines for sloped roof design (with an emphasis on the importance of vapour retarders for moisture control) are not sufficiently adequate to prevent the occurrence of moisture problems.

In the database of moisture problems in roofs, interstitial condensation is the major cause of problems (46%), primarily as a result of air leakage and convective vapour tranfer. The second important cause of problems is rain infiltration (25%), related to inadequate detailing, workmanship or choice of materials.


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0

10

20

30

40

before '90 after '90

flat roofs sloped roofs

0 10 20 30 40 50

rain infiltration

interstitialcondensation

surfacecondensation

buildingmoisture

membranedamage

other

Figure 1: Types of moisture problems in flat and sloped roofs


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2 Different climate zone, different problems

Although fire protection and noise control work independent from climate, there is no simple way in general to apply national requirements for thermal insulation and moisture protection in other countries.

The improvement of thermal insulation in warm and humid regions does not result in the same energy savings in comparison to our conditions as a rule, as the greatest energy losses occur by the conditioning (drying) of supply air. In this case, it is worth investing in regenerative sorption drying (Desiccant wheel) to relieve air-conditioning systems. In order to avoid damage caused by humidity, it is necessary to consider not only the energetic but also the hygric behaviour of buildings under local climate conditions1.

Figure 2: Microbial growth on the façades of a typical residential building in Bangkok.

Figure 2 gives an example of mould growth on the external surfaces of typical residential buildings in Bangkok. In contrast to German conditions, the Thai climate causes lower surfaces temperatures due to cooling of the interior. The result is a higher humidity, which results in mould growth, possibly occurring at a temperature of 30°C and a relative humidity from approx. 75 %. In Middle Europe, condensation on the interior side of windows occurs in winter, whereas in these climate zones condensation occurs on the outside of windows in case of air-conditioning throughout the year. Moreover, vapour barriers, which are installed inside buildings in the tropical climate of Florida (U.S.) for example probably, cause damage due to humidity, because vapour diffusion penetrates from outside over a period of nine months. In addition, ventilated and diffussion-open constructions, which are very popular in our climate, cannot be recommended for this kind of climate conditions.

1 K. Sedlbauer, A. Holm, H.M. Künzel und A. Saur: Bauen in anderen Klimazonen – ändert sich die Bauphysik? Bauphysik 25 (2003),

H. 6, S. 358-366.


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Great problems also arise concerning constructions with a high rate of trapped building moisture. Increased ventilation by opening the windows is rather counterproductive in this case, as the dew point of the exterior air is mostly higher than that of indoor air. Therefore, building moisture can only be diverted by means of air-conditioning systems. If the capacity is not designed for this purpose, indoor humidity will rise during the drying phase in a way that mould growth will probably occur on large parts of the surface. Thus, constructional planning including the design of thermal insulation and moisture protection must be adjusted to the respective climate zone and the habits of the users. Building services, too, are entirely different.

Figure 3 shows the Köppen Climate Classification System. It is the most widely used for classifying the world's climates. Most classification systems used today are based on the one introduced in 1900 by the Russian-German climatologist Wladimir Köppen. Köppen divided the Earth's surface into climatic regions that generally coincided with world patterns of vegetation and soils. The system recognizes five major climate types based on the annual and monthly averages of temperature and precipitation. Each type is designated by a capital letter. In this project severel countries located in different climate zones around the world participated. Table 1 gives an overview of the partners, their corresponding climate zones and the moisture engineering problems.

Figure 3: Climate classification according to Köppen-Geiger2

To find solutions for the above-mentioned problems, it is not only necessary to know the local climate boundary conditions, but also to be acquainted with and take into consideration cultural differences, among them, of course, the habits of users concerning the application of air-conditioning, cooling and dehumidification.

2 Köppen-Geiger


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Table 1: Overview of the participant countries and their moisture engineering problems

Moisture Engineering Problems

Indoor RH others

Country Participant

Clim

ate

zone

s (K

oepp

en)

Fros

t dam

age

Win

d dr

iven

R

ain

Sola

r driv

en

moi

stur

e

Mou

ld g

row

th

Inte

rnal

co

nden

satio

n

low

(w

hen)

high

(w

hen)

HVA

C

Cfb Dfb Dfc

Austria TUW

ET Belgium KUL, UG Cfb

Af Am As Aw BSh Cfa

Brazil PUCPR

Cfb Cfa Csa Dfb Dfc Dsc ET

Canada BCIT, CON, NRC, UofS

EF Denmark DTU, Sbi Cfb

Dfb Finland TTU, TTY, VTT Dfc

Cfb Cfc Csb

France CETHIL, ULR

Csa Germany FhG, TUD Cfb Jan Aug no

Cfb Great Britain

GCU, UCL Cfc

Cfa

Japan BRI, KIU,

KYL, NILIM, SHK, TGC, THU Dfb

Nether-land

TUE Cfb

Cfb Cfc Dfc

Norway NTNU

ET Csa Portugal UP Csb Cfb Slovak SAB Dfb BSk Csa

Spain UDC, UFSC

Csb Cfa Dfb Dfc

Sweden CTH, KTH, LTH, SP

ET BSh BSk BWk BWh Cfa Cfb Csa Csb Dfa Dfb Dfc

United States

ORNL

Dsb


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2.1.1 Example: Influence of the Outdoor Climate on the Necessary Air-Conditioning of Interiors

In the following, we discuss the comparison of the necessary heating and cooling performance as well as the possible dehumidification rates for the different climate zones, in order to explain the possible climatic impact on air-conditioning installed in buildings. In the process, Munich is compared with Lisbon, Oslo, Chicago, Miami, Seattle and Tokyo. Table 2 shows the respective mean values per year for the temperature and relative humidity of the outdoor air in the seven above-mentioned places.

Table 2: Basic climatic data for the seven different locations.

Munich Lisbon Oslo Miami Chicago Seattle Tokyo

Mean temperature

[°C] 8.0 15.6 6.8 25.1 11.0 11.3 16.1

Max temperature

[°C] 30.6 37.0 29.3 33.9 35.6 37.2 35.3

Min. tempereratur [°C] -17.9 1.2 -14.8 6.7 -17.2 -2.8 -0.8

Mean relative humidity

[%] 78 75 73 71 71 73 62

Normal Rain [mm/a] 1177 675 605 1246 1089 955 915

Figure 4 above shows - in consideration of these climate boundary conditions - the progression of dehumidification rates, determined by means of the computation method WUFI® Plus, as well as the necessary heating and cooling performance for a model room of aerated concrete for the three locations. The room is suggested to have a flat roof (R=5.4 m2K/W) and an exterior wall of 36.5 cm thickness (R=2.7 m2K/W). The structure of the wall construction consists from outside to inside the of lime external plaster, aerated concrete and gypsum plaster. A window with a surface area of 50 m² is installed on the southern side of the room, whereas the surface area of the other windows in other directions is 20 m² respectively. The room has a volume of 1625 m³ and a ground area of 250 m². The suggested boundary condition is that the temperature is 20 °C in this room with between 8 a.m. and 4 p.m. During this time 40 students are present. The suggested humidity production rate is 16 kilo per day. During the rest of time, i.e. at night, the temperature is 18 °C with no humidity production and heat sources. During operation the air change rate is suggested at a value of 1 h-1, i.e. once in an hour the complete indoor air is exchanged. The strict keeping of temperature values is guaranteed by means of an idealized air-conditioning in the programme WUFI® Plus.


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Figure 4: Computed progression of dehumidification rate as well as necessary heating and cooling performance for a model room of aerated concrete.

Figure 4 above clearly shows that the predetermined boundary conditions in Munich and Oslo result in the lowest dehumidification rates, whereas they are higher, primarily in the third quarter, in Chicago. The figure at the right side shows that dehumidification rates of 10-15 kg per hour are common in Miami throughout the year. In this case, expenditure for dehumidification is higher during the first weeks caused by the building moist still trapped in the construction, which must be dried over a certain period of time, first of all.

Figure 5 gives an example of the temporal progression of water contents in the aerated concrete used for the west wall for the seven locations. It is obvious that a very rapid drying takes place in Miami. This amount of moisture is supplied to the room so that this will cause higher dehumidification rates in the beginning. This effect is negligible from the second year. But this also means that, dependent on the local climate, building moist may have an influence in form of higher requirements for ventilation or dehumidification. In Miami, the required capacity of the air-conditioning system would thus be double as high during the drying phase. This fact must always be taken into consideration, when designing and operating a building. To avoid an increased moisture load in a room, it is possible to predry the aerated concrete or to dry it during construction by means of special drying devices, which divert the produced moisture. Special warning goes to the over-dimensioning of air-conditioning systems, as the result may be a reduced dehumidification performance of the system under normal operation so that even then there is the risk of mould growth.


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Figure 5: Computed temporal progression of water contents in aerated concrete in the roof construction for the seven different locations.

Figure 4 in the middle shows the necessary heating and cooling performances for the model room for the seven locations. The total annual heating and cooling demand is plotted in Figure 6. Heating systems must be used in almost all places in winter, air-conditioning systems must be used for a short period in summer. But cooling performance could be neglected for Oslo, Chicago, Seattle and Munich if the strict requirements of 20 °C during the day and 16 °C at night did not exist. In Lisbon and Tokyo the annual heating demand is slightly lower than the cooling demand. The situation in Miami is quite different. Heating is not necessary, but cooling throughout the year. To reduce the cooling performance, it must be recommended to design wall and roof insulations as effective as possible as well as to install external shading of the windows. Moreover, ventilation at night or, if necessary, the cooling of the external air supply by means of a geothermal heat exchanging device could also be recommended for the design.

Figure 6: Computed annual heating (left) and cooling (right) demand as function of the annual external mean temperature.


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3 Acceptable Moisture Levels (Performance Indicators)

In buildings, there are four aspects that limit acceptable moisture levels:

Human health and comfort Deterioration of a building’s content Need to safeguard the building’s structure Minimize the energy consumption

3.1 Human health and comfort

Thermal environment is a parameter composed of several factors and not directly measurable. The four most important elements are:

Ambient air temperature Ambient radiant temperature Air flow velocity Relative humidity

The reaction to the thermal environment – thermal comfort/discomfort – is dependent on two personal variables:

physical activity clothing

Besides the above-mentioned so-called primary influencing factors there are further physical and physiological influencing factors, but their ranking is of secondary importance as concerns the subjective feeling of indoor climate.

Human beings react on the entire climatic situation. A certain climatic feeling may be achieved by means of different combinations of the individual elements. But this subjective climatic feeling can be considerably influenced by changing only a single factor. The feeling of the environment to be cool, for example, caused by a low air temperature can be eliminated by increasing the radiation temperature. The feeling of sultriness at high air temperatures can be reduced by lowering air humidity and air temperature. Low radiation temperature (e.g. cold wall causes cold room temperature) can be compensated by increasing the air temperature. High loads due to thermal radiation can be compensated by blowing in cool air.

Fanger 3 combined the six variables (air temperature, radiation temperature, humidity, air flow velocity, activity, and clothing) in a complex way and developed the equation of thermal comfort. The computation method is based on the suggestion that the human thermal regulation system automatically changes the average skin temperature and perspiration to keep the thermal balance. The equation thus computes the more or less balanced energy balance of the human body. If this physiological reaction of test persons is related to climate assessment, the result is the Predicted Mean Vote Index (PMV Index). The equation of thermal comfort is practically applied either by iterative solution of an equation4 or by means of thermal comfort diagrams. Figure 7

3 Fanger, P. O. (1970): Thermal Comfort. Reprint 1982 by R. E. Krieger Publishing Company, Florida, 244 pp. 4 DIN EN ISO 7730 (1994): Moderate thermal environments – Determination of the PMV and PPD indices and specification of the

conditions for thermal comfort. International Organization for Standardization.


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gives an example, which indicates the dependence of air temperature, humidity and air flow velocity as well as the intensity of muscular activity for a clothing insulation value. Figure 8 shows the frequently applied thermal comfort for the values of room temperature and relative humidity according to Leusden and Freymarck, and Figure 9 shows the limit of sultriness in dependence of temperature and humidity.

Figure 7: Diagram of thermal comfort in dependence of air temperature, humidity and air flow velocity as well as the intensity of muscular activity

From [5]

5 Hettinger, Th. (1995): Klimawirkungen auf den Menschen. In: Handbuch der Arbeitsmedizin, Konietzko und Dupuis (Hrsg.), ecomed

verlag Landsberg, Kap. III-4.3, S. 10.


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Figure 8: Thermal comfort for room temperature tL and relative humidity ϕ; valid for a surrounding surface temperature of 19.5 – 23 °C and air flows of 0

– 20 cm/s (Leusden and Freymarck 6).

Figure 9: Limit of sultriness in dependence of air temperature and humidity

according to Fiedler7

6 Leusden, F. P. und Freymarck H. (1951): Darstellungen der Raumbehaglichkeit für den einfachen Gebrauch. Gesundheits-Ingenieur

72, S. 271 – 273 7 Fiedler, K. (1995): Hygiene, Präventivmedizin, Umweltmedizin. Kap. 15: Wohnungshygiene. Uni-Med Verlag, ISBN 3-89599-101-5,

576 Seiten.


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Figure 10: Acceptable ranges of operative temperature and absolute humidity for people in typical winter (left) and summer (right) clothing during light acitivity according to ASHRAE standard 55.

Present standards and recommendations for thermal comfort allow a relatively broad range for humidity. The integral effects of all climate parameters are taken into consideration, when determining the PMV Index according to EN ISO 7730 8, requirements concerning humidity are not defined.

In the zone occupied by people engaged in light, primarily sedentaty activity (≤ 1.2 met), the humidity shall conform with the limits shown in Figure 10. In the Version from 1992 the humidity shall conform with the limits shown in Figure 10the upper and lower limits of relative humidity are based on considerations of dry skin, eye irritation, respiratory health, microbial growth, and other moisturerelated phenomena. It is esplicit notetd, that temperatures of building surfaces and materials (e.g., windows, ductwork) must be controlled to avoid condensation. In the ASHRAE 55-9standard the range of operative temperatures for zones occupied by people engaged in light, primarily sedentaty activity (≤ 1.2 met), presented in Figure 10 are for 80% occupant acceptability. This is based on a 10% dissatisfaction criteria for general (whole body) thermal comfort basedon thePMV-PPD index, plus an additional 10% % dissatisfaction that may occur on average from local (partial body) thermal discomfort. Figure 10 specifies the comfort zone for environments that meet the above criteria and where the air speeds are not greater than 0.20 m/s. Two zones are shown - one for 0.5 clo of clothing insulation and one for 1.0 clo of insulation. These insulation levels are typical of clothing worn when the outdoor environment is warm and cool, respectively. In the Version from 1992 the humidity shall conform with the limits shown in Figure 10. The upper and lower limits of relative humidity are based on considerations of dry skin, eye irritation, respiratory health, microbial growth, and other moisturerelated phenomena. It is esplicit notetd, that temperatures

8 DIN EN ISO 7730 (1994): Moderate thermal environments – Determination of the PMV and PPD indices and specification of the

conditions for thermal comfort. International Organization for Standardization. 9 ASHRAE (1992): ANSI/ASHRAE Standard 55-1992, Thermal environmental conditions for human occupancy. American Society of

Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta.


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of building surfaces and materials (e.g., windows, ductwork) must be controlled to avoid condensation.

However there are now no lower limits in the revised ASHRAE 55-200410 standard anymore, which in section 5.2.2 states that “Systems designed to control humidity shall be able to maintain a humidity ratio at or below 0.012, which corresponds to a vapour pressure of 1910 kPa at standard pressure or a dew-point temperature of 16.8°C. There are no established lower humidity limits for thermal comfort. However non-thermal comfort factors, such as skin drying, irritation of mucus membranes, dryness of the eyes, and static electricity generation, may place limits on the acceptability of very low humidity.”

The relative humidity between 30 % and 70 % has usually only a low impact on the global feeling of temperature (thermal comfort). The limit of thermal comfort can be defined – largely independent of air temperature - as 30 % r. h., occasional values below this limit are acceptable. In case of a high share of speech communication (e.g. teachers, call center agents) the relative humidity must not fall below 40 % 11. If the values for relative humidity are higher than 70%, the climate is felt to be sultry.

But various studies show that humidity has a clearly higher impact on local thermal comfort.

3.1.1 Investigation on the Importance of Humidity for Thermal Comfort, Health and Performance

Feeling of moist air and health effects

Low indoor humidity can have direct and indirect effects. The direct effects concern the thermal feeling and the feeling of dry skin and mucosa as well as irritations of the eyes and the respiratory tract in combination with volatile organic compounds. The indirect effect of moist or dry air may be the growth of micro-organisms (mould and mites) as well as the incidence of colds. Further effects concern emissions of organic compounds from furniture as well as electrostatic charge.

There is evidence for the connection of high relative humidity (> 50 % r. h.) or moisture in the interior environment and health problems/diseases (Table 3). The negative effects on health are of indirect nature and can be referred to an increase in the growth of micro-organisms. In contrast, health risks caused by dry air (< 20 – 30 %) are not yet discussed as a problem. Thus, a series of studies is presented in the following, dealing with the perception and impact of dry indoor air. Investigations refer to climate chambers, living spaces, air-conditioned work places; several investigations refer to air craft cabins.

10 ASHRAE (2004): ANSI/ASHRAE Standard 55-2004, Thermal environmental conditions for human occupancy. American Society of

Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta. 11 BIA-Report (2003): Grenzwerte-Liste 2003 – Thermische Belastungen. www.bia.de.


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Table 3: Symptoms and health problems associated to moist living spaces polluted by mould growth (according to Johanning et al. 12)

Health problems Symptoms headache, tiredness, loss of concentration, amnesia, general indisposition, susceptibility to infection

Burning eyes, irritation of nasal mucosa, recidividing sinusitis, hoarseness, roughness of the throat, cough, wheezing, dyspnea, bronchitis, asthma

Human beings do not possess any sensors to directly perceive humidity. The humidity values, however, indirectly influence thermal comfort by means of a latent thermal transfer. The thermal loss via the respiratory tract and the skin is dependent on the vapour pressure of water in the air. Higher humidity means lower thermal loss; the air is felt to be warmer. The air is generally felt to be less dry at low temperatures than at high temperatures. The epidemiological investigation of Jaakkola et al., which was carried out with 329 employees in an office building with health problems due to the indoor climate, is given as an example in table 3. The table shows that there was a linear association between indoor temperature and the feeling of dryness. The relative humidity of 10 - 15 % was very low during the investigations.

Table 4: Feeling of dryness at different temperatures and humidities of 10 -

15%*( shortened excerpts from Jaakkola et al 13 )

temperature (°C) judgement (in %) �22 > 22 �23 > 23 � 24 > 24

satisfied 26 22 17 5 dry from time to time 55 40 36 35 always dry 19 38 48 60

In Scandinavian countries, the indoor air is very dry during the winter months and air-conditioning systems with humidification are not standard in office and similar buildings. The relative humidity in naturally ventilated buildings is generally between 10 and 30 % during the winter months. Controlled field studies in office buildings in Finland proved that the comment “dry air” was significantly reduced, if the indoor air was artificially humidified

14/15

. The differences in relative humidity between the groups “with humidification / without humidification” amounted to a maximum of 20 to 30 %. Similar results were presented by a Swedish longitudinal study in a hospital16. The frequency of perception of dry air was clearly reduced from 73 % of all interviewed persons (at the beginning of the study) to 24 % (after four months of humidification) in the part of the building with air-conditioning (40 – 45 % r.h.). This parameter was only slightly changed in the control group. Table 5 gives a survey on these studies.

12 Johanning, E., Landsbergis, P., Gareis, M., Yang, C. S., Olmsted, E. (1999): Clinical Relevance and Results of a Sentinel Health In-

vestigation Related to Indoor Fungal Exposure. Environ. Health Perspect. 107, 489-494. 13 Jaakkola, J. J. K., Heinonen, O. P., Seppänen, O. (1989): Sick building syndrome, sensation of dryness and thermal comfort in

relation to room temperature in an office building: need for individual control of temperature. Environment Int. 15, 163-168. 14 Reinikainen, L., M., Jaakkola, J. J. K., Hainonen, O. P. (1991): The effect of air humidification on different symptoms in office

workers – an epidemiologic study. Environment International 17, 243-250. 15 Reinikainen, L. M., Jaakkola, J. J. K., Seppänen, O. P. (1992): The Effect of air Humidification on Symptoms and Perception of Indoor

Air Quality in Office Workers: A Six-Period Cross-Over Trial. Arch. Environ. Health 47, 8-15. 16 Nordström, K., Norbäck, D., Akselsson (1994): Effect of air humidification on the sick building syndrom and perceived indoor air

quality in hospitals: a four month longitudinal study. Occup. Environ. Med. 51, 683-688.


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The findings of the cited studies clearly contradict a great number of surveys on the Sick-Building-Syndrom (SBS), which found out a solid connection between the occurrence of SBS symptoms and ventilation and air-conditioning systems with humidification. Comparative investigations at air-conditioned work places by Kröling 17 show that people more frequently complain about dry air than at naturally ventilated work places (control building). The humidity values in the air-conditioned buildings were in the recommended range between 40 and 60 %, and never fall below the humidity values in the control buildings. In the extensive investigation of Sundell 18 with the participation of more than 5.000 office workers the perception of “dry air”, which was frequently expressed by affected persons, correlated to the other SBS symptoms, but in no way with the measured indoor relative humidity.

The feeling of dryness or the perception of dry respiratory tract, dry eyes and skin may also be caused by chemical and biological air pollutants. Thus, test persons reported besides irritations and other adverse effects on health also on dry respiratory tract in chamber investigations with a mixture of volatile organic compounds (VOC) (Rev. von Molhave 19).

Dry Air and Health Interferences

Irrespective of whether human beings are able to perceive differences in relative humidity or not, it is generally suggested that dry air has or at least can have adverse impacts on health. Relative humidities of less than 30 % are regarded as risks for the moist mucosa of the respiratory tract and the eyes as well as for the skin.

According to the frequently cited chamber investigations of Andersen et al. ( 20, 21) a four-hour exposure to dry air (10 % r. h., 23 °C) did not cause any interference of the nasal myxorrhea with healthy and young test persons, as the 8-hour exposure in moist air (68 % r. h.) did not provide any improvement of the important mucosal function. These results were confirmed in a further study, in which the test persons lived at a relative humidity of 9 % over a period of three day22. The authors conclude from the tests that in case of pure nasal respiration the capacity of humidification of the nose is sufficient even at a longer exposure to very dry air to compensate the lack of humidity. The tests by Anderson et. al. were carried out with filtered and thus particle-free chamber air, and therefore could not by transferred to “normal” ambient air, which always contains air-polluting substances.

17 Kröling, P. (1985): Gesundheits- und Befindensstörungen in klimatisierten Gebäuden. Zuckschwerdt, München (zit. in Knorr 1993). 18 Sundell, J. (1994): On the Association between Building Ventilation Characteristics, Some Inoor Environmental Exposures, Some

Allergic manifestations and Subjective Symptom Reports. Indoor Air Suppl. 2, pp 1-148 (zit. in. Brightman and Moss, 2000). 19 Mølhave, L.: Sensory irritation in humans caused by volatile organic compounds (VOCs) as indoor air pollutants: a summary of 12

exposure experiments. In: Indoor Air Quality Handbook, Mc Graw Hill Comp. (2000), S. 25.1-25.28. 20 Andersen, I., Lundqvist, G. R., Proctor, M. D. (1971): Human Nasal Mucosal Function in a Controlled Climate. Arch. Environ. Health

23, 408-420. 21 Andersen, I., Lundqvist, G. R., Proctor, D. F. (1972): Human Nasal Mucosal function under Four Controlled Humidities. Am. Rev.

Respir. Disease 106, 438-449. 22 Andersen, I., Lundqvist, G. R., Jensen, P. L., Proctor, D. F. (1974): Human Response to 78-Hour Exposure to Dry Air. Arch. Environ.

Health 29, 319-324.


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Table 5: Field Studies on the Perception and Effect of Low Humidity

Reinikainen, Jaakkola, Heinonen (1991)

Reinikainen, Jaakkola, Seppänen (1992)

Nordström, Norbäck, Akselsson (1994)

Period of study country

Feb. – Apr. 1988 Finland

Jan. – Feb. 1988 Finland Dec. – Apr. 1992 Sweden

Study population 257 office workers. 211 office workers. 48 % male 24 – 55 years old

90 hospital personnel < 8 % male 36 - 40 years old (med. age/group)

Study design Cross section study; 3 surveys in 2 comparable building parts with and without humidification

cross-ove study; 6 periods (1 week respectively) in 2 comparable building parts with and without humidification

Longitudinal study, 4 relatively new hospital units, 2 with humidification, 2 without humdification for 4 months respectively

Humidity (r. h.) 45 – 55 % (with humidification) 10 – 20 % (control, Feb.) 20 – 25 % (control, March, April)

30 – 40 % (with humidification) 20 – 30 % (natural ventilation) temp. 21 – 23 °C

35 - 45% (average 40 %) vs. 28 – 38 % (average 31 %) temp.21 – 23 °C

Parameter (questionnaire)

Score for perception and symptoms of dryness

Score for perception and symptoms of dryness; allergical symptoms; air quality

Assessment of climate and SBS symptoms

Results Feeling of dryness, dry skin, throat and nose as well as irritations of the nasal mucosa were significantly reduced at higher humidity

Feeling of dryness, dryness symptoms and allergical symptoms are reduced with humidification, feeling of the air being stale rises with humidification

Feeling of dryness, respiratory tract symptoms and electrostatic charge are reduced with humidification, no change with controls


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Table 6: Field studies on objective and subjective effects of low humidity.

Authors (year)

Wyon (1992)

Norbäck, Wieslander, Nordström, Walinder, Venge (2000)

Period of study /counry

Feb. – March 1990

Jan. – March 1997 / Sweden

Study population

339 hospital personnel

32 hospital personnel

Study design

experimental study

experimental study

humidity (r. h.)

25 % r. h. for 3 weeks (control) 40 % r. h. for 3 weeks (with humidification)

35 % r. h.; 22.5 °C (control) 43 % r. F.; 22,5 °C (with vapour humidification) duration: 6 weeks; after 6 week repetition

Parameter (physiological tests, questionnaire)

Scale for reported SBS symptoms; Expert assessment of skin etc.Tear Film Brake-up Time (BUT)

Physiological tests: stability of tears, open nose, biomarker in nose lavage; Questionnaire on symptoms;

Results

Subjective feeling of discomfort (eyes) was linked with reduced Tear Film Brake-up Time; Humidification reduced SBS symptoms; Reduction of symptoms of dry skin (reported and objectively determined)

Reduced feeling of dryness and improvement of skin symptoms by humidification; No measurable physiological changes

Participants in the controlled field studies (Table 6) reported on less dryness symptoms, if the originally low relative humidity was elevated to approx. 40 – 45 % r. h. Significant improvements due to humidification were determined for the following fields:

Nasal and pharyngeal cavities eyes skin feeling of heaviness allergical symptoms

General health problems, e.g. headache, tiredness etc., were not influenced by changes in humidity in most cases. The humidification systems of air-conditioning systems were free of microbiological contaminations during the


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cited investigations and the presence of air pollutants in the rooms was negligible.

The study of Lindgren et al 23 gives information on the correlation of dry air and the symptoms described above. Discomfort and irritative symptoms during the winter months were investigated with employees of the Scandinavian Airline. The reported health problems of the crew were compared to those of the office worker of the same Airline (Table 7). Humidity was not measured, but other investigations proved that relative humidity is very low in air craft and amounts to 17 % in average (see 2.3.3). General symptoms (e.g. tiredness, headache etc.) occurred in both groups with a similar frequency, whereas it was significant that health problems of the respiratory tract and the skin occurred more frequently in case of the air line crew. The symptoms were most serious in case of persons with allergical disposition and during long-distance flights. Since smoking is allowed during long-distance flights, there may be an interaction of dry air and the effects of passive smoking (ETS), because being exposed to passive smoking (ETS) frequently results in irritations and a feeling of dryness of the eyes, skin and respiratory tract.

Table 7: Symptoms and health problems in air craft – comparison of crew and office workers (Lindgren et al.)

Symptomsa

Air line crew (n=1513)

in %

Office workers (n=168)

in %

Comparison crew vs. office workers ORb significance

Rapid fatigue 21 22 0.94 n. s. Feeling of heaviness 7 8 0.70 n. s. Headache 6 4 1.04 n. s. Poor concentration 1 3 0.49 n. s. eyes (itching, burning etc.)

11 8 1.08 n. s.

nose (problems with the nasal mucosa)

15 6 3.12 p < 0.01

throat (hoarseness, dry throat)

8 1 5.75 p < 0.05

cough 4 0 – Facial sikn (dry and red)

12 7 2.03 p < 0.05

scalp (dandruffs, itching)

8 7 1.92 n. s.

Skin of the hands (dry, red, itching)

12 4 3.68 p < 0.01

a: data on symptoms during the last 3 months by means of a 3-step scale (frequently/each week – sometimes – never). b: OR = Odds Ratio, adjusted to age, gender, atopy, smoking and psychosocial variables. n. s.: differences between crew and office workers are not significant. p > 0,05 or p < 0,01: The differences are significant.

23 Lindgren, T., Andersson, K., Dammström, B.-G., Norbäck, D. (2002): Ocular, nasal, dermal and general symptoms among

commercial airline crews. Int. Arch. Occup. Environ. Health 75, 475-483.


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3.1.2 Indirect Effects of Humidity

As humidity may have an influence on the growth and spread of pathogens in different ways, indirect effects could cause the interaction of dry air and colds. In addition, infections show seasonal progession even in countries, where there are no great fluctuations in humidity. According to the assessment of Arundel et al. 24 of the indirect health effects of dry air, optimal humidity is between 40 and 60 % r.h. to minimize all investigated effects.

Figure 11 gives a survey of the importance of humidity for the occurrence of different biological and chemical factors.

Figure 11: Optimal relative humidity to minimize adverse health effects (from Arundel et al.)

Humidity and the Growth of Micro-Organisms

As already mentioned, bacteria and viruses prefer different humidities according to the species. Rhino-viruses (frequent pathogens for the so-called cold) for example are rapidly inactivated at a relative humidity below 50 %, other viruses survive at a relative humidity higher than 50 %. Mould growth requires a relative humidity of > 80 % , mites better grow at a relative humidity of 60 % and more (Figure 11). The occurrence of mould and mites in indoor environments is associated with a series of adverse health effects, e.g.

24 Arundel, A. V., Sterling, E. M., Biggin, J. H., Sterling, T. D. (1986): Indirect Health Effects of Relative Humidity in Indoor En-

vironments. Environ. Health Perspect. 65, 351-361.


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symptoms of the respiratory tract, irritations of the mucosa etc. 25. Some symptoms are similar to those of the direct effects of very dry air.

Regulations fix the upper limit of humidity at 70 %, causing increased occurrence of mites and mould, or indirect effects. No negative health effects are identified at high humidities below the limit of sultriness.

Humidity and Perceived Air Quality

Human beings perceive air quality by two senses: the sense of smell (olfactory sense) is in the nasal cavity, the chemical sense is on the nasal mucosa and the mucosa of the eyes. The common reaction of these two senses determines, whether the air is perceived as fresh and pleasant or stale, muggy and irritating. The nasal thermal receptors were not observed over a longer period of time in relation to the air quality perceived. But recent studies show that air temperature as well as humidity have a clear influence on the acceptability of indoor air.

With rising temperature and/or increasing humidity under controlled conditions, even pure air was generally perceived less acceptable (26, 27, 28, 29, 30). Based on chamber investigations, Berglund and Cain developed a regression equation by means of the variables of air temperature, dew point and metabolism to assess the characteristic “stale”. Fang et al. established a connection between olfactory sensation, acceptability and enthalpy of the air in a similar way. The thermo-dynamic parameter of enthalpy, or energy content of the air, links the combined effects of temperature and humidity. The olfactory sensation of “stale” increased with rising enthalpy of the air and the acceptance of the chamber air decreased in both investigations. The effect of temperature was stronger than the effect of relative humidity. The investigations of Fang were based on the exposure of the entire body to a temperature range of 18 – 28 °C and a relative humidity of 30 – 70 % (Figure 12).

25 Bornehag, C.-G-, Blomquist, G., Gyntelburg, F., Järvholm, B., Malmberg, P., Nordvall, L., Nielsen, A., Pershagen, G., Sundell, J.

(2001): Dampness in Buildings and Health. Nordic Interdisciplinary Review of the Scientific Evidence on associations between Exposure to “Dampness” in Buildings and Health Effects (NORDDAMP). Indoor Air 11, 72-86.

26 Berglund, L. G. und Cain, W. S. (1989): Perceived air quality and the thermal environment. In: Proceedings of IAQ ´89, Atlanta, S. 93-99 ((zit. In Nagda und Hogson, 2001).

27 Fang, L., Clausen, G., Fanger, P. O. (1998a): Impact of Tempera¬ture and Humidity on the Perception of Indoor Air Quality. Indoor Air 8, 80-90.

28 Fang, L., Clausen, G., Fanger, P. O. (1998b): Impact of Temperature and Humidity on perception of Indoor Air Quality During Im-mediate and Longer Whole-Body Exposures. Indoor Air 8, 276-284.

29 Gwosdow, A. R., Nielsen, R., Berglund, L. G., Dubois, A. b., Tremml, P. G. (1989): Effect of thermal conditions on the acceptability of respiratory protective devices on humans at rest. Am. Ind. Hyg. Assoc. J. 50, 188-195.

30 Toftum, J., and Jorgensen, A. S. (1998): Effect of humidity and temperature of inspired air on perceived comfort. Energy and Buildings, 28, 15-23 (zit. in Fang et al. 1999).


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Figure 12: Perceived air quality as function of temperature and humidity, adapted according to Fang et al.

Investigations at the Fraunhofer Institute for Building Physics showed that the impact of relative humidity on the perceived indoor air quality (olfactory sensation) occurs in dependence of temperature. Figure 13 gives data to show interconnections.


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Figure 13: Presentation of results from test series 1: assessment of pure air at different

temperatures and relative humidities. The shaded area marks the range, where the perceived air quality was determined to be < 3 decipol.

The increase of relative humidity from 0 to 50 % resulted in a higher emission of volatile organic compounds (VOC) from several materials in the beginning in investigations of Wolkoff 31 on temperature-dependent emission of volatile organic compounds (VOC) from different building materials. After three weeks of ventilation, emissions in the temperature range from 23 to 35 °C were independent of temperature and humidity to a large extent. The effect of humidity on the emission of volatile organic compounds from textile floor coverings – with the exception of aniline – were negligible32 according to other investigations. The increase of relative humidity from 30 % to 70 % (at 22 °C), however, doubled the emission of formaldehyde from particle boards. But in case of ozone, low humidity causes higher indoor concentrations, as the adsorption of ozone molecules on surfaces is reduced under these conditions.

Further indirect or undesired effects of low humidity are a higher dust load of the indoor air and a higher electrostatic charge of furniture materials.

3.2 Deterioration of a building’s content

There are three different modes of deterioration that are influenced by the relative humidity:

Change of size and shape Chemical reaction Biodeterioration

31 Wolkoff, P. (1998): Impact of air velocity, temperature, humidity, and air on long-term VOC emissions from building products. At-

mospheric Environment 32, 2659-2668 (zit. in Fang et al. 1999). 32 Sollinger, S., Levsen, K., Wünsch, G. (1994): Indoor pollution by organic emissions from textile floor coverings: climate test

cham¬ber studies under static conditions. Atmospheric Environment 28, 2369-2378 (zit. in Fang et al. 1999).


- 26 -

For the storage of hygrostatic materials and for some industrial processes, specialized temperature and RH levels may be required. These must be known to the designer before attempting to design the building envelope, but no general guidelines can be provided.

3.2.1 Preservation of cultural heritage

Buildings containing antiquities and precious artworks have their own moisture and temperature requirements. In general, somewhat higher levels of RH will be desirable. Plenderleiht and Werner 33 recommend RH limits of a minimum of 50% to a maximum of 65% within a temperature range of 16 to 25°C, except for picture galleries for which a constant RH of 58 and 17°C is recommended. However, it appears that artifacts are more sensitive to rapid changes in temperature and RH levels than to absolute values.

When comparing the recommendations for museum indoor-environment conditions to those concerning churches and collections kept in historical buildings, it becomes quite clear that there are distinct differences in the respective specifications. In museums, high importance is attributed to the preservation of the collections and the recommended, narrow margins of climate fluctuations are mostly observed very strictly. In the sector of architect-ural monuments, recommendations concerning the indoor environment quality are specified with more regard to the objects' actual conditions. Artifacts that have overcome centuries in a church without suffering any damage have good chances to endure further in the same environment. Also, regular repair and restoration works are quite usual in the preservation of historical buildings and monuments, which may explain the fact that the ideas of Preventive Conservation have not yet been widely spread here.

Based on the very severe specifications for indoor environment conditions that have existed since the second half of the 20th century, museums nowadays tend to adopt a more differentiated approach; thus, different materials are differently rated with regard to their resistance against fluctuations of climate. Particularly the range and the frequency of fluctuations of temperature and relative humidity are considered to be influential factors for the preservation of works of art. The application of statistical analyses allows to assess potential damages. Concerning the conditions that are really required for the preservation of artifacts, a considerable amount of further research is still needed.

Certainly, the crucial task for museums and the conservation of historical monuments is the preservation and protection of cultural heritage. However, there are fundamental differences in how to deal with this task and what approaches to use in practice. Museums often make high investments for technical installations in order to provide optimum indoor environment conditions for their exhibits (depending on the value and status of the artifacts); in monument conservation, severe restrictions must often be obeyed in connection with the preservation of historical buildings. Here, compromise solutions must be found very frequently. Also, repair work or regular restoration works are general practice here. The maintenance of churches, castles, patrician houses etc. requires renovation and restoration measures to be performed at regular intervals; often, the objects of art that are treasured in

33 Plenderleith, H., and A. Werner. 1971. The conservation of antiquities and works of art, 2nd ed. London: Oxford University Press.


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these buildings will be included in the restoration activities within the framework of a large-scale project.

There are also fundamental differences in handling the indoor environment. Since the mid-1950s, rigorous climate values have been postulated for museum environments, whereas monument conservators follow a clearly pragmatic approach, which is often geared to a building's actual situation. Actually, neither museum conservators nor monument conservators succeeded to establish general standards for the indoor environment. There are, however, numerous recommendations concerning indoor environment conditions34, the majority of which is based on empirical experience. In the following sections, the fundamentals of damage processes will be presented along with the most important recent risk-assessment approaches.

3.2.2 Causes of damages and risks

Most damages to artifacts can be ascribed to climatic impacts (apart from human activities). A low percentage of relative humidity allows materials to shrink and become brittle, while a high level of relative humidity induces the intake of water and thus swelling. Due to changes in the conditions tensions can build, which may result in damages and the progressive destruction of works of art. Most artifacts are inhomogeneous because they combine different materials in one object. This fact is liable to enhance inherent tensions and associated potential degradation. Furthermore, the state of an artifact is subject to construction and age. Also, the chemical material properties can deteriorate due to presence of moisture (examples: oxidation of metals, glass corrosion). In addition, the presence of moisture is the crucial precondition for fungal or microbial growth.

Significant changes in humidity and temperature involve dimensional changes of materials that are liable to cause tensions within the compound material, which can induce damages, in turn. As a consequence, short-term (hourly and diurnal cycles) fluctuations in humidity and temperature must be minimized in order to prevent damages. Accordingly, it is necessary to maintain a constant indoor environment to ensure the durable preservation of historical cultural heritage.

Usually, the dimensional changes due to temperature fluctuations are significantly smaller than those induced by a change in the relative humidity. This is why temperature changes are considered less critical than changes of moisture content. Partially, low temperatures are conducive to the conservation of materials, as some chemical processes are slowed down at low temperatures..35 Nevertheless, various organic materials tend to embrittle at temperatures below freezing point. If uncombined water is present in the composite material, there is also some risk of frost splitting.

The key control variable in museums and historical buildings is relative humidity. Its upper limit is set by the risk of microbial growth. Risk assessment with regard to micro-organisms basically requires to consider the climatic conditions prevailing near the surface; likewise, thermal bridging must be considered, too. Under thermal aspects, thermal bridges are the weakest points of the building

34 HOLMBERG, JAN G.: Relative Humidity, RH, in historic houses, museums and museum storage rooms, a

literature study. In EUROCARE Eureka Project EU 140, EU 1378 PREVENT, Preventive Conservation, Report No.1 from Swedish Partners. Stockholm, 1995

35 ASHLEY-SMITH, JONATHAN: Risk Assessment for Object Conservation. Oxford, 1999


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construction as the content of relative humidity is substantially higher at the surface of cold building components than in the center of the room. For existing buildings it is therefore recommended that a value of 50 % rh should not be exceeded during the heating season. In summer, however, values between approx. 60 and 65 % rh are considered to be noncritical. ASHRAE recommendations (2003)36 specify 60 % rh as the minimum level enabling microbial growth on the material surface. For a more precise assessment of the risk of mould formation, the isopleth model proposed by Sedlbauer (2001) is available (see Fig. 1), for which the growth conditions of more than 150 moulds affecting buildings were compiled and analyzed. 37 Also, the chemical and mechanical properties of some materials can deteriorate in environments with higher RH levels (examples: oxidation of metals, glass corrosion). However, dryness may also be detrimental to artifacts. Values below 40 % rh are estimated to be critical, as the mechanical embrittlement of some organic materials sets in at this point (e.g. polymers of animal glues). In the presence of salts feared to be detrimental to the building, the admissible relative humidity must be determined separately, also taking into account crystallization processes.

3.2.3 Museum environments

In museums, the so-called ICOM directions38 were considered to be authoritative for a long time. These directions had emerged from a survey conducted among several institutions. On the basis of this survey, the generally recommended indoor environment values of 50 % rh and 20 °C were derived, mostly with minor deviations of only ± 3 % rh and ± 1 K, for instance. Today, the trend is towards a much more differentiated approach, on the one hand distinguishing between the materials, on the other hand admitting seasonal climate changes in the course of the year 39, 40, 41 . It seems reasonable to accept the seasonal adjustment of climate specifications, as the required humidification (which is necessary to achieve a level of 50 or 60 % rh at 20 °C) can cause substantial moisture problems and damages in historical buildings, e.g. by formation of condensation and mould particularly in winter. Moreover, this kind of air-conditioning requires an extremely high energy input.

A fundamental work, which influences the handling of climatic data to this very day, is 'The Museum Environment' by Thomson (1986)42. Thomson describes that the choice of the indoor environment should be governed by the geographic circumstances of the respective location. In cold regions, where the air is very dry in winter, humidification may induce condensation problems in buildings. Therefore, a lower RH level than in the tropics should be targeted in those regions, for instance. For Class 1 museums of special or national importance, Thompson proposes a higher standard; Class 2 museums should observe climate values between 40 and 70 % rh, ensuring particularly sensitive materials to be stored separately in vitrines or cabinets.

36 ASHRAE Applications Handbook (SI), 2003 37 Sedlbauer (2001). 38 MICHALSKI, STEFAN: Relative Humidity: A Discussion of Correct / Incorrect Values. In ICOM Committee for

Conservation 10th Triennial Meeting Washington, DC, USA Preprints. Paris, 1993, 624–629 39 BURMESTER, ANDREAS: Die Beteiligung des Nutzers bei Museumsneubau und -sanierung: Risiko oder

Notwendigkeit oder Welche Klimawerte sind die richtigen? In Raumklima in Museen und historischen Gebaeuden. Bietigheim-Bissingen, 2000, 9–24)

40 Hilbert (2002) 41 KOTTERER, MICHAEL: Standardklimawerte fuer Museen? Restauro – Zeitschrift fuer Kunsttechniken,

Restaurierung und Museumsfragen, March 2004, no. 2, 106–116 42 THOMSON, GARRY: 'The Museum Environment', 2nd edition, London, 1986


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Table 8: Indoor environment setpoint values according to Thomson (1986)

Class 1 Class 2

Temperature Winter 19 ± 1°C Summer up to 24 ± 1°C

Sufficiently constant to stabilize rh

RH 50 or 55 ± 5% rh 40 - 70% rh

RH, heterogeneous collections

45 - 60% rh 40 - 70% rh

The upper limit for relative humidity is set by the risk of microbiological growth (see 38, 37). Further, the chemical and mechanical properties of many materials will undergo changes if these setpoint values are exceeded. Values below 40 % rh must be assessed as critical value, as the mechanical embrittlement of glue (chalk base) and paint layers starts already here. 43

In the German-speaking countries, Hilbert's publication entitled "Sammlungs-gut in Sicherheit" (1981, 1996, 2002) is attributed similar importance as Thomson's book internationally. Here, 35 % rh and 65 % rh are given as a general margin of safety for the relative humidity. However, different materials require different optimum conditions. Based on experience, it is known that metal and paper should preferably be kept in rather dry storage, whereas wood and canvas are optimally conserved in a RH range between 50 and 60 % rh. For artifacts combining diverse materials, a compromise must be found for the selected margin. With respect to the visitors' comfort, the temperature in a museum should vary from 18 to 25° C; temperatures around 16° C are considered to be reasonable for some materials, but not practicable (see (40)). Concerning the indoor environment in buildings that are classified as critical in terms of building physics, the seasonal fluctuation of temperatures (20 to 24° C) and relative humidity (45% to 55% rh) is recommended. All in all, not the absolute RH values are decisive, but the frequency of the fluctuations experienced. This is why short fluctuations that are due to variations in the adjustment control should be minimized to max. ± 2 K for the indoor air temperature and max. ± 2% rh for the level of relative humidity.

Burmester [39] takes up this discussion and relates it to the use of museums. Here, an approach is described for the first time that tries to assess the importance of extreme values according to their statistical frequency of occurrence.

In the ASHRAE Applications Handbook (2003), a differentiated approach is described in the Chapter 'Museums, Libraries, and Archives', which was prepared under the auspices of Stefan Michalski (Canadian Conservation Institute). Similar to Thomson's book, considerations are based on a target value of 50% rh (and a set point temperature between 15 and 25° C, respectively) or a 'historic' annual average for permanent collections. Depending on the envisaged indoor environment quality, it is differentiated between Classes AA and A through D, which are attributed different admissible

43 ERHARD, DAVID AND MECKLENBURG, MARION: Relative Humidity Re-Examined. In ROY, ASHOK UND

SMITH, PERRY (HRSG.): Preventive Conservation Practice, Theory and Research. Preprints of the Contributions to the Ottawa Congress, 12–16 September 1994. London, 1994, 32–38


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margins of short fluctuations, seasonal adjustment of the indoor environment parameters and various risk categories.

Table 9: Temperature and relative humidity specifications for museum buildings, libraries and archives, ASHRAE (2003)

Holmberg (2001) conducted investigations in several Swedish storage depots, including Skokloster Castle. His results recommend indoor environment conditions to vary between 40 - 60% rh and 5 - 18°C to ensure optimum preservation. Any values below 30 - 35% rh should be avoided, as irreversible damages will occur in that case. Likewise, values exceeding 65 - 70% rh should be avoided, as a higher RH level would give rise to mould growth. According to


- 31 -

Holmberg, variations of ± 15% rh during 24 hours are considered tolerable for objects made of wood.44

3.2.4 Indoor environment conditions requested for loan exhibitions

Last, but not least it is the conditions required for loan exhibitions that contribute to the proliferation of strictly formulated climatic standards in museums. In the case of loan exhibits, the lender's loan contract will require a Facility Report in most instances, which always includes the air-conditioning of the exhibition rooms. Usually, values between 50% and 55% or 60% rh ± 5% rh at 21° C are required (see Table 8)

3.2.5 Recommendations concerning the preservation of historical buildings and monuments

In the recommendations for the preservation of historical monuments and their furnishings, issued by the Bavarian State Office for the Preservation of Historical Monuments 45 , a constant indoor environment featuring a relative humidity between (optimally) 50 and 65% rh is proposed. It is recommended not to fall below a level of 50% rh, which is an empirical value derived from negative experience with heating churches and historical buildings. High levels of humidity should be primarily avoided in order to minimize the risk of microbiological growth.

It is generally recommended that a space be only moderately heated and, if necessary, with due regard to comfort and conservation. A basic temperature of 6 - 8° C is stated. Space heating and cooling (before religious services, for instance) must be done very slowly (1 K per hour, with a maximum total of 12 K).

Also, an experience made by conservators and restorers is quoted, which is quite close to the current discussion: "An existing indoor environment - irrespective of the prevailing humidity and temperature conditions - that does not cause damages to the furnishings or artifacts, needs no change.“46

3.2.6 Example: Indoor environment of an unheated church in Southern Bavaria

After the overall retrofitting and restoration of St. Margaretha's subsidiary church at Roggersdorf had been completed in September 2004, the church warden discovered recurrent moisture damages at the walls. The church, a solid gothic building constructed in field stones, is furnished with paintings, sculptures, and a high altar in the neo-gothic style. It is neither heated nor air-conditioned. The building is only naturally vented by the church warden as he thinks best. Subsequent climate measurements found that the damages were due to condensation problems, which occur primarily in the interseasonal period in spring. At this time of year the church building is still cold from the wintertime while warm, more humid air is introduced by natural air change or uncontrolled ventilation and condenses at the cold wall surfaces. However,

44 HOLMBERG, JAN G.: Environment Control in Historical Buildings. Royal Institute of Technology Building Services

Engineering, Bulletin No. 53, Stockholm, 2001 45 Bayerisches Landesamt fuer Denkmalpflege (eds.): Vorsorge, Wartung und Pflege, Empfehlungen zur

Instandhaltung von Baudenkmaelern und ihrer Ausstattung. Denkmalpflege Informationen, Munich, October 2002.

46 Bayerisches Landesamt fuer Denkmalpflege (2002).


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problems due to condensed moisture may also occur in summer and fall during periods of warm and humid weather.

Figure 14: Scatter diagram presenting climatic data measured in unheated St. Margaretha's church during 12 months, critical ranges of frost, microbiology acc. to Sedlbauer (2001), and dryness compared to the recommended indoor environment for important museums and collections (Museum Class 1) acc. to Thomson (1986)


- 33 -

Figure 15: Box plots of annual RH levels inside St. Margaretha's church, according to the meteorological seasons. 80 % of the values measured in the respective period are within the boxes.

Figure 16: Statistical analysis of the daily change in relative humidity (histogram) in St. Margaretha's church. Several days show RH fluctuations exceeding 15 % rh, and 7 of these days even show fluctuations exceeding 20 % rh.


- 34 -

The statistical analysis of the climate data recorded in St. Margaretha's church at Roggersdorf47 reveals the annual humidity level to be very high (Figure 14). 50 % of the measured indoor-air humidity values exceed 75% rh, thus ranging in an area where the growth of micro-organisms becomes probable. The risk of mould growth in the unheated chapel prevails mainly in the warmer seasons; despite a high level of relative humidity it is too cold inside the church in winter. For about 6 weeks, the internal temperatures are below 0° C. After the recently completed retrofitting and restoration work, the indoor environment still remains in a critical range, with long periods of frost in winter and high RH levels all the year round. Based on the ASHRAE approach, the church even fails to achieve Class D, as RH values are constantly above 75% rh (Figure 14). For 2005, the median is equal to 76% rh; for the temperature it is equal to 10° C. Above all, the fluctuations caused by venting the church are critical (Figure 16).

To prevent new damages, simple measures like controlled ventilation - mainly with regard to short fluctuations – or minimal heating (through electric heating cables, for instance) could be taken.

3.2.7 Concepts for preventive conservation

There is a great variety of approaches dealing with the risk assessment of climatic data. In central Europe, the key challenges to the conservation of historical buildings and monuments are frequently high RH levels inside historical buildings and the stability of indoor-environment conditions, which is difficult to be achieved. The museum sector provides several statistical approaches for risk assessment, some of which may be successfully transferred to the sector of monument preservation methods. In contrast to museums, decentralized air-conditioning solutions appear to be preferable when choosing methods for preserving historical buildings.

Because the recommended RH levels are above those recommended for buildings in general, the storing and presenting of important artifacts in climate-controlled cabinets should be considered.

Empirical experience gained from conservation practice and concerning the historic climate provides an important basis for understanding the processes that cause different materials to be damaged, in relation to the indoor environment and its admissible range of fluctuation.

All measures intended to improve the environmental conditions of artifacts can be subsumed under the notion of 'Preventive Conservation'. With regard to the indoor environment, Preventive Conservation must avoid extreme conditions and, most of all, reduce fluctuations. As most artifacts consist of diverse materials (e.g. wood with a base coat and paint layer) an optimum or appropriate range of environmental conditions must be identified for all materials. In addition, artifacts vary with regard to their manufacturing methods. This is why such defined ranges can only relate to the respective combination of materials. A general statement that is valid for all groups of materials proposes that an artifact will be optimally preserved in its environment if the climatic conditions are constant. Here, the crucial questions are: how strictly should climate fluctuations be limited and where exactly is the limit that allows to permanently preserve objects free of damage, while keeping the use of resources at an economically justifiable level and ensuring a

47 Kilian (2007).


- 35 -

satisfactory quality of user comfort. To answer these questions, further research is needed to proceed from the current empirical approach to an advanced, scientific approach. Or, to close with Robert Waller's words:

"Ideally, preventive conservation should be able to identify and quantify all risks to a collection or set of collections. We do not currently have the knowledge required to do this precisely. Nevertheless, by attempting to quantify all risks to collections, useful estimates can be obtained of the relative magnitudes of most risks."48

For the storage of hygrostatic materials and for some industrial processes, specialized temperature and RH levels may be required. These must be known to the designer before attempting to design the building envelope, but no general guidelines can be provided.

3.3 safeguard the building’s structure

The temperature and RH levels recommended for health and comfort of building occupants in general are acceptable for the building structure. Also, the prevention of mold growth will also reduce the potential for premature deterioration of interior finishes. Where significant and regular condensation is allowed to form on steel constructions, corrosion could become a problem. This is one reason that the thickness of any one part of structural steel sections as a rule should not be less than 0.5 cm. although thinner, cold-formed light-gage galvanized steel shapes are routinely used as secondary elements, such as for supporting metal curtain walls.

Failure has a fatal effect on the service life and durability of materials. Durability of materials to different agents varies: weather, water, heat, biodeterioration (mold and decay fungi, bacteria). Performance requirements of materials and structure should be different for inside surface, inside structures (isolation layer), and outside surface or structures, and the effect of different organisms on the service life should be verified carefully.

In order to predict failures we first have to define the term. Failure involves direct changes in the properties of materials or structures. The changes or deformations can be of various degrees: excess moisture can cause reversible or irreversible deformations or degradation in performance resulting from physical changes, chemical or biological processes. One type of failure is increased heat loss caused by high moisture contents in materials and airflow through building envelope systems. Other types are mold growth, rot damages, freeze-thaw cycles resulting in structural failures, dimensional changes, corrosion, emissions of volatile organic compounds (VOC), etc.

Some of these failures affect only the appearance of the systems under consideration, but some may have severe consequences such as risk to the health of occupants (sick huilding syndrome caused by VOC and mold) or structural collapse of the whole building. 49

48 WALLER, ROBERT: Conservation risk assessment: a strategy for managing resources for preventive conservation. In: Preventive conservation: practice, theory and research. Preprints of the contributions to the Ottawa Congress, 12-16 September 1994 (1994), pp. 12-16 49 Hannu Viitanen and Mikael Salonvaara: Failure Criteria: in Moisture Analysis and Condensation Control in Building Envelopes by

Heinz R. Trechsel, Astm Manual Series, Mnl 40, Astm Intl, 2001.


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3.3.1 Mould

Life on earth would not be conceivable without fungi, bacteria and other microorganisms. These organisms are responsible for the fast decomposition of dead material, splitting it up into its components and thereby giving it a new access to a further life cycle. Therefore microorganisms as fungi and bacteria are important components of our ecosystem. In buildings, however, favourable growing conditions for mould fungi can also occur and cause fungus infestation. Despite the quality of house building having improved over the last decades, especially by measures aiming at the reduction of heat losses due to transmission and ventilation, the number of reports on building damages caused by microorganisms, especially by mould fungi is still increasing. The “Third Report on Building Damages” by the Federal Government of Germany in 1995 50estimated the costs resulting from mould fungi damages to amount to more than 200 million Euro per year. Different causes, as for example the critical combination of the airtight construction method with insufficient ventilation of the building are given as reasons for the recent increasing occurance of mould fungi in dwellings. Whereas before the energy crisis in the 1970’s regulating the temperature was mainly operated by opening the windows, today, because of energy saving reasons, airing is not done as frequently. Especially the unintentional ventilation due to leakages was reduced considerably. As a result the air humidity in rooms rises. Thereby mould fungi do not only occur on the inside surface of external building components, but even inside construction parts. The danger for the occupants of dwellings lies in the settling and spreading of pathogens (disease causing agents) through microorganisms. Therefore, consequent measures have to be taken to avoid health dangers that come from mould fungi on the surface of building components. For example, when selling a building in the USA proof has to be furnished guaranteeing that the dwelling is free of the mould fungus Stachybotrys atra. In comparison to health aspects, building damages caused by mould fungi – i.e. the destructive effect the fungi have on building materials, like bio-corrosion or bio-fouling – only is of minor importance.

Mould in dwellings is a persisting complaint in moderate climates. Nine parameters define the likelihood for meld to develop:

1. climate, 2. inside temperature, 3. vapour release, 4. ventilation, 5. layout, 6. envelope thermal performance, 7. moisture buffer capacity inside, 8. presence of preferential condensation surfaces 9. type of finish.

Exterior climate acts as boundary condition while inside temperature, vapor release and ventilation belong to living habits. The five others parameters are design and construction related.

50 DRITTER BERICHT über Schäden an Gebäuden, Bundesministerium für Raumordnung, Bauwesen und Städtebau, 1995


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Hens51 reanalyzed thirty five mold (original study carried out at the Laboratory of Building Physics, KU Leuven from 1972 to 2002) cases with major objective to evaluate if the nine parameters explained the problem. The answer is yes. Especially lack of ventilation, large surfaces of exterior walling, low inside temperatures and poor envelope thermal performance rank high. A low inside temperature anyhow is typically the result of a rebound effect, the fact that the dwelling has such poor thermal performance that people accept lower comfort to keep heating payable. Ventilation in turn is to a large extent building-related. In most cases, no intentional ventilation system is provided. In insulated dwellings, thermal bridging is the main cause. If inside buffer capacity, preferential condensation surfaces and type of finish have an impact, could not be confirmed.

During the years 1998-2002 a major research campaign was carried out by altogether 13 institutions in Denmark to uncover a wide range of scientific and practice oriented issues related to the problem of mould in buildings 52. This report presents the results of a multidisciplinary research programme that lasted four years: “Moulds in buildings, 1998–2002”. The purpose of the programme was to gain more knowledge of the conditions that affect mould growth and human health. Furthermore the aim was to create basic knowledge of safe and cost efficient solutions for solving problems and for preventing mould growth in buildings during planning, operation, maintenance and renovation.

The programme covered health aspects, microbiological aspects and building aspects.

Health aspects – results

From clinical examinations and results from questionnaires of adolescents and young pupils, teachers and others employed in the schools, as well as from laboratory investigations, the following was found:

– A clear and statistically significant correlation between the level of moulds in floor dust and the number of irritation symptoms from eyes and upper airways along with general symptoms such as headaches, dizziness and difficulties with concentrating was demonstrated. The higher the level of moulds, the higher the level of symptoms.

– The correlation between exposure to moulds from floor dust and symptoms was significantly stronger among pupils with asthma and hay fever. This condition was especially pronounced for irritation of the mucous membranes of the eyes.

– The prevalence of airway allergies (hay fever and asthma) and the presence of airway infections was, however, not related to the level of moulds in floor dust.

– Laboratory investigations indicate that moulds affect the organism via the immune system. Activation of the immune system may actually induce the general symptoms observed in this study.

51 Hens, H.: Mold in dwellings: field studies in a moderate climate, Annex 41, Subtask 4, Kyoto meeting, April 2006. 52 Documentation 026, Moulds in buildings - A survey of the projects of the programme, Danish Building Research Institute, 2002


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– A larger number of water damages and an increasing extent of mould growth on walls and in building constructions were neither associated with an increasing frequency of the symptoms investigated nor with susceptibility to infections, hay fever or asthma.

– The laboratory investigations as well as the field studies indicate that the exposure routes for moulds are sedimentation of spores to floor dust either liberated from mould growth on building materials or brought in by footwear. The sedimented fungal particulates along with other dust particles may be re-suspended to the ambient air and inhaled when reaching the breathing zone.

– Accumulation of dust and mould must be minimized in rooms and buildings, as this can have adverse effects on human health.

Microbiological - and building aspects – results

– 15-20 % of Danish homes show visible signs of moisture or mould growth. Spot checks in owner-occupied apartments revealed that 13 % had visible mould growth but in most of these apartments the extent of the mould growth was small.

– The presence of moisture and mould in Danish public schools was recorded and showed that 9 % had water damages. 20 % showed indications of earlier water damage. However, spot checks revealed that half of the schools were actually water damaged. The water damages were generally of a limited extent and were primarily found in roof constructions.

– Roof constructions are generally vulnerable due to lack of maintenance and especially complicated roof constructions are vulnerable to bad workmanship or poor planning. Bathrooms with light constructions, basement constructions, e.g. crawl spaces; older buildings with rising damp and poorly maintained fronts are vulnerable constructions.

– Schools or other buildings with water damage do not necessarily have mould problems. This depends on the time of humidification; the resistance of the material used and on the infestation of materials prior to their installation. Humidified materials will, however, always present a risk of mould growth.

Moulds and building materials - results

– Experiments have shown that wood and wood-based materials; wallpaper and materials containing glue (starch) have the lowest resistance to mould infestation.

– Plaster and concrete, which have a very modest content of organic material, can be infested with mould growth, provided these have a high moisture content (water activity 0,95) which is a prerequisite for growth.

– High temperatures (up to around 28°C), rough surfaces and dust generally facilitate mould growth.

– A big difference has been demonstrated concerning liberation and dispersal of spores to the ambient air according to morphology of moulds and


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physical conditions of the buildings. The drying out of fungal growth spots is often a prerequisite for dissemination. This might partly explain why moulds in dust and not the extent of mould growth on the constructions were associated with the described symptoms on the occupants in this study.

– Experiments with controlled dispersal of spores from Penicillium chrysogenum and Trichoderma harzianum cultivated on plaster boards demonstrated fungal particles smaller than spores liberated to the air.During inhalation these particles will more easily reach the lungs compared with larger spores.

– Detailed studies of the toxic mould Stachybotrys chartarum, known to produce mycotoxins (satratoxins) during growth in buildings, have demonstrated that only 40 % of the isolates produce this kind of toxins, but all isolates produced other toxic metabolites.

3.3.1.1 Growth conditions for mould

A prerequisite for preventing mould fungus without the use of biocides is the knowledge of the boundary conditions under which fungus growth takes place. In reference to the boundary conditions for the growth of fungus it turns out that the decisive parameters of influence like humidity53, temperature 54 as well as the substrate 55 have to be available over a certain period of time simultaneously in order to enable the formation of mould fungi.

For the construction sector, literature often states a relative humidity of 80% at wall surfaces as decisive criterion for mould growth, independent from temperature. Sometimes it is mentioned that many types of mould can also thrive at lower humidity (see for example the draft of DIN 4108-X, Mould 56). Other growth conditions, namely a suitable nutrient substrate and a temperature within the growth range are usually taken for granted on all types of building elements.

The three factors required for growth – nutrients, temperature and humidity – must exist simultaneously for a certain period of time. This is the reason why time is one of the most important influence factors. It can be assumed that germinable spores are present in most cases. This means that mould growth will occur when hygrothermal growth conditions are fulfilled.

Important factors for mould growth: Moisture and temperature

For each mould species and temperature level there will be a minimal amount of moisture needed for the mould. The growth conditions for mould may be described in so-called isopleth diagrams 57. These diagrams describe the

53 Grant, C.; Hunter, C. A.; Flannigan, B.; Bravery, A. F.: The moisture requirements of moulds isolated from domestic dwellings.

International Biodeterioration 25, (1989), S. 259 - 284. 54 Smith, S. L.; Hill, S. T. 1982. Influence of temperature and water activity on germination and growth of aspergillus restrictus and

aspergillus versicolor. Trans. Br. Mycol. Soc., 79(3), S. 558-560. 55 Ritschkoff, A.-C.; Viitanen, H.; Koskela, K.: The response of building materials to the mould exposure at different humidity and

temperature conditions. Proceedings of Healthy Buildings (2000), Vol. 3, S. 317 - 322. 56 Deutsches Institut für Normung 1999. Wärmeschutz und Energie Einsparung in Gebäuden, Teil x: Vermeidung von Schimmelpilzen

(heat protection and energy saving in buildings, part x: prevention of mould growth), Beuth Verlag, draft 10.05.99. 57 Ayerst, G. 1969. The Effect of Moisture and Temperature on Growth and Spore Germination in some Fungi. J. stored Prod. Res., 5,

S. 127-141.


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germination times or growth rates. Below the lowest line (isopleth) every mould activity ceases. Under these unfavorable temperature and humidity conditions spore germination or growth can be ruled out. The isopleths are determined under steady state conditions, i.e. constant temperature and relative humidity.

Important factors for mould growth: Time dependence

Besides moisture and temperature is the time a critical factor for microbiological growth. The growth can be divided in a number of phases 58.. Figure 17 shows the microbiological growth under ideal conditions in a liquid with limited amount of nutrition and the colony growth on a material surface. Significant is the time lag for the start of growth. This is important for cases with a water damage or rain in the building period. For some materials it is possible to dry it out before microbiological growth starts. See Chang et al., 1995 59, Horner et al., 200160 or Johansson, 200361 .

Figure 17: Models of microbiological growth under ideal conditions. a) growth in a liquid with limited amount of nutrition b) colony growth on a material surface. Picture from [58].

3.3.1.1 Viitanen Model

Mathematical modeling of mould growth has been researched in the VTT Technical Research Centre of Finland for several years. The recent model of mould growth consists of a mathematical basic model that takes into account the delay and influence of fluctuating humidity conditions. In addition to wood, the model also contains correction coefficients for some other materials. Hannu

58 Cooke, R.C. and Whipps, J.M., 1993. Ecophysiology of Fungi. Blackwell Scientific Publications. 59 Chang, J.C.S., Foarde, K.K. and VanOsdell, D.W., 1995. Growth Evaluation of Fungi (Penicillium and Aspergillus spp) On Ceiling

Tiles. Atmospheric Environment, 29(17): 2331-2337. 60 Horner, E., Morey, P.R., Ligman, B.K. and Younger, B., 2001. How Quickly must gypsum board and ceiling tile be dried to preclude

mold growth after a water accident?, ASHRAE Conference IAQ 2001. Moisture, Microbes and Health Effects: Indoor Air Quality and Moisture in Buildings, San Francisco.

61 Johansson, P., 2003. Mögel på nytt och begagnat byggnadsvirke, SP Energiteknik, Borås.


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Viitanen has created regression models for a mathematical model on simulation of growth of fungi on wooden material in his dissertation62. The mathematical model was generated by Hukka and Viitanen 63for calculating the development of mould growth, which is expressed as mould index. The conditions favourable for initiation and temperature dependent critical relative humidity are shown in Figure 18.

Figure 18: Conditions favourable for initiation of mould growth on wooden material (left) and temperature dependent critical relative humidity needed for mould growth at different values of mould index (right) after Hukka and Viitanen (1999).

The mathematical model of mould growth is based on the regression model for mould growth. In Figure 19 predicted mould growth is expressed with a characteristic curve as a function of time 64.

Mould growth on materials is detected using microscopy or visually and expressed as mould indexes developed by Viitanen (1996):

Index for mould growth on materials - 0 = no growth - 1 = some growth (microscopy) - 2 = moderate growth (microscopy) (coverage > 10 %) - 3 = some growth (visually detected) - 4 = visual coverage > 10 % - 5 = coverage > 50 % - 6 = tight coverage 100 %

The mould growth model is combined with building physic models for analysing the critical humidity and temperature conditions 65, 66

62 Viitanen, H. 1996. Factors affecting the development of mould and brown rot decay in wooden material and wooden structures.

Effect of humidity, temperature and exposure time. Dissertation. Uppsala. The Swedish University of Agricultural Sciences, Department of Forest Products. Thesis. 58 p

63 Hukka, A, and Viitanen, H. 1999. A mathematical model of mould growth on wooden material. Wood Science and Technology. 33 (6) 475-485.

64 Viitanen, Hannu; Hanhijärvi, Antti; Hukka, Antti; Koskela, Kyösti. 2000. Modelling mould growth and decay damages Healthy

Buildings 2000: Design and Operation of HVAC Proceedings. Espoo, 6 - 10 August 2000. Vol. 3. SIY Indoor Air Information Oy (2000), s. 341 - 346


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I

Figure 19: The predicted mould growth at +5 and 40 °C in different humidity conditions (Viitanen et al 2000)

3.3.1.2 sopleth Systems

Significant differences exist among the various fungus species. Therefore, when developing common isopleth systems all known fungi were included that can be detected inside buildings. Quantitative statements on the growth preconditions temperature and humidity have been set up for more than 150 species that fulfill both features, as far as they are given in literature 67. Within the Isopleth model the prerequisites for the growth of mould fungi in dependence of temperature and relative humidity are given at first for the optimum culture medium. These isopleth systems are based on measured biological data. The resulting lowest boundary lines of possible fungus activity are called LIM (Lowest Isopleth for Mould).

In order to regard the influence of the substrate, that is the building materials or possible soiling, on the formation of mould fungus, isopleth systems (Figure 20, left side) for 4 categories of substrates are suggested that are derived from experimental examinations:

Substrate category 0: Optimal culture medium;

Substrate category I: Biologically recyclable building materials like wall paper, plaster cardboard, building materials made of

65 Viitanen, H and Salonvaara, M. 2001. Failure criteria. In Trechsel, H. ed. Moisture analysis and condensation control in building

envelopes, MNL40. ASTM USA. pp. 66-80 66 Viitanen H, Ritschkoff A-C, Ojanen T and Salonvaara M. Moisture conditions and biodeterioration risk of building materials and

structure. Proc. 2nd Int. Symp. ILCDES 2003 Integrated Life-time Engineering of Buildings and Civil Infrastructures. Kuopio, 1-3. 12. 2003

67 Sedlbauer, K. 2001. Vorhersage von Schimmelpilzbildung auf und in Bauteilen (Prediction of mould manifestation on and in building parts). Thesis, University of Stuttgart.


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biologically degradable raw materials, material for permanent elastic joints;

Substrate category II: Biologically adverse recyclable building materials such as renderings, mineral building material, certain wood as well as insulation material not covered by Ι;

Substrate category III: Building materials that are neither degradable nor contain nutrients.

For the substrate category III no Isopleth system is given since it can be assumed that formation of mould fungi is not possible without soiling. In case of considerable soiling, substrate category I always has to be assumed. Persistent building materials with high open porosity mostly belong to substrate category II. The basic principle of the new method and of defining the building material categories is to assume a worst case scenario, therefore always being on the safe side in respect to preventing the formation of mould fungi. To what extent the isopleth systems can be corrected for individual building material categories towards a more increased relative humidity with clear conscience, will have to be proved by further measurements. Especially in regard to risk assessment for individual art materials with a high risk of mould growth like canvas, book, leather, etc. further research is needed and planned.

Figure 20: Isopleth systems for 3 categories of substrates (left), in order to regard the influence of the substrate on the formation of mould fungus [67] and Isopleth systems for the so called critical fungus species (Class K, right, for optimum culture medium).

In order to differentiate the mould fungi according to the health dangers they may cause, a so called hazardous class K has been defined as follows: The isopleth system K applies to mould fungi, which are discussed in the literature because of their possible health effect (Figure 20 right). For the dangerous species Aspergillus fumigatus, Apergillus flavus and Stachybotrys chartarum growth data is available from [67]. The isopleth system for the fungi estimated as critical to health is based on the available data on optimum culture medium. To compile an adequate substrate specific isopleth for the critical fungus species precise measurements are missing.


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Examples for measured isopleth-ranges:

Desrciption of Figure 21

Figure 21: Isopleth-ranges dor three different materials, according to the determined

measurements. It is disdinguished between the inoculation area (where traces of nutrients may be) and original material surface. The “lowest isopleth for mould” (LIM 0) indicates conditions below which no growth can occur. left: Gypsum render with coating middle: Areated concrete right: wallpaper

3.3.1.3 Biohygrothermal Model

For transient boundary conditions of temperature and relative humidity, either spore germination time or the rate of mycelia growth can be determined with the help of these Isopleth systems. Yet the assessment of spore germination alone on the basis of the Isopleth model has the disadvantage that an interim drying out of the fungi spores cannot be taken into account in the case of transient micro-climatic boundary conditions. Therefore in these cases, this process will predict the germination of spores more often than the following Biohygrothermal Model. In order to describe the fundamental influences on the germination of spores, i.e. the humidity available at certain temperatures, this new model was developed.

The decisive condition for the germination of the spores is the ambient humidity which determines the course of the moisture content within a spore. The objective of the so called “Biohygrothermal Model“[67] is to predict this moisture balance as affected by realistic unsteady boundary conditions as they are found in buildings in order to permit predictions of growth probabilities. Of course the moisture content of a spore is also determined by biological processes, but the current knowledge is far from sufficient to allow the realistic modeling of these. It is safe to assume that a spore begins to germinate only above certain minimum moisture content and that no biological metabolic processes occur before. Until the end of the germination process the spore may be considered as an abiotic material whose properties are subject to purely physical principles (see Figure 22). The Biohygrothermal Model describes the development of the spore only up to this point. Due to the small size of the


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spore an isothermal model is sufficient, so that liquid transport processes (such as capillary suction) can be lumped together with diffusion transport. Under these assumptions only the moisture storage function of the spore and the moisture-dependent vapour diffusion resistance of the spore wall are needed as material parameters68. According to the assumptions noted earlier the germination is principally affected by thermal and hygric conditions only. Therefore it should be independent of the substrate. But normally the starting point of germination is defined by the first visible growth and not by the start of metabolism. The apparent start of germination depends on the quality of the substrate according to these considerations. This influence of the substrate is taken into account by using the LIMs (Figure 20) in order to calculate the so called critical water content.

Figure 22: Schematic diagram of the Biohygrothermal Model [67].

3.3.1.4 Testing mould resistance

There are two types of methods that are normally used 69, 70, 71.

A. The material is exposed for a fixed climate (relative humidity and temperature) and exposed to the normal types of spores found naturally on the material. The growth of this mix of species can then be tested.

B. The material is exposed for spores from one or more known mould species (so called “mould cocktail”). This is then placed in a fixed climate and the mould can growth. To prevent that spores on the material influence the result the material must be sterilized before the testing.

Different materials are attacked by mould in different extent and with different growth rate for the same climate. This makes it very difficult to compare measurement done in the laboratory and also to real exposures.

Different species can attack different part of the material as for instance gypsum plates. The testing result can depend on the mould species that is used in the tests.

68 Krus, M. 1996. Moisture Transport and Storage Coefficients of Porous Mineral Building Materials. Theoretical Principles and New

Test Methods. IRB-Verlag Stuttgart, S. 1-172, ISBN 3-8167-4535-0. 69 Hyvärinen, A., Meklin, T., Vepsäläinen, A. and Nevalainen, A., 2002. Fungi and actinobacteria in moisture-damaged building

materials - concentrations and diversity. International Biodeterioration & Biodegradation(49): 27-37. 70 Doll, S.C. and Burge, H.A., 2001. Characterization of Fungi Occuring on "New" Gypsum Wallboard., Conference Proceedings IAQ

2001. Moisture, Microbes, and Health Effects: Indoor Air Quality and Moisture in Buildings, San Francisco, California. 71 Clarke, J.A. et al., 1996. Energy Systems Research Unit in collaboration with Department of Bioscience/Biotechnology, Energy

Systems Unit. Dept of Bioscience and Biotechnology, University of Stracthclyde.


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3.3.1.4.1 Application Example: Mould Growth Prediction in the “Ruiksmuseum Amsterdam“

Historic buildings and museums are the most visible and important foundation of the European cultural heritage and contribute significantly to the attractiveness and identity of Europe for its citizens and visitors. Therefore we must take care that these invaluable testimonies of our past are maintained and protected in a sustainable manner. Although huge progress has been made with air conditioning and heating technologies while saving energy for modern buildings, most of the damages to collections of works of art in historic buildings are still caused by unfavorable climate conditions.

The change of use of rooms, storeys or whole buildings will in most cases lead to a change of internal climatic conditions. This is always given if a HVAC System or additional heating is implemented. Already during the phase of planning of substantial refurbishment measures in the museum it could be foreseen that decisive changes of the interior climate had to be expected, also due to the change of use. Rooms which were used only sporadically or as storage rooms were now to be used for the exhibition of objects of art and as a consequence would have numerous visitors. Additionally it was planned to fit the climate of the rooms to the requirements of the exhibited objects. This means temperatures between 19 °C and 23 °C at a relative humidity of up to 60 %. Furthermore the single glass windows were replaced by modern double glazing to reduce heat losses as well as to improve the security concerning solar irradiation and burglary. The upgraded insulation of the windows results in higher surface temperatures of the windows than of the walls. This means that now the dew point temperature is exceeded at first on the walls and not on the windows as before. With this the probability of condensate as well as of mould growth is increasing enormously. Already before the refurbishment the existing building showed moisture problems due to surface condensation. After the realisation of all measures planned, even more adverse relative humidity conditions had to be expected in the room and at the inner surfaces of the exterior walls.

For this reasons the risk of condensate and mould growth was calculated for critical building details by one- and two-dimensional simulations 72. Measured data were used for the outdoor climate. In case of predicted RH and temperatures, which are too high respectively too low, an internal heat insulation made of diffusion open calcium silicate was projected, because the historic façade had to remain unchanged. This kind of insulation has the effect of higher surface temperatures resulting in lowered RH at the wall surface. Because of its low diffusion resistance the drying of the wall to the interior is enabled. Resulting condensate will be spread due to its high capillarity.

As an example, results for the reveal of a window and for an outside wall will be shown. The reveal of the windows is made of sandstone from inside to outside. The simulation results show that because of its high thermal conductivity, especially during winter time, low surface temperatures occur as well as a high relative humidity (10.8°C and 97% RH, Figure 23 ). An interior

72 Häupl, P.; Petzold, H.; Finkenstein, C.: Feuchteschutztechnische und energetische Bewertung der Gebäudesanierung mit

raum¬sei¬tiger Wärmedämmung aus Calciumsilikat (Energetic and hygric assessment of an interior insulation with calcium silicate boeards), Abschluss-For¬schungs¬bericht, TU Dresden, Februar 2003.


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insulation of calcium silicate with a thickness of 40 mm will improve the conditions: the temperature reaches 13.1°C at 87% RH.

Figure 23: Course of temperature (left side) and relative humidity (right side) during the winter time at the reveal of a window and behind a canvas painting on the wall (72).

It was also planned to fix canvas paintings at the thinner sections of the exterior wall of rooms which had been unused until then. If the wooden frame of a picture is fixed holohedrally to the wall, it can be assumed that because of the missing air exchange the air layer behind the picture has a function of an additional insulating layer resulting in lowered surface temperatures. The courses of temperature and relative humidity (Figure 23, red and green colour) correspond to the inside surface of the exterior wall behind a picture. The positive influence of the interior insulating layer is obvious. During winter time the surface temperature is increased about 2°C and the RH is lowered about 10 to 15 %. Figure 24 shows the result of the mould growth prediction. Whereas with the interior insulation no mould growth is predicted, without additional insulation increasing problems have to be expected after the planned change of use.

Figure 24: Predicted Mould growth behind a picture fixed on the outside wall and at the reveal of a window without insulation (72).

3.3.2 Wood deterioation

by: Dominique Derome (Concordia University)


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When moisture transport occurs constantly through the building envelope, moisture can be adsorbed by wood and wood-based building materials where it may accumulate. Excessive moisture movement within these wooden materials may have the following consequences:

1. augmentation of the risk of fungus development and biodegradation under appropriate temperature and time exposure,

2. subjecting the materials to the effects of series of shrinkage/swelling cycles,

3. decrease of stiffness and strength,

4. deformation, warping and appearance of cracks

5. decrease of their thermal resistance, and

6. various impacts due to the corrosion of fasteners.

This section addresses the first point of this list. First, the wood anatomy is presented, following by some introduction to moisture storage and transport, then conditions leading to biodeterioration are presented, and the section ends with the concept of damage probability functions.

Wood anatomy

Wood is an orthotropic, composite material of biological origin. At the meso-scale level, growth rings can be observed consisting of early- and latewood layers (also called spring and summer wood). The center region of the trunk is referred to as heartwood while the wood at the perimeter is the sapwood.

The microstructure of wood is cellular, consisting of longitudinal cells, called tracheids and thin walled cells, called parenchyma or rays, located in the radial direction in an approximate proportion of 93 % to 7 %. In hardwood (deciduous trees), another structure of large openings, vessels, may be present.

The average length of tracheids in early- and latewood is similar, between 1800 and 4500 μm. In tangential direction, tracheid and lumen dimensions are almost equal for late- and earlywood. In cross section, earlywood is characterized by larger tracheids and lumen dimensions in radial direction compared to the latewood. For example, in spruce, the cell cross dimension may vary from 20 (earlywood) to 50 μm (latewood), with cell wall thickness between 8 and 12 μm.

Figure to come

Wood rays in radial direction cross through several growth rings and contribute to the moisture transport. However, more importantly for liquid transport, adjacent cell walls have openings, called pits, that play a major role in terms of intercellular water transport. Pits are more numerous on the radial surfaces thus enhancing permeability from cell to cell in the tangential direction. The overall diameter of pits is approximately 7 μm between tracheid, and quite smaller when connecting a tracheid with a ray. Typically, a bordered pit houses in its center plane a membrane made of a porous mesh on the perimeter around a central solid core named the torus.


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The wood cell walls show a multilayered, slightly porous structure. The cell wall density is usually reported to be 1500 km/m3, with porosity in the range of 18-20%. The layers are named S1, S2, S3 from the inside out. Layer S2, the thickest of the three layers, is the main contributor to the wood longitudinal strength due to its macrofibrils parallel organization, at a slight angle (10-30°) from the vertical. The macrofibrils (diameter 14-23 nm, making-up approximately 50% of the S2 layer material,) are composed of several microfibrils (diameter 2-4 nm) of crystalline (hydrophobic) cellulose wrapped in an unstructured binding matrix composed of lignin and different hemicelluloses. Of similar components, layers S1 and S3 act as thin layers of unorganized fibrils circling the S2 layer. Unstructured lignin and hemicellulose matrix is also found between the cells.

Figure to come

Density of most wood may varies from 320-720 kg/m3, with average construction wood density is the 400-500 kg/m3 range. Surface area of wood matter is difficult to determine; values of 29 m2/g (Poots and McKay 1979), and even to values above 200 m2/g have been reported (Annex paper A41-T2-Sl-06-1 Matiasovsky and Takacsova).

Moisture storage in wood

Equilibrium moisture content (EMC) in wood is determined macroscopically in terms of gravimetric (kg/kg), volumetric (m3/m3) or mass per volume (kg/m3) units, for quantity of water per quantity of dry wood for different steady-state conditions. Given that wood changes dimensions in function of moisture content, it is preferably to use the gravimetric EMC (kg/kg).

The bound water found in cell walls is attracted to wood with stronger forces than the free water held within the cell lumen. When wood is dried, the free water is lost first, as it is held with weaker forces due to capillary action. The traditional concept of fiber-saturation point (FSP) refers to a moisture content at which all free water of a cell has exited, but the cell walls are still saturated with water. FSP ranges from 20% to 40% of the dry weight. This concept of FSP is being revisited as different methods used to determine location and quantity of moisture in wood – calorimetry, NMR, neutron scattering – have indicated that water in both liquid and bound forms may coexist, even at EMC considered to be low (10%EMC). Nevertheless, the FSP concept is useful as a threshold moisture content below which shrinkage occurs and where stiffness and strength vary with moisture content. The average volumetric shrinkage is 9-12% from FSP to dry conditions but the change is anisotropic, e.g. for SPF wood, the change in the radial direction is in the 3-5% range, in the tangential direction 6-9% and in the longitudinal direction of less than 0.2%. Moisture content variations in the hygroscopic range may lead to mechanical damage like deformation, crack initiation, crack propagation and warping.

The variation of EMC versus moisture quantity in the environment (in terms of RH, capillary pressure or free energy) is reported using sorption curves, developed for a range of conditions at one temperature (therefore also named isotherms). Wood exhibits a hysteretic behavior as, during desorption, it retains more moisture than it can adsorb at any given relative humidity. The processes of full desorption (from wet to dry conditions) and full adsorption (from dry to wet conditions) of the same specimen, at the same temperature, yield different sorption curves. Also, wood exposed to series of different moisture conditions,


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alternating adsorption and desorption processes, result in intermediary or scanning curves. In the Annex work, sorption and desorption curves for spruce and walnut were measured (Annex paper A41-T2-Sl-06-1 Matiasovsky and Takacsova).

Figure to come

Hysteresis of wood has different potential explanations: bottle-neck effect of pores with liquid, reduced availability of sorption sites along the amorphous cellulose molecules in adsorption versus desorption.

Modeling of wood hysteresis still requires more attention. In modeling hysteresis, different approaches can be distinguished. Two major classes are: a. network models which are based on the both filling and accessibility rules, and b. independent domains theory, where the emptying of a pore is based on the size of the entry pore.

In the Annex 41, Carmeliet et al (A41-T1-B-04-8) compared two approaches to model hysteresis of wood:

1. empirical approach using the weighted values of the slope of the adsorption and desorption curves (Pederson, 1990)

2. phenomemogical hysteresis model, based on the independent domains approach (Mualem 1974, Everett 19..).

The second approach is further developed by tracking separately bound and free water.

More to be added here.

Vapor transport

Transport properties of moisture in wood are function of moisture content and dependent on wood orientation. Vapor permeability function can be approximate with an exponential law. An example of wood transverse diffusion coefficient function of moisture content and temperature is found in Hemeury, 2005 (also A41-T1-S-05-4). Vapor transport properties can be determined by up-scaling from an elementary cell (Siau 19.., demonstrated in A41-T1-B-05-6).

Examples of values depending on orientation to be added.

Biodegradation of wood

The first colonization of wet wood is by bacteria that decompose the sugars in the sap, then follow the molds that grow mainly on the surface of wood, finally, rot fungi attack the cellular structure of wood, decomposing first cellulose and hemicelluloses, and finally lignin.

Several studies of biodegradation of wood have been done. In what has become a reference piece of work, Viitanen (1996) found that one fungus of the brown rot decay family, a family of decay fungi found typically in wood structure exposed to moisture and water damage, required EMC of 25 to 28% for growth. These moisture content values result from equilibrium with air at relative humidity of 94 to 96%. For these moisture conditions, it was found


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that growth was activated at temperatures as low as 5°C after several months of exposure and was more rapid at 20°C after several weeks of exposure. In terms of temperature exposure, reported temperature limits for growth of decay fungi vary between –5° and +45° C. Lethal temperatures for fungi vary between 35°C and 80°C. At the conditions of 100% RH and 20°C to 30°C, the decay rates of wood accelerate. The minimum moisture requirement, for brown rot fungi to develop, is about the fiber saturation point of wood which corresponds to a relative humidity above 94 to 98%, depending on temperature. Other works by Holzkirchen (reference?), Yang , etc.

The widespread rule-of-thumb is that from a practical point of view (e.g. National Building Code of Canada), the general rule for wood protection in construction is to keep the wood moisture content below 20% independently of temperature and duration of moisture exposure. No fungi can grow at conditions below 20% EMC. In the next range from 20% to fiber saturation point or around 27% EMC, it is possible that, locally, a wood specimen has its fibers saturated with moisture. An example of the large variations in moisture content found within a sample of two centimeters is found in Hemeury, 2005 (also A41-T1-S-05-4), where the use of NMR for high resolution moisture content determination is demonstrated. In locations of high EMC, fungi growth would be possible. Above fiber saturation, and with temperatures in the 10° to 40°C range, conditions become very favorable for rot development.

Towards damage probability functions

Efforts to provide global methods to evaluation the durability of envelope assemblies look at the combination of parameters into so-called damage probability functions (also called damage functions, etc.). Examples of such work are Nofal and Kumaran, 1999 and in Japan, Suzuki et al (A41-T4-J-05-1), wood specimens exposed steady-state vs varying wetting conditions that may lead to rot were compared in term of longitudinal strength over a period of one year. Rot was observed mainly on the steady conditioned samples, for which reduction of strength was put into evidence. Rot was not observed on the samples exposed to varying conditions. Work is progressing to try to relate realistic conditions to damage of wood, in the larger objective to develop damage function for wood components.

References to come

Suzuki, H., Kitadani, Y., Iwamae, A., Suzuki, K., Nagai, H. 200?, Study on damage function of wood rot for building envelope design, Contribution to IEA Annex 41.

Zillig et al 2006 3rd IBP

STUDY ON DAMAGE FUNCTION OF WOOD ROT FOR BUILDING ENVELOPE DESIGN

From Astushi IWAMAE, Associate professor of Kinki University, Dr.Eng.

https://www.kuleuven.be/bwf/projects/annex41/protected/data/KIU%20May%202005%20Paper%20A41-T4-J-05-1.pdf


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Impact of Hygrothermal Conditions on Wood Rotting in Cedar Sapwood

Hiroaki Saito

https://www.kuleuven.be/bwf/projects/annex41/protected/data/BRI%20Oct%202005%20Prese%20A41-T4-J-05-8.pdf

3.3.3 Corrosion

Text from Achilles and Mika

3.3.4 Algae growth on façades

In recent times more and more complaints about algae growth on façades have been made. Due to the higher insulation standards the temperature and moisture situations have been changed on façades. This growth occurs mostly in the first years after completion which leads to displeasure from the building owner. Up to now there are different opinions about the reasons of growth of microorganism because the physical causes are not yet clarified.

Requirements for Growth

Algae are spread over the whole world and they are a major part of the ecosystem. They are reckoned as the biggest oxygen producer worldwide. A lot of research is done in the field of aquatic algae. But aerophytic algae have been recognised in the last years as important part in the microbiological growth on façades. The current opinion is that mainly blue and green algae grow on façades. Sporadically some red or gold algae are identified. There is a lack of knowledge about the growth conditions and the biotic influences. But some common demands can be specified. For photosynthesis sufficient light, water, temperature, carbon dioxide and some mineral nutrients must be present. Some algae need some trace elements (as Fe, Mn, Si, Zn, Cu, Co, Mo, B, V) for growth, which are normally available in our environment (rain, dust), so that the local micro climate is the determining factor for biological growth on façades. The most important climate conditions are humidity and temperature.

Humidity is fundamental for algae growth as it is needed for photosynthesis. Because algae don’t have any roots, the water uptake must occur directly through the cell wall by osmosis. The growth limit for green algae is at least 70 – 80 % RH (Denffer 198373) and for blue algae is 100 % RH (liquid water) (Scherer 199374). Wind driven rain and dew water are the main reason for wetting of façades with liquid water. Algae can survive dry periods without any harm and can restart their growth when enough humidity is available. Therefore a drying of façades during the day is not sufficient to prevent algae growth.

73 Denffer v., D., et al. 1983. Lehrbuch der Botanik für Hochschulen. Begründet von E. 74 Scherer, S. 1993. Anpassungen von Cyanobakterien in Wüsten. In: Hausmann, K. & Kremer, B. P.: Extremophile: Mikroorganismen

in ausgefallenen Lebensräumen. VCH. Weinheim; New York; Basel; Cambridge; Tokyo, S. 179-193.


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Venzmer describes the optimal growth conditions on façades for green algae within a temperature range from 0°C to 40°C (Venzmer 200175). Under dry conditions algae can withstand extreme thermal conditions (heat or cold stress) much better than in humid conditions. Therefore the prediction of algae growth on façades is an unsolved problem. This approach describes a first attempt to solve this problem.

Hygrothermal boundary conditions

The hygrothermal boundary conditions on the outdoor surface are very important for the growth of microorganism on the façades. These conditions are influenced by several and concurrent physical phenomena. The irradiation between the outdoor surface and the surrounding leads to a permanent loss of energy of the surface. The surface temperature increase by solar radiation during the day and therefore the relative humidity on the surface decreases and the wall gets drier. In the night there is no solar radiation and because of the thermal irradiation the surface temperature decreases enhancing the risk of condensation on the façade.

As mentioned above suitable temperature and humidity conditions at the outer surface of walls are necessary for biological growth. Moulds need a relative humidity of approximately 80 % (depending on temperature) for a longer period of time for example whereas algae need higher humidity for their growth or even free water. Whereas an interim drying out do not harm them. The fact that microbiological growth mostly occurs on the northern oriented façades, where very low wind driven rain arise, shows that the surface condensation due to natural long wave irradiation is an important moisture source on the façades. Therefore the periods of surface condensation during the night time may be suitable as one criterion to assess the risk of growth.

If the surface temperature gets colder than the dew point temperature of the outdoor air then condensation of the surface occurs. In Figure 25 he courses of surface temperatures of an old building (U-value = 1,1 W/(m²*K)) and a well insulated façade (U-value = 0,35 W/(m²*K)) are shown for a night in September. The effect of an additional insulation on the temperature of the outdoor surface is clearly visible. The surface temperature of the old building is higher than the dew point temperature during the whole night whereas the temperature of the well insulated façade decrease under the dew point temperature from 3 to 8 in the morning. The result is a higher amount of condensation on the surface of a well insulated façade.

75 Venzmer, H. 2001. Grüne Fassaden nach der Instandsetzung durch WDVS? Nicht bestellt und dennoch frei Haus. 3. Dahlberg-

Kolloquium


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Figure 25: Courses of the surface temperature of an old building and a well insulated façade with ETICS, compared to the dew point temperature of the outside air.

Beneath wetting by driving rain, condensation occurs in consequence of long wave radiation in clear nights by reaching temperatures below the dew point of the air. The periods of surface condensation and the accumulated degree of cooling below dew point temperature are taken as criterion to classify the results (Krus 200376). The following two construction types are compared by hygrothermal calculations with climate data for Holzkirchen in order to compare the surface temperatures with the dew point temperature of outdoor air. The first one is a wall made of concrete with an ETICS. The other wall consists of aerated concrete. Both walls have a similar thermal transmittance of about 0,35 W/m2K. Starting from a standard ETICS construction different surface properties were examined. Beneath the colour (radiation coefficient) of the rendering the influence of the long wave emissivity was of special interest. In addition numerous field tests have been conducted on walls with ETICS facing to the north as well as to the west to verify the calculated results.

Results by modelling

Spring and especially autumn are the most critical times of year, as in winter it’s mostly too cold and in summer mostly too hot and dry. Figure 26 sows the summarised hours below dew point in autumn for both constructions and two orientations. The difference between the construction with ETICS and the monolithic one is obvious. The monolithic wall gets more condensation on the eastern orientation in contrast to the wall with ETICS. That’s because the stored solar energy of the morning sun is partially gone until dawn, while on the western side the energy can be saved during night time. With ETICS this effect is not important because of its low thermal capacity. In total the amount of condensation is obviously higher with ETICS compared to the monolithic wall of aerated concrete. This corresponds with observations concerning biological growth on facades in practice. Therefore further investigation concentrate on ETICS.

76 Krus, M. & Sedlbauer, K.: Instationärer Feuchtegehalt an Außen¬oberflächen und seine Auswirkungen auf Mikroorga¬nismen.

Tagungsbeitrag zur IBK-Bau-Fachtagung 288 Bau¬schäden durch Schimmelpilze und Algen, Berlin, 27. Feb. 2003, S. 5/1 – 5/15.


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Figure 26: Comparison of the summarised hours below dew point in autumn for a construction with ETICS and a monolithic one for two orientations.

To get the influence of irradiative properties of the surface calculations have been conducted with a dark colour of the rendering instead of a white one (radiation coefficient of 0.6 instead of 0.4) and with a lowered infrared emissivity (long wave radiation coefficient of 0.6 instead of 0.9). As expected the surface temperature of the dark coloured plaster is reaching the maximum temperature during the day as shown in Figure 27. Also the version with a bright plaster but low IR-emissivity is getting warmer than the standard case. This means that even during the day every surface has anenergy loss due to long wave emission, which is not neglectable.


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Figure 27: Courses of surface temperatures for west facing walls with ETICS and different surface properties. Data for a sunny summer day (13th Sept.). The curves for outdoor air temperature and dew point temperature are shown additionally.

During nighttimes the thermal irradiation causes the cooling down of the wall. The surface temperature of the dark coloured facade, which shows the highest temperatures during the day, is cooling down below due point temperature for the same time as the white one. This shows that the thermal mass of the rendering on top of the insulation is too low to save enough solar energy for the prevention of condensation. The surface temperature of the ETICS with low IR-emissivity remains above dew point temperature instead.

Figure 28 shows the accumulated duration of condensation for walls with ETICS within the main growth period autumn in dependence on different surface properties. It is evident that a dark colour only causes a slight improvement, which means a reduction of condensation of about 5%. The advantage of a dark colour lies in the higher surface temperatures after sunrise, which enhance the drying of the rendering. The good performance for ETICS with infrared active colour compared to conventional paints demonstrates that this is a promising possibility to reduce the risk of algal growth on ETICS.


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Figure 28: Accumulated duration of condensation for walls with ETICS within the main growth period autumn in dependence on different surface properties.

Experimental results

Different field tests have been conducted to verify the calculated results. In the following two test facades are discussed, one with northern orientation is a standard ETICS and different top paints, the others with western orientation are monolithic constructions. Figure 29 shows a photographic view of the test facades. Beneath the outer surface of the walls the course of temperature has been measured to get the duration and the intensity of the condensation by comparing this surface temperature with the measured outdoor dew point temperature. Figure 30 shows an exemplary night time course of these measurements on the west oriented test facade. Here a standard ETICS with a white colour and a thin rendering of about 5 mm is compared to a system, where a thick rendering of about 15 mm is applied to enhance the heat capacity. Due to the higher thermal capacity of the thicker system the surface temperature drops later below dew point temperature than the thin system. But on the other side it exceeds the dew point temperature later, too. The grey coloured IR-paint shows a quite different behaviour. In this case the dew point temperature hasn’t been crossed at any time. It has to be mentioned that up to now no white IR-paint exists because the IR-effect is caused by adding metallic pigments. For this reason a grey coloured paint is chosen.


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Figure 29: Photographic view of the northern (left) and western (right) oriented test façade.

Figure 30: Courses of surface temperatures for west facing walls with ETICS and different spectral properties. The curves for outdoor air temperature and dew point temperature are shown additionally.

A comparison of condensation times for the described ETICS with monolithic constructions is shown in Figure 31. For the monolithic constructions, made of aerated concrete with a density of 400 kg/m³ and brick with a density of 600 kg/m³, the measured duration of condensation is with only 120 resp. 85 hours compared to the standard ETICS with 290 hours very low. The colouring of the ETICS brings a slight improvement of about 3 %. The IR-paint, which was available for these tests, reaches a long wave emissivity of 0.78 instead of 0.6, which were assumed for the computational investigations. With this the difference to the standard colour with an emissivity of 0.9 was less than half. With this IR-paint more than 20 % less hours of condensation have been reached, which corresponds well to the calculations.


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Figure 31: Accumulated duration of condensation for the different wall constructions and paints within the main growth period autumn.

Not only the duration of condensation and the amount of condensate is important to assess the risk of microbial growth, because for the microbial growth only the water on top of the surface of the façade is disposable. Therefore on the western façade the time course of this amount of water has been measured, too. For this dry paper towels were pressed on the different surfaces at certain times and the amount of absorbed water was determined by weighing. In Figure 32 the results of the measurements in the morning after a clear night can be seen. On the surface of the standard ETICS with a white colour nearly twice as much water has been obtained as on top of the thick rendering. It is important to take into account that this results are strongly depending on the hygric material properties of the rendering and paint, too. With the thick rendering more of the condensate is absorbed below the surface. The most impressive result comes from the surface with IR-paint. At this morning nearly no water could be measured on top of the surface. Nevertheless these IR paints have to be optimised for a practical use, because up to now all tested available IR-paints are getting a metallic glance after weathering. This is due to the special binding agent, which has to be transparent for long wave radiation but isn’t stable enough for the use outside.


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Figure 32: Courses of surface water for west facing walls with different ETICS.

As it is obvious that already small differences in the mean or long-term level of humidity cause visible differences in growth, a control of the hygric surface parameters could lead to a profound reduction of the biological infestation. In the context, it remains to be clarified in how far hydrophoby of the surface, providing for lower water absorption of the external render, will have the commonly expected positive effect of a reduction of pollution and microbial growth. It is also possible that a certain degree of absorbency of the substrate will help avoid microbial growth by deviating condensation water from the surface deeper into the building components so that it is no longer available for micro-organisms.

To determine the temporal progression of surface moisture under natural boundary conditions, the already described method of dabbing with a subsequent weighing of the fibrous web was used at the western-oriented external façade with different thermal insulation composite systems and coatings. The results are presented in Figure 33 for a two-hour interval in September. At the first measuring time at 8:00 a.m., the amount of condensation water on the thick and thin plaster system with mineral coating was remarkably lower than on the thin plaster system with silicone resin paint and ultra-hydrophobic coating. The amount of condensation water is still growing in the beginning concerning the specimen with the ultra-hydrophobic coating, whereas it remains almost constant with other specimens. The specimens with thick and thin plasters with water-repellent coating dry in the temporal progression, whereby the specimen with the thick plaster dries out more slowly than the specimen with the thin plaster. At 10:00 a.m., both walls are already dry at the surface, whereas on the silicone resin paints and especially on the ultra-hydrophobic coating some condensation water is still left.


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Figure 33: Gravimetric determination of the surface moisture on a test wall with different coatings.

The temporal progression of surface moisture may be calculated by applying the specific material characteristics of the plasters and coatings on the façade by means of a calculation method described in Krus & Rösler 200677. Figure 34 shows the amount of water on the external façade for an ultra-hydrophobic paint and a dispersion silicate coating, accumulated during the main growth period of micro-organisms in autumn. It is easily discernible that the dispersion silicate coating provides only half of the amount of water for micro-organisms. When assessing these findings, it must be taken into consideration that other influencing factors, e.g. self-cleaning effects, can also avoid microbial growth despite a high surface humidity.

Conclusions

The most essential criterion for the risk of microbial growth on façades is the availability of sufficient amounts of water. Dewing at night is of special importance, as it is the only possible explanation for an increase in microbial growth on the northern side with only small amounts of driving rain. In direct comparison of the monolithic walls made of aerated concrete or brick to the walls with ETICS the advantages of monolithic walls is remarkable. For the energetic improvement of existing buildings in most cases only the use of ETICS is applicable. Therefore solutions against microbial growth have to be found for these systems, too. The choice of a darker colour brings only a slight improvement, because of the lack of thermal capacity of the standard rendering. With renderings of higher thickness the amount of condensate is reduced a bit more, but the most advantage of this system lies in the capability of absorbing part of the condensate below its surface. The most effective method instead is the use of IR-paints, which reduce the physical effect responsible for microbial growth. Unfortunately up to now these paints are not stable enough for long-term weathering.

77 Krus, M. & Rösler, D.: New model for the hygrothermal calculation of condensate on the external building surface. Third

International Building Physic Conference in Montreal.


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Figure 34: Amount of water on the external façade for an ultra hydrophobic paint and a dispersion silicate coating, accumulated during the main growth period of micro-organisms in autumn.

But according to the present state of knowledge, it is obvious that the amount of condensation water is not decisive for the risk of microbial growth, but the condensation water on the external surface, which is available for micro-organisms in the initial phase of growth. Absorbent substrates could offer remarkable advantages in this respect.

An exact assessment cannot yet been made on the length of time condensation water must occur on a façade, before the growth of algae begins. Considering that despite dewing at night most installed thermal insulation composite systems remained free of damage, it is to be supposed that already a small decrease in condensate of approx. 20 % or more can mean a considerable reduction of the risk of microbial growth. But in special cases, for example with shady façades near a forest and a river or alike, the only applicable measure is the use of biocides.

3.4 Minimize the energy consumption

Something general about energy and moisture

3.4.1 Moisture and thermal conductivity

Most standards calculate thermal transmittance of envelope parts as if all materials used are airdry. Even for parts with initially wet layers, that is not too bad, as long as these parts are well insulated and the insulation material stays dry. For parts where the wet construction material also serves as insulation layer and for parts where the wet layer sits between two vapour retarding layers, together with a vapour permeable insulation, things are different.

In the first case, the part starts at a higher thermal transmittance than projected. For aerated concrete for example the starting value may be twice the final one, i.e. an increase with 100% (Figure 35). As drying progresses, thermal transmittance drops to its final air-dry value at a rate which depends on the thickness of the wall, the temperature and relative humidity in- and outdoors and the water vapour diffusion resistance of the inside finish.


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Figure 35: Measured heat conductivity as a function of the materials moisture content

In general, thermal conductivity is a function of moisture content which is determined by guarded hot plate measurements of dry samples and of samples conditioned in climatic chambers at 80 % R.H.78. For common insulation materials (mineral wool, polystyrene foam) the hygrothermal material properties are well-known but not enough data are available for ecological insulation materials. Since they usually have rather high hygroscopic moisture contents building standards prescribe that a formal increase be added to measured dry values to arrive at the design value for thermal conductivity. For example the German standard a 20 % supplement is assumed for determining the design �-value.

In Figure 36 the temporal evolution of the thermal conductivity for cellulose fiber is plotted as determined from the transient heat flux in the guarded hot plate at different temperature levels. Due to the hygroscopic nature of cellulose fiber, the dry sample absorbs minor amounts of vapor from the ambient air. This effect will be included in the evaluation of the measured results. The bottom graph shows the extrapolation of the heat conductivity for the temperature range between 10 °C and 40 °C. From the difference of these two linear functions the moisture related increase of the heat conductivity can be calculated. The results show that the moisture supplement at 80% R.H. related to the dry conductivity attains 6 %. This is much less than the 20 % supplement assumed in the German standard. The discrepancy can be explained as follows:

78 DIN 52616: Bestimmung der Wärmeleitfähigkeit mit dem Wärmestrom¬meßplatten-Gerät. Beuth-Verlag. Berlin, November 1977.


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Figure 36: Measured development of the heat conductivity for cellulose fiber. The results of the top graph represent the measurement of a dry sample, the ones in the middle those of a sample conditioned at 80 %RH. The bottom graph shows the extrapolation of the heat conductivity for the temperature range between 10 °C and 40°C. From the difference of these two linear functions the moisture related supplement of the heat conductivity can be calculated.


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Figure 37: Comparison between measured and, with WUFI calculated average heat flux for a sample of cellulose fiber conditioned at 80 % RH. top: calculation, where the measured moisture supplement and latent heat effects are taken into consideration. middle: calculation, where the measured moisture supplement is neglected but latent heat effects is taken into consideration. bottom: calculation, where the measured moisture supplement is taken into consideration, but latent heat effects neglected.

Figure 37 shows the measured and the calculated average heat flux through a sample of cellulose fiber conditioned at 80 % RH. The calculations are carried out in three different ways. The first run includes latent heat effects by moisture evaporating at the hot plate and condensing at the cold plate as well as a moisture supplement of 6 % on the thermal conductivity. In the second and


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third run only the latent heat affects or the moisture supplement is taken into account. It should be mentioned, that it is physically not correct to exclude latent heat effects during hygrothermal calculations, but here it is necessary in order to estimate its importance. From the results the conclusion can be drawn, that the measured moisture supplement of 6 % is only a latent heat effect. This means that the real thermal conductivity of the insulation material in equilibrium with 80% R.H. is not higher than in the dry state. But the temperature gradient in the insulation material during the test leads to a redistribution of the sorption moisture in the sample by vapor diffusion. This entails an enthalpy flow which affects the guarded hot plate test in a way similar as the heat flow by conduction. However, there is a distinct difference between the enthalpy flow and the conduction heat flux. The enthalpy flow is transient in nature and reversible. That means it ceases as soon as the local sorption equilibrium is achieved. When the temperature gradient is reversed the enthalpy flow changes direction and the lost energy is recovered. Therefore, the latent heat effects should not be included in the thermal conductivity.

Figure 38: Calculated interior heat flux of a south orientated roof construction (inclination 50°) with cellulose insulation during 4 winter days with high radiation. The Figure shows the calculation with and without latent heat effects.

There is also some redistribution of moisture in an annual cycle where the moisture tends to accumulate at the exterior side of the insulation in winter, but this redistribution only occurs once per heating period and overall latent heat losses are thus negligible compared to the heat transmission. As a practical example Figure 38 shows the HAM-calculated interior heat flux of a roof construction with cellulose insulation and orientation to the south during 4 winter days with high radiation 79. The results shows that the mean heat fluxes of the calculation with and without latent heat are equal. If we integrate this heat fluxes over one heating period the difference between these heat losses is only 0.5 % and therefore these transient heat effects should not be included in the design value of thermal conductivity insulation materials.

79 M. Kehrer, H.M. Künzel, K. Sedlbauer: Dämmstoffe aus nachwachsenden Rohstoffen - ist der Feuchtezuschlag für die

Wärmeleitfähigkeit gerechtfertigt? IBP Mitteilung 390 (2001)


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3.4.2 Infuence of build in moisture on the energy performance

Buildings face several moisture sources, among them built-in moisture. Its presence reflects the history a building and the materials used face during production, transport and processing at the building site. Some materials are humid at delivery, an example being aerated concrete. Rain wets walls and floors during construction. Water is used in lager quantities than needed to facilitate brick-laying. Some chemical reactions have water as one of the reaction products. As a result, especially stony buildings start their service life at quite a high humidity. The only performance criterion forwarded in relation to initial moisture is that it should dry without causing harm. When drying is retarded, drawbacks include a lasting, higher thermal transmittance than planned, built-in moisture that migrates to other, more moisture-sensitive layers in the construction part to condense there and possible remoistening of already dry layers. But also drying itself could cause nuisances. A typical one is a several months long transient of high relative humidity indoors, favouring mould growth in edges, corners and on window reveals. And, whether slow or fast, drying of built-in moisture costs extra energy.

The application of internal finishing materials to a masonry wall with built-in moisture is going to affect the building's drying behaviour. As the moisture trapped inside the wall construction will reduce the efficiency of thermal insulation, rapid drying is important also with regard to energy aspects. The effects of two different finishing systems (gypsum plaster and tiles) regarding the duration of drying processes and resulting annual heating demand are determined by calculation.

The building is suggested to have a flat roof (R=5.4 m2K/W) and an exterior wall of 36.5 cm thickness (R=2.7 m2K/W). The structure of the wall construction consists from outside to inside the of lime external plaster, aerated concrete and gypsum plaster. A window with a surface area of 50 m² is installed on the southern side of the room, whereas the surface area of the other windows in other directions is 20 m² respectively. The room has a volume of 1625 m³ and a ground area of 250 m². The suggested boundary condition is that the temperature is 20 °C in this room with between 8 a.m. and 4 p.m. During this time 40 students are present. The suggested humidity production rate is 16 kilo per day. During the rest of time, i.e. at night, the temperature is 18 °C with no humidity production and heat sources. During operation the air change rate is suggested at a value of 1 h-1, i.e. once in an hour the complete indoor air is exchanged. The strict keeping of temperature and relative humidity values is guaranteed by means of an idealized air-conditioning in the programme WUFI® Plus.

Figure 39 presents the influences due to the inner surfacing and the insulating material on the anural heating demand, the interior relative humidity and the water content of the AAC. The variation featuring files on the inside produces the slowest drying process. By contrast, the system using interior gypsum finish dries fastest. Here, drying-up occurs both at the inside and the outside. The impact on the heating demand is indicated at the top of the diagram,


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Figure 39: Influence of inner surfacing on the drying behaviour (bottom) of the AAC with

inherent moisture; interior relative humidity (middle) and the effect of moisture on the achieved heating demand (top).

The investigations suggest that the diffusion resistance of the interior finishing is a key parameter. The simulations show that it will take several years to reach the hypothermal equilibrium, particularly in the case of tiles. The high moisture


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content of the AAC is strongly changing the U-Value but also ventilation heat losses must also be added to the increased energy consumption caused by build-in moisture. These ventilation losses are due to the necessity of venting out the building moisture that is migrating towards the inside.


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4 Indoor climate control

4.1 Active Strategies

Something general

4.1.1 Humidity controlled ventilation strategies

Background

Ventilation plays an essential role in the indoor environment. Ventilation is needed to maintain good indoor air quality by diluting and removing pollutants emitted within the ventilated space. There are different needs to ventilate the rooms including to remove of human bio effluents (CO2 levels can be used as an indicator) and odour to the acceptable level, to limit pollution and humidity that is produced through normal activities of occupants, to limit pollutants produced by emissions from building materials and furnishing; sometimes the dominant need for ventilation may be to meet cooling needs.

Reducing energy, used for ventilation of buildings, should be making without compromising the indoor air quality. In ventilation systems with variable air flow controlled by any criteria representing demand, timer or occupancy detection, the ventilation rate may vary between maximum and minimum depending on the occupancy and pollution load such as humidity production.

A high-performance low-energy ventilation system needs to combine two opposite demands: an adequate air flow rate for a satisfactory IAQ and a minimal air flow rate to reduce ventilation heat loss. The basic question is, how does moisture buffering affect the performance, and how much energy can be saved by using humidity controlled ventilation strategy?

The energy savings possible with carbon dioxide Demand Controlled Ventilation (DCV) 80, 81 systems have well been established (10-80%), but on the other hand questions have risen concerning IAQ if only CO2 is taken into account82. Unlike CO2, the humidity in a building is not solely dependent on the presence of occupants, but also on ventilation with outdoor air, cooking, showering, washing and drying laundry. Furthermore high indoor humidity levels often lead to health related concerns and building damage. Possibly humidity DCV can overcome some of the flaws of CO2-based DCV.

Humidity Demand Controlled Ventilation

Some backroundtext about how the system works (Areco ?) and measured results

Measurements on humidity controlled ventilation systems in 55 occupied apartments

S. Berthin, J.L. Savin and M. Jardinier

80 Persily A., Musser A., Emmerich S., Taylor M. 2003. Simulations of indoor Air Quality and Ventilation Impacts of Demand Controlled

Ventilation in Commercial and Institutional Buildings NISTIR 7042. NIST. 81 Emmerich S.J., Persily A.K. 1997: A literature review on CO2-based demand controlled ventilation, ASHRAE Transactions., Atlanta,

GA, 1997. 82 Afshari A., Bergsøe N.C. 2003: Humidity as a Control Parameter for Ventilation, Indoor and Built Environment 2003; 12:215-216


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https://www.kuleuven.be/bwf/projects/annex41/protected/data/Aereco%20Apr%202007%20Paper%20A41-T4-F-07-1.pdf

Ventilation controlled by a dew point sensor

Adequate ventilation, intended to avoid mould growth, must avoid surface boundary conditions as well suited for mould growth, at least in the long run. From the energetic point of view, forced permanent ventilation does not solve the problem. The solution must be found in dependence of boundary conditions occurring on surfaces and caused primarily by transient humidification processes (cooking, showers etc.). Continuous experimental determination of the surface humidity and temperatures is not applicable due to the sensors and costs. Thus, ventilation systems, which are operated at present, are mostly regulating constant air change rates. Moisture load and the risk of mould growth remain unconsidered.

The idea of the development is to install an artificial thermal bridge at a selected point of the external wall. With the help of different levels of hygrothermal modelling this “thermal bridge” may be thermally designed in a way that condensation occurs, if the surface humidity has reached a value in the problematic areas of the internal surfaces of external walls (e.g. corners) to expect mould growth. Figure 40 shows the schematic design of such an artificial “thermal bridge”.

Figure 40: Schematic design of an artificial “thermal bridge” with dewing sensor.

If there are simple, switching dewing sensors on the internal surface of such an artificial “thermal bridge” (structure e.g. Figure 41 but also simple resistive sensors or others), ventilation systems can be operated, as long as there is


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condensation. Without condensation, the ventilation system will be automatically switched off according to requirements.

Figure 41: Schematic design of a dewing sensor for the regulation of ventilation.

The advantage is, that with the application of such a dew point switch the ventilation is only operating, if the external air temperature is lower than the internal air temperature, what is correct from the building physical point of view. Thus, ventilation is only working, if the indoor air humidity is high and the internal wall temperature is so far below the indoor air temperature that the indoor surface humidity on the outside walls is too high. Unnecessary and partly false ventilation is thus prevented. The special advantage of the application of such a dew point switch is that it is working without any measurement equipment. The dew point switch is cost-efficient and almost maintenance-free, regular calibration measures are unnecessary.

The investigations are carried out in an old test building with two small rooms and bad insulation. The wall structure is made of 27 cm pumice concrete blocks with internal and external plaster. A window is at the south side, which is covered during the investigations in order to guarantee that there is no influence of solar radiation during the measurements. Measuring sensors were installed at different places to determine temperature and humidity conditions as well as the energy consumption of the heating and the effectiveness of the ventilation system on the indoor climate. Heating energy consumption and ventilator performance is determined by an energy performance transmitter. Artificial thermal bridges of metal cylinders of different length were installed in the wall. Each dew point sensor on the thermal bridges can be used to regulate the ventilation system.

Dew point sensor system versus commercial humidity controlled system

Besides the newly developed ventilation system with dew point switch another commercial and retrofit ventilation system was investigated for comparison. This system has an air supply element with opening dampers, which are


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regulated by an installed humidity sensor. The ventilator in the air outlet element is regulated to a constant pressure difference. With increasing opening of the dampers in the air supply element, the pressure difference is falling and the ventilator must work under higher performance

For an appropriate assessment of the influence of the ventilation system it is important to know the indoor climate conditions in the test house in case that the ventilation system is not operating. In this case, the supplied humidity can only be absorbed or ventilated by the sorption behaviour of the room-enclosing materials and by natural air exchange. The courses of the temperature and humidity in the test room are shown in Figure 42.

Figure 42: Courses of temperature and humidity in the test room, if air exchange is only affected by natural infiltration.

Figure 43 presents as an example the indoor humidity courses (red curve), the humidity on the thermal bridge surface (black curve) as well as the switch points of the dew point switch (blue curve) for the 15 cm artificial thermal bridge. It is discernible that the dew point switch correctly switches on the ventilation, if 100 % humidity is reached at the surface and condensation occurs. The indoor humidity (red line) increases during humidification by approx. 10 % to a maximum of approx. 55 % of relative humidity. In the process, the striking effect is that the dew point switch again and again switches on and off the ventilation during the periods of humidification. One reason is probably the fact that the performance of the installed ventilator is too high. It can be expected that the restriction of the ventilator results in a clear reduction of the switching operations. This remains to be rechecked in further test series.


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Figure 43: Courses of humidity for two subsequent selected days and of the switch points with ventilator regulation by the dew point switch of the 15 cm thick thermal bridge.

The course of indoor air humidity with similar external air conditions in case of ventilator regulation by the commercial ventilation system is presented in Figure 44. It shows a higher range of fluctuations as the course with the application of a dew point switch, presented in Figure 43. Besides the measured ventilator performance (black curve) the damper position (green curve) is also presented in which the value at the low end of the diagram indicates completely closed and at the upper end completely open. It is discernible that the ventilation system is constantly working in this case, thus providing a permanent air exchange, which is higher than the infiltration air exchange.


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Figure 44: Courses of indoor r.H. and depending reactions ventilator regulation by the commercial ventilation system for two subsequent selected days.

The effect of the electrical room heating was also recorded during the measurement periods. Figure 45 ows indoor and outdoor temperatures for both regulation systems as well as the course of the heating output. As is obvious, the heating output clearly increases during the humidification phases in case of the regulation with dew point switch (left) as well as in case of the regulation with humidity-regulated air supply element. Yet this fact is not only due to an increase in air exchange at that point of time. In this case, the evaporation heat of the humidity supplied by the ultrasound evaporator must also be added. If comparing both figures, it is easily discernible that there is a lower heating output between the humidification phases in case of ventilation regulation by means of the dew point switch. It is also obvious that heating output increases to similar value during the humidification phases. During these measurement periods with similar mean outdoor temperatures of approx. – 5 °C the result for the medium heating output was 1,320 W in case of heating with the regulated air supply element. In case of applying the dew point switch, the result was only 1,160 W, although the adjacent room was not heated. But long-term measurements would be necessary to give a secure assessment of the energetic savings by using the dew point switch.

The essential difference of the dew point sensor system in comparison to other ventilation systems is that the purpose of this device is not to care for ventilation in favour of the resident and to patronize him or her. The system is only operating, if the ventilation behaviour of the users is insufficient, or so to speak a kind of automatic emergency ventilation. In case of sufficient ventilation, the resident will not take any notice of the device. These are the reasons, why such a ventilator regulation is promising for the area of social housing, as especially in this field conscious handling of living space and ventilation cannot be taken for granted.


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Figure 45: Courses of temperatures and heating output for two subsequent selected days with ventilator regulation by means of the dew point switch (left) and by means of the humidity-regulated air supply element (right).

4.1.2 Minimum ventilation requirements for buildings from the mould point of view

To reduce unnecessary thermal losses by uncontrolled air exchange in buildings due to leakages and to avoid damages of buildings by condensation in the building structure, the air-tightness of buildings is increasingly improved. In particular, improper manual ventilation by short-term opening of the windows may cause mould fungus growth due to high internal moisture loads. Ventilation according to requirements reduces the risk of mould fungus growth due to adequate air exchange without causing unnecessarily high ventilation heat losses. „Ventilation according to requirements means the optimized operation, when the air flow rate … is adjusted to requirements“ (abridged quotation from [83]).Due to different utilization and moisture loads, ventilation requirements are different for each room [84].Krus et all [85]determined an optimized ventilation strategy in dependence of different moisture loads by coupling the hygrothermal indoor climate model WUFI®Plus as well as the bio-hygrothermal model WUFI®-Bio.

A characteristic 3-room-apartment was selected for the computation of a ventilation strategy to avoid the growth of mould fungus. Constructions with ETHICS (variation 1) and highly thermal insulating masonry (variation 2), both constructions with an insulation standard according to the energy-saving directive, as well as an example for an old building, masonry with a minimum thermal insulation (U-value 1.4 W/m²K) with old windows (variation 3) and new and airtight windows (variation 4) were used [86]. The moisture load is not generalized in this investigation and determined by a mean value for the apartment, but each room is considered separately. Calculations are carried out

83 Hartmann, Th.: Bedarfsgeregelte Wohnungslüftung.(Airing according to requirements.) Tagungshandbuch Hermann-Rietschel-

Colloquium 2002, S. 79-87. 84 Richter, W.; Hartmann, Th.: Mindestluftwechsel zur Verhinderung der Schimmelpilzbildung in Wohnungen.(Minimum air change for

the prevention of mould growth in flats) VDI-Berichte Nr. 1603, (2001), S. 121-130. 85 Martin Krus, Kristin Lengsfeld, Andreas Holm, Klaus Sedlbauer: Minimum ventilation requirements for buildings from the mould

point of view, IEA Annex 41 Workmeeting, Tronheim October 2005 86 Kainz, E.: Lüftungskonzepte zur Erhaltung der Raumluftqualität und gleichzeitiger Vermeidung von Schimmelpilzen. (Ventilation

strategies for the maintenance of the air quality and prevention of mould at the same time.) Diplomarbeit, Fraunhofer Institut für Bauphysik Holzkirchen, Fachhochschule Rosenheim (2004).


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for each room separately. As a result, the moisture exchange between the rooms, for example by opening the doors, is not determined. The characteristic moisture generation was analyzed for each room showing realistic utilization conditions. Moisture generation due to residents, plants, hygiene and cooking is normally considered. In a worst-case determination, moisture generation is additionally increased by drying laundry. In this case, the moisture generation of one dried load of a washing machine per day was divided according to the room volume and added to the normal moisture load. According to utilization, all rooms were based on a respective moisture generation. The determination of air exchange rate requirements or of the duration of short-term ventilation by opening the windows for air exchange is carried out iteratively by determining the temperatures and moisture of the room corners at a given ventilation profile by means of the modified hygrothermal indoor climate model. Based on this determination, the risk of mould fungus growth is assessed by means of the bio-hygrothermal model with the subsequent adjustment of the ventilation profile until adequate ventilation requirements are reached (Figure 46).

Climate Building part Humidification Ventilation

BiohygrothermalModel

Whole Building Model

Mould ?

Yes

No

Evaluation

Figure 46: Diagram of the individual computation steps to determine ventilation requirements to avoid mould fungus growth.

As an example Figure 47 describes the resulting ventilation requirements in order to avoid mould fungus growth in case of constant ventilation or short-term ventilation by opening the window for air exchange for a sleeping room of an apartment with ETICS. In case of the constant minimum air change rate (red line) or minimum air change rate by short-term opening of the window (violet line), surface moisture occurs in the external room corners, which is just below the required conditions of mould fungus growth according to the computation by the bio-hygrothermal model (WUFI-Bio). The green line describes the infiltration air change rate, defined and contained in the constant minimum air exchange rate (0.5 h-1 in case of an old building with old windows, otherwise 0.1 h-1). The impact of an increased moisture load caused by drying laundry is evident. If laundry is dried, ventilation by opening the window for 15 minutes in the morning with an air exchange rate of 10 h-1 is no


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longer sufficient to avoid mould growth. In this case, the windows must be opened three times a day over a longer period of time.

airin

g ra

te [h

-1]

moi

stur

e pr

oduc

tion

[g/h

] normal cloth drying bedroom

time [h] time [h]

permanent ventilationintermittent ventilationinfiltration

airin

g ra

te [h

-1]

moi

stur

e pr

oduc

tion

[g/h

] normal cloth drying bedroom

time [h] time [h]



Figure 47: Above: Course of moisture generation in a sleeping room of an adequately insulated apartment without (left) and with (right) drying of laundry in the living room. Below: Determined ventilation requirements to avoid mould fungus growth in case of constant ventilation or short-term ventilation by opening the window for air exchange. The infiltration air change is also described.

Figure 48 shows the constant ventilation requirements to avoid mould fungus growth for all variations and all rooms. An additional ventilation requirement in case of drying laundry is obvious. Due to a higher moisture generation in the bathroom and in the kitchen, especially in old buildings, the ventilation requirements are clearly higher. But the use of an extractor hood with exhaust air is normally sufficient to remove the moisture (without consideration in the context). According to the results, the clearly higher ventilation requirements of the kitchen of variation 2 in comparison to variation 1 with constant air change are obvious. The two variations only differ in the structure of the wall. Variation 1 has an ETICS, and variation 2 is built of highly iinsulating masonry. The reason for the different thermal performance is the location of the kitchen, which is the only investigated room exposed to driving rain on the western wall. Due to the capillary absorption of the external rendering and brick used for the calculations, lower internal surface temperatures and consequently higher ventilation requirements occur for variation 2 caused by temporarily higher wall moisture contents. The application of a hydrophobic plaster would result in reduced ventilation requirements. As was to be expected, a lower insulation standard causes clearly higher ventilation requirements. But with the exception of the kitchen and bathroom even in this case the generally required air change rate of 0.5 h-1 is sufficient to avoid mould growth, as long as laundry is not dried in the apartment.


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Constant air exchange Short-term ventilation

Figure 48: Required constant minimum air change rates (left) for different rooms to avoid

mould fungus growth and in comparison required ventilation by opening the windows for hourly air change rates as mean value per day (right) for all variations of the model apartment.

Figure 49 shows the influence of different ventilation behaviour (ventilation during the day or at night) for a sleeping room on the heating energy demand of the four variations. It is obvious that if an apartment is adequately insulated, short-term ventilation by opening the windows for air exchange results in lower


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energy consumption despite a slightly higher air change rate. In case of an apartment in an old building with airtight windows, however, constant ventilation is clearly more favourable. In general, this also applies, if the drying of laundry in the apartment is an additional constant source of moisture. It must be mentioned that the result of an energetic determination of the entire apartment shows that the drying of laundry in the apartment causes additional energy consumption due to an increase in ventilation requirements, which is frequently higher than the drying of the same amount of laundry by a condensation dryer. It must, however, be considered that the dryer is operated by primary-energetically more unfavourable electric current. From the energetic point of view, ventilation by short-term opening of the windows for air exchange is neither suited nor reasonable for the ventilation of an apartment, where laundry is dried (without dryer).

Figure 49: Comparison of the heating energy requirement for a sleeping room during the heating period from October to March for the different variations. The right side shows the energy demand, if laundry is dried in the apartment. V1: construction with ETICS V2: highly isolating masonry V3: old building V4: old building with new airtight windows

4.1.3 Influence of different ventilation strategies on the indoor climate

Kalamees simulated the influence of different ventilation strategies on indoor climate 87. Three different ventilation strategies (the constant air mass flow, relative humidity (RH) controlled system, and CO2 controlled ventilation system) in a test room with different internal surface hygroscopic material properties were analysed (aluminium foil, gypsum board and wood fibreboard). The simulations are based on the real test room which is located at the outdoor testing site of the Fraunhofer-Institute of building physics in Holzkirchen and were used in IEA Annex 41 Subtask 1 Common Exercise 3 (CE3). The simulation program IDA Indoor Climate and Energy (IDA ICE) was used to calculate the results. The material properties, external and internal climates as boundary conditions, as well as moisture release are those from CE3 steps 1, 2 and 3. There is no solar radiation transmission through the windows. The envelope of

87 Targo Kalamees: Influence of different ventilation strategies on indoor climateAnnex 41 MOIST-ENG, Working meeting, 16-18 April

2007, Florianopolis, Brazil


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the room was supposed to be maximum airtight. Leak area and infiltration rate through envelope were set as minimum as possible.

Three different ventilation strategies with two different indoor surface materials during three different periods were analysed. Ventilation systems were: the constant air mass flow (Run A), relative humidity (RH) controlled system (Runs B and D), and CO2 controlled system (Runs C and E). Indoor surface materials were: gypsum board and wood fibreboard. Calculations were made in three periods: Step 1 (cold period, the room was covered with aluminium foil), Step 2 (cold period, all walls of the room (~50m2) were covered with gypsum board or wood fibreboard), and Step 3 (mild period, all walls and ceiling of the room (~65m2) were covered with gypsum board or wood fibreboard).

Ventilation implementations

There was constant air mass flow with air change rate close to 0.66 ach in Run A. The ventilation system with the airflow controlled by relative humidity (RHC) adapts the airflow to changes in the indoor RH:

– when RHindoor < 25%, then the flow is set to the minimum value of Qmin = 10 m3/h

– when RHindoor > 60%, then the flow is set to the maximum value of Qmax = 40 m3/h

– when RHindoor is in between the min and the max, the airflow rate in between is linearly interpolated.

The ventilation systems with the airflow controlled by carbon dioxide adapts the airflow to changes in the indoor CO2:

– when CO2 < 600ppm, then the flow is set to the minimum value of Qmin = 10 m3/h

– when CO2 > 1500ppm, then the flow is set to the maximum value of Qmax = 40 m3/h

– when CO2 is in between the min and the max, the airflow rate in between is linearly interpolated

The diurnal CO2 and humidity production pattern used for the calculations is shown in in Figure 50.


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Figure 50: Diurnal CO2 and humidity production pattern used for the simulations

Table 10: Percent of the time, when the indoor climate parameters were over and under acceptable RH (25&60%) and CO2 (1200ppm) limit values and average heating energy consumption.

Step 1: cold period, the room was covered with aluminium foil)

Step 2: cold period, all walls of the room were covered with gypsum board or wood fibreboard

Step 3: mild period, all walls and ceiling of the room were covered with gypsum board or wood fibreboard

>12

00pp

m

<RH

25%

>RH

60%

Hea

ting

ener

gy

>12

00pp

m

<RH

25%

>RH

60%

Hea

ting

ener

gy

>12

00pp

m

<RH

25%

>RH

60%

Hea

ting

ener

gy

A:Constant mass flow Aluminium foil

0 39 5 674 0 20 1 650 0 0 10 392

B: RH controlled vent. system Gypsum board

33 13 4 571 35 0 1 562 1 0 6 388

C: CO2 controlled vent. system Gypsum board

0 20 6 589 0 3 3 570 0 0 31 351

E: RH controlled vent. system Wood fibreboard

33 13 4 571 45 0 0 550 0 0 0 390

F: CO2 controlled vent. system Wood fibreboard

0 20 6 589 0 0 0 558 0 0 5 348


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From the results (Table 10) the following can be concluded. In the case of continuous ventilation, there was the lowest CO2 level but also the highest energy consumption and RH deviation. During cold there was similar energy consumption in the case of CO2 controlled and RH controlled ventilation systems. Hygroscopic indoor surface materials (wood fiberboard compared to gypsum board) dampened fluctuation of indoor RH in cases of all ventilation systems. During cold period, in the case of RH controlled ventilation, there was the longest period when the CO2 was >1200ppm. During cold period, in the case of continuous ventilation, there was the longest period when the RH<25%. During warm period, in the case of CO2 controlled ventilation there was the longest period when the RH>60%.

The RH sensitive ventilation may be a good and simple demand controlled ventilation system. It reduces heating energy demand due to lower ventilation rate, when the building is not used (low moisture production) and relative humidity maximum values are still kept at a correct level. In cold climate the absolute humidity of outdoor air is low wherefore indoor air is also drier. This may cause unacceptable low ventilation rate. Indeed, reducing energy used for ventilation of buildings should be made without compromising the indoor air quality. Therefore should also check that other pollutants (such as CO2) are within correct limits. The compromise could be the combination of CO2 controlled and RH controlled ventilation systems.

4.1.4 Application in residential buildings

Van den Boscsche et all 88 determines the potential energy savings due to humidity controlled ventilation. The performance evaluation of ventilation systems has to consider both CO2 as well as humidity. According to EN 1377989 (Ventilation for non-residential buildings – Performance) the relative humidity has to lie between 30% and 70% and the IAQ concerning CO2 is commonly expressed by IDA-classes.

More on : https://www.kuleuven.be/bwf/projects/annex41/protected/data/UG%20Oct%202006%20Paper%20A41-T4-B-06-11.pdf

4.1.5 Application in schools

Using Wufi ®plus, the hygrothermal indoor environment modelling programme developed by scientists of the Fraunhofer Institute for Building Physics (IBP) at Holzkirchen, the nonsteady-state behaviour of a classroom (year of construction: 1964) is simulated for the period form December 1 until March 31. The exterior building components have a low level of thermal insulation: the U-values of the external walls are ranging between 0.6 and 1.8 W/m²K, the windows have a U-value of 2.7 W/m²K. The windows are projected windows. The sunblinds in front of the windows are sufficiently vented due to a wide space towards the façade, yet was the fabric's efficiency assessed to be insufficient. The blower-door test resulted in an n50-value of 11.0 h-1 for Room 205 (phase of construction 1964, space volume 170 m³). In

88 N. Van Den Bossche, M. Steeman, L. Willems, A. Janssens: PERFORMANCE EVALUATION OF HUMIDITY CONTROLLED VENTILATION

STRATEGIES IN RESIDENTIAL BUILDINGS, Annex 41 Meeting, Lyon, October 2006. 89 EN 13779 Ventilation for non-residential buildings - Performance requirements for ventilation and room-conditioning systems;

German version EN 13779:2007 Standard, Publication date : 2007-09 , German


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this room, the poor airtightness is particularly due to obvious leakages in the window area.

Figure 51: Photographic view of the simulated classroom build in 1964. The orientation of the windows is south-east. The room has an Ana of 55 m2.

The actual condition is compared to 6 different retrofit variations. Hourly data of the Holzkirchen weather station were chosen as climatic boundary conditions.

Concerning the internal thermal loads, moisture loads, and CO²-loads the classroom is assumed to be continuously occupied by 30 individuals (Monday through Friday between 8 am and 1 pm, except for a 30-minute break from 10:15 am to 10:45 am).

Table 11: Variations of Wufi ®plus calculations

Case Infil-tration [h-1]

Ventilation Duration controlled by HRS* Heating Demand [kWh]

1.Dec. 31.Mar. Base 0.3 5 h-1 during breaks - - 1924 C1 0.1 5 h-1 during breaks - - 590

C2 0.1 15

m³/(hm²) 7am 2pm - - 2360

C3 0.1 15

m³/(hm²) 7am 2pm - 80% 314

C4 0.1 max. 15 m³/(hm²)

RH > 55% - 381

C5 0.1 max. 15 m³/(hm²)

CO2 > 1500 ppm - 443

C6 0.1 8 m³/(hm²)

plus 5 h-1

7am 2pm

during breaks - 237

HRS=Heat Recovering System

Table 11 presents the classroom ventilation strategies that were investigated for the six variations of retrofitting, compared to the current situation. For all varia-tions (except for the actual condition), calculations considered an improved level of thermal insulation (external walls: U = 0.2 W/m²K) and new windows (U = 1.3 W/m²K). This causes a substantial improvement in the heating energy demand. If the air change set according to the required 5 m³/h m² (approx. 30 m³/h pers), the heating energy demand rises sharply, in spite of the clearly improved level of thermal insulation (compared to the current situation). How important it is to use a heat recovery system becomes quite clear on directly


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comparing Case #2 and Case #3. This is why any further considerations are based on a heat recovery system with an efficiency of 80 %. The predicted profiles of indoor air humidity and indoor air temperatures are represented in Figure 52 for basic casic and Case #6.

Figure 52: Calculated profiles of relative humidity and temperature inside the classroom for the original classroom and the suggested retrofitting (case 6).

In Figure 53 the resulting concentrations of CO2 are compiled for an exemplary day. In the upper part of the diagram the corresponding air change rate is plotted, the calculated indoor-air concentration of CO2 is indicated in the lower part. Limiting the maximum permissible relative humidity to 55 % achieves the most favourable situation regarding the heating energy demand - but the concentration of CO2 is hardly improved at all. If the maximum permissible concentration of CO2is limited to 1,500 ppm (0.15 vol%) the total heating energy demand amounts to almost 450 kWh. Under aspects of energy use, this is the least favourable solution at all. This is why case 6 was computed for a variation assuming the classroom to be vented at a reduced air change rate of 8.5 m³/ (h m²) between 7 am and 2 pm, with additional thorough window ventilation during the breaks. This variation also produces clearly improved CO2

concentrations. Temporarily, 0.15 vol% are slightly exceeded after three hours of teaching lessons. However, this situation can be improved by additional thorough venting after every lesson (i.e. after 45 min the classroom will be thoroughly vented). The calculations presented in this context are intended to facilitate the assessment of different ventilation concepts by way of nonsteady-state computation. However, it is not possible to assess the demand of final and/ or primary energy on the basis of these calculations.


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Figure 53: Assumed outdoor air change and calculated CO2-concentration profile resulting inside the class room

4.2 Passive Strategies

Influence of Moisture buffer materials


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5 Moisture Safty Aspects

5.1.1 Recommendations for sloped roof design based on HAM-modelling

by:

Arnold Janssens (Department of Architecture and Urban Planning, Ghent University) Filip Dobbels (Department of Building Physics and Indoor Climate, BBRI)

The moisture response of building components has traditionally been evaluated by means of the Glaser method90. This method only considers moisture transport by water vapour diffusion and is therefore only valid in airtight components consisting of non-capillary materials. In tiled, slated and metalic sloped roofs the first condition is rarely met (contrary to flat membrane roofs), and so the Glaser method is not applicable. To evaluate the moisture behaviour of sloped roofs there is need of an additional method which takes convective vapour transfer into account. This way it would be possible to design and construct sloped roofs with an effectively reduced risk of moisture problems.

For this purpose a reseach project was initiated entitled ‘Moisture problems in roofs: influence of current boundary conditions and technology in Belgium’. The research was conducted by BBRI (Belgian Building Research Institute), in cooperation with UGent (Ghent University), K.U.Leuven (Catholic University Leuven) and WenK Sint Lucas Gent (Superior Institute for Science and Art, Department of Architecture). The following subjects have been investigated:

– Which level of air tightness is feasible in sloped roofs with the current construction practice in Belgium?

– Which calculation method may be applied to predict the amount of interstitial condensation taking convective vapour transfer into account?

– How can the selection of materials and the choice of roof lay-out contribute to a reduced risk of interstitial condensation?

Air tightness of building components

In conventional sloped roof construction in Central Europe the air barrier function is often combined with the vapour retarder. For this a vapour and air retarding membrane may be used (most often polyethylene film or building paper,...) which is continuously sealed at joints and intersections. Since in the traditional roof lay-out the air and vapour retarder is attached at the inside of the structural framing, it is called an ‘interior air barrier’ (see Figure 54). In a so-called warm roof lay-out it is possible to apply an ‘exterior air barrier’ on a rigid support at the outside of the structural framing (Figure 54). In this position the continuity of the barrier is better achievable since interference by major structural elements, internal walls and service penetrations is minimised. Because of the rigid support, bituminous membranes may be used as the air barrier material.

90 NBN EN ISO 13788


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Ridge detail for a compact tiled roof with interior

air barrier Ridge detail for a warm tiled roof with

exterior air barrier

Figure 54: Difference between interior and exterior air barrier

Measured air permeance of lightweight constructions with interior air barrier.

In order to investigate which air tightness criteria are effectively achievable in the existing building practice, the literature on measured air leakage characteristics in lightweight components has been reviewed. Table 12 and Figure 55 give a survey of reported air leakage data. The data contain measurements on attic ceilings, sloped compact roofs and wood frame walls. All components contained conventional air-vapour barriers applied at the inside of the structural cavity (PE-film, kraft paper). The reported leakage characteristics included the effects of joints, intersections and perforations found in practice. For comparison also the much referenced measurements on curtain wall air leakage has been included.

The table lists the measuring method, the original reporting format of the leakage characteristics and the measured range of air permeances. To make a comparison possible the reported data were converted into a single format: the air permeance at a reference pressure difference of 4 Pa. This reference is in the range of mean pressure differences that usually occur across building components.

The values listed in Table 12 show that the air permeance achieved by systems of this type is in the order of 1⋅10-4 m3/m2/s/Pa in current construction practice. In systems with the air-vapour barrier designed and installed for continuity of airtightness lower values are achievable. However measured air permeances smaller than 1⋅10-5 m3/m2/s/Pa are not reported. These observations appear to be independent of the component type (ceiling, roof or wall) or building tradition (North American or European). This suggests that the figures of 1⋅10-4 and 1⋅10-5 m3/m2/s/Pa may be held as default air permeance values of lightweight components with interior air barrier of typical and best achievable construction respectively.

Figure 55compares the air permeance values achieved in practice to the air barrier requirements according to the Canadian Building Code. These requirements are based on computer simulations of the effects of air leakage on moisture accumulation and heat losses in timber frame walls. They ensure a


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safe moisture performance and simultaneously limit the heat loss associated with air leakage (Di Lenardo et al. 1995)91. The comparison shows that the effectively achieved air permeance in lightweight constructions in practice is one or two orders of magnitude higher than the Canadian requirements. Even when considering that these requirements have been developed for building applications in the cold Canadian climate, these observations show the discrepancy between the air leakage characteristics achieved in common building practice and the characteristics needed to safely prevent moisture problems due to air leakage.

Table 12: Measured effective air permeance of lightweight building components

Component Measuring method

Reporting format*

Air permeance** (10-4 m3/m2/s/Pa)

Source

min avg max Ceilings unspecified AL(4) 0.5 1.2 1.8 ASHRAE 199792

of vented attics

unspecified c , n 0.3 25%

0.8 median

1.4 75%

Liddament 199693

field testing AL(4) 1.0 Burch 199694 Sloped Field+lab. K(4) 0.3 0.9 1.8 Janssens 199895

compact roofs

laboratory c , n 0.1 good

practice

1.9 poor

practice

Hens et al. 198596

Wood field testing va(50) 0.2 0.5 0.7 Bassett 198797 Frame walls laboratory raw data 0.6 1.0 1.8 Jones 199598

Curtain walling

field testing c , n 0.3 0.7 1.1 Tamura and Shaw 197699

* Reference pressure difference between brackets ** Linearized value, based on the air flow rate at 4 Pa pressure difference and flow exponent n = 0.67 (except when n is reported)

91 Di Lenardo, B., W.C. Brown, W.A. Dalgliesh, K. Kumaran and G.F. Poirier. (1995) Technical guide for air barrier systems for exterior

walls of low-rise buildings. Canadian Construction Materials Centre, Institute for Research in Construction, National Research Council Canada, Ottawa, Ontario.

92 ASHRAE. (1997) Handbook of Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Atlanta GA.

93 Liddament, M.W. (1996) A guide to energy efficient ventilation. International Energy Agency. Air Infiltration and Ventilation Centre. Oscar Faber Partnership, UK.

94 Burch, D.M., G.A. Tsongas and G.N. Walton. (1996) Mathematical analysis of practices to control moisture in the roof cavities of manufactured houses. Report NISTIR 5880. National Institute of Standards and Technology. Gaithersburg ML.

95 Janssens, A. (1998) Reliable control of interstitial condensation in lightweight roof systems: calculation and assessment methods. Doctoral dissertation. K.U.Leuven, Laboratorium Bouwfysica, Heverlee.

96 Hens, H., J. Lecompte, G. Mulier and P. Staelens. (1985) Metodiek voor de beproeving van dakkomplexen: globale vochtgedrag van hellende daken. (Methodology for testing roof constructions: moisture performance of sloped roofs, in Dutch). WTCB-K.U.Leuven-IWONL. Overeenkomst nr. 4213.

97 Basset, M. (1987) Air flow resistances in timber frame walls. Airborne Moisture Transfer: New Zealand Workshop Proceedings and Bibliographic Review. Technical Note AIVC 20, Paper 3. Air Infiltration and Ventilation Centre. Oscar Faber Partnership, UK.

98 Jones, D.C.. (1995) Impact of airflow on the thermal performance of various residential wall systems utilizing a calibrated hot box. Thermal performance of the exterior envelopes of buildings VI. Proceedings of the sixth ASHRAE/DOE/BETEC Conference. ASHRAE Special publications, Atlanta GA. 247-260.

99 Tamura, G.T. and C.Y. Shaw. (1976) Studies on exterior wall air tightness and air infiltration of tall buildings. ASHRAE Transactions. Vol. 82, Pt. 1, 122-.


- 93 -

1E-6 1E-5 1E-4 1E-3EFFECTIVE AIR PERMEANCE (m 3/(m2.s.Pa))

ATTIC

CE

ILING

SC

OM

PA

CT

RO

OFS

WA

LLS

AIVC 96

ASHRAE 97

Burch 96

Hens 85

Jones 95

EXISTING LIGHTWEIGHTCONSTRUCTION PRACTICE

Janssens 98

Bassett 87

Tamura&Shaw 76

AIR

BA

RR

IER SYSTEM

REQ

UIR

EMEN

TSN

ational Building C

ode of Canada

Figure 55: Comparison of air permeance data of lightweight components with

interior air barrier, and air barrier requirements according to the Canadian Building Code (linearised at 4 Pa)

Calculation method

Since lightweight building components are rarely air tight, their moisture performance should be evaluated by means of a calculation method that also accounts for convective vapour transfer. It is proposed to use the one-dimensional steady-state diffusion-convection method. The calculation method is described and its validity is analysed by comparison with results of a 2D numerical calculation tool.

The diffusion-convection method

The basic differential equations describing the steady-state one-dimensional heat and vapour transfer in air permeable materials may be analytically solved. This solution is the basis of the so-called diffusion-convection method to assess the effect of air leakage on the accumulation of condensation in building components. The method may be implemented by means of spreadsheets. The development of the method has been described in detail by Janssens (1998) [95]. The analytical solution leads to expressions for the variables in the form of exponential functions of the dimensionless Peclet-number of the problem. The condensation flow rate gc (kg/s/m2) on a single condensing surface in a multi-layer construction is given by:

⎥⎥⎦

⎤

⎢⎢⎣

⎡

−−

−θ−

−

θ−ρξ=

1)Peexp(

p)(p

)Peexp(1

)(ppvg

ce

ecsatic

csatiaac (3)

met: NDvZvPe2

1iiiaa

2

1aa

21 ∑∑

=

μρξ=ρξ=


- 94 -

with va the air leakage rate (m³/s/m2), ρ the air density (1.2 kg/m3), ξa the specific vapour capacity of the air (6.1⋅10-6 kg/kg/Pa), pi and pe the vapour pressures of interior and exterior respectively (Pa), psat(θc) the saturation vapour pressure at the temperatur of the condensing surface θc (Pa), Z the vapour diffusion resistance (m/s), μ the vapour resistance factor (-) and D the thickness of a material layer (m), and N the specific vapour diffusion resistance (5.4 109/s). In Equation 3 the air leakage rate va assumes a negative value in case of air exfiltration from the inside to the outside.

In the limit where the air leakage rate equals 0, the well known equations of the Glaser method are found (EN ISO 13788):

∑∑==

−θ−

θ−=−=

c

ejj

ecsati

cjj

csatiuit,vin,vc

Z

p)(p

Z

)(ppggg (4)

In case of combined conductive and convective heat transfer, the temperature of the condensing surface is given by:

)Peexp(1)Peexp(1

)(tot

ce

eiec −−

θ−θ+θ=θ (5)

met: ⎥⎥⎦

⎤

⎢⎢⎣

⎡

λ+

αρ=ρ= ∑∑

=

c

1i i

i

eaa

c

eaa

ce

D1cvRcvPe

with θi and θe the temperature of inside and outside resp. (°C), R the thermal resistance (m²K/W) and λ the thermal conductivity (W/mK) of a material layer, and αe the transfer coefficient for heat transfer at the exterior surface (W/m²K). Also in this equation the air leakage rate assumes a negative value in case of exfiltration from inside to outside.

For the application of the steady-state diffusion-convection method adequate design climate conditions have been developed (Janssens 1998). Based on the significance of the outside temperature for the occurrence of interstitial condensation due to air leakage, a steady-state moisture performance analysis for a typical cold spell has been proposed as a first-order assessment of the risk of interstitial condensation in lightweight roof systems. Table 13 lists the proposed weekly mean design climate values based on the coldest week contained in the Test Reference Year for Uccle, Belgium. The combination of a low temperature, high humidity and negative radiation heat flux represents severe conditions for condensation.

Table 13: Design climate conditions (weekly mean values) for steady-state analysis of

interstitial condensation due to air leakage. (Ukkel, Belgium)

θe ϕe qr αce vmet

-2.5°C 95% -30 W/m² 17 W/(m²K) 3.8 m/s


- 95 -

Comparison with numerical calculation methods.

In order to evaluate the validity of the diffusion-convection method to predict interstitial condensation, it was compared to calculation results with the numerical model 2DHAV for 2-dimensional heat air and vapour transfer (Janssens 2001100). Both models were used to predict the condensation flow rate occurring in a sloped roof under steady-state conditions.

The roof lay-out was from outside to inside: – Tiles with vented cavity: thermal resistance 0.08 m²K/W, diffusion

resistance neglected; – Underlay: diffusion thickness is a parameter (μd = 2 m or 0.02 m) – Air cavity 5 mm: thermal resistance 0.1 m²K/W – Glass fiber 12 cm: λ = 0.035 W/m²K, μ = 1.5 – Gypsum board including vapour retarder: thermal resistance 0.1 m²K/W, μd

= 2 m. The U-value of the roof is consequently 0.26 W/(m²K).

Boundary conditions: – Outside: Table 2 – Inside: θi = 20°C, αi = 8 W/m²K, pi = 1251 Pa (Borderline between indoor

climate class 3 and 4, see Janssens and Hens 2003101)

Table 14 lists the specific material properties used as an input for the 2DHAV-tool; in addition some assumptions on geometry were made: – Roof slope 30° (parameter for the calculation of buoyancy related air

flows); – Roof length from eaves to ridge is 4.55 m (91 calculation nodes)

In order to calculate the effects of air leakage with the 2D-model, also a leakage geometry is required. Four different leakage geometries were introduced, differing only in the position of cracks in the internal lining (Table 15). This way the difference between a concentrated and a more distributed air leakage was investigated.

100 Janssens, A. 2001. ‘Advanced numerical models for hygrothermal research: 2DHAV model description’ Moisture analysis and

condensation control (H.R. Trechsel, editor). ASTM Manual 40. American Society for Testing and Materials, West Conshohocken, PA, USA, ISBN 0-8031-2089-3, 177-178.

101 Janssens, A. en H. Hens. 2003. Development of indoor climate classes to assess humidity in dwellings. Proceedings of the 24th AIVC-Conference ‘Ventilation, humidity control and energy’, Washington DC, ISBN 2-9600355-4-2, 41-46.


- 96 -

Table 14: Material properties of the sloped roof layers in 2DHAV

Material nx d m

λx W/(mK)

λy W/(mK)

ka,x m²

ka,y m²

μ

Tiles+underlay 2 0.01 0.12 0.12 2.00E-13 2.00E-17 200 or 2 Air cavity 1 0.005 0.05 0.05 0.5 Glass fiber 8 0.12 0.035 0.04 4.00E-09 6.00E-09 1.5 Gipsum board 2 0.01 0.1 0.1 5.00E-14 2.00E-17 200

n: number of calculation nodes, d: thickness, λx en λy: thermal conductivity perpendicular and parallel to roof

slope, ka: specific permeability and μ: vapour resistance factor.

Table 15: Four different leakage geometries

aa bb cc

dd

2 cracks of 1 mm in gipsum board at 0.5 m from eaves and

ridge

2 cracks of 1 mm in gipsum board at 2.2 m from eaves and

ridge

2 cracks of 1 mm in gipsum board near

eaves and ridge

2 cracks of 1 mm in gipsum board and of 5 mm in glass fiber

near eaves and ridge 1 air cavity of 5 mm in between glass fiber and underlay

5 joints of 1.5 mm in underlay every 1.15 m

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0.00E+00 1.00E-04 2.00E-04 3.00E-04 4.00E-04 5.00E-04

Gemiddeld debiet (m3/m²/s)

Con

dens

atie

OD

(kg/

m²/w

eek)

a

b

c

d

1D

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0.00E+00 1.00E-04 2.00E-04 3.00E-04 4.00E-04 5.00E-04


Con

dens

atie

OD

(kg/

m²/w

eek)

a

b

c

d

1D

Figure 56: Predicted amounts of condensation at the underlay (μd = 2.0 m) during

a cold week as a function of the air leakage rate through the roof (Indoor climate class 3-4).


- 97 -

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0.00E+00 1.00E-04 2.00E-04 3.00E-04 4.00E-04 5.00E-04


Con

dens

atie

OD

(kg/

m²/w

eek)

a

d

1D

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0.00E+00 1.00E-04 2.00E-04 3.00E-04 4.00E-04 5.00E-04


Con

dens

atie

OD

(kg/

m²/w

eek)

a

d

1D

Figure 57: Predicted amounts of condensation at the underlay (μd = 0.02 m) during

a cold week as a function of the air leakage rate through the roof (Indoor climate class 3-4).

The predicted condensation amounts are presented as a function of the air leakage rate in Figure 56 and Figure 57. The effect of an increasing air leakage rate was analysed in the range of 0 to 5.10-4 m3/m²/s. These are the air leakage rates occurring in roofs with a typical air permeance (1⋅10-4 m3/m2/s/Pa) at an air pressure difference up to 5 Pa. Hence the investigated range may be considered representative of the air leakage rates occurring in low rise buildings. In the diffusion convection method the air leakage rate is a direct input parameter. With the 2D-HAV-model successive calculations were performed for different values of the air pressure difference; for these calculations the air leakage rate is one of the output parameters. The results of the 2D-calculations were averaged per unit of roof surface.

-5.0

0.0

5.0

10.0

15.0

20.0

0 0.05 0.1

Dikte (m)

Tem

pera

tuur

(°C

)

400

600

800

1000

1200

1400

0 0.05 0.1

Dikte (m)

Dam

pdru

k (P

a)

Figure 58: Predicted temperature (left) and vapour pressure profile (right) with the 1D-diffusion convection method: va = 3.10-4 m3/m²/s (full line), va = 0 (dashed line).


- 98 -

Figure 58 shows the temperature and vapour pressure profiles in the roof as calculated with the diffusion convection method.

Following conclusions can be drawn from the results:

– The results of all different models and leakage geometries show the same trends. In the investigated range of air leakage rates there is a postitive relation between air leakage rate and condensation rate. In all cases the condensation amounts are reduced by 20 to 30% when a vapour permeable underly is applied in stead of a vapour retarding one.

– The predicted condensation amounts strongly depend on leakage path geometry: when air leakage is more concentrated, the condensation amounts decrease. This is clear when comparing case a to d. In case of concentrated leakage such as case d the condensation amounts are a factor 4 to 5 smaller than in case of air leakage through a longer leakage path (case a), although the air leakage rates may be the same. This is explained by the fact that in case of concentrated leakage the humid exfiltrating air remains quite warm on its path through the roof, and consequently deposits less water vapour.

– Physically the 1D-diffusion convection model represents a situation where the air flows in a diffuse way through the building component. This explains why the results of the 1D-method coincide with the most negative results of the 2D-model (case a: distributed leakage geometry), thus representing a ‘worst case scenario’ of interstitial condensation due to air leakage. In case of a vapour retarding underlay the agreement between the 1D- and 2D-results (case a) is perfect. In case of a vapour permeable underlay, the 1D results tend to overestimate the condensation rates at increasing air leakage rates.

– Only at small air leakage rates (< 5.10-5 m3/m²/s) the 2D-models predict more condensation than the 1D-model. This is explained by the fact that the 2D-models account for other air flow patterns than only air leakage through the component. Due to buoyancy indoor air may enter the roof cavity through the upper crack in the internal lining and cause condensation, even though the air leakage rate to the outside remains small.

As a general conclusion the model comparison shows that the 1D- diffusion convection method is suitable to estimate the maximal impact (worst case) of air leakage on the steady state moisture performance of building components. The results of the 1D-model are in agreement with results of more complete numerical models and may be considered a safe prediction. Therefore in the following only the diffusion convection method will be used to derive criteria for the use of air barriers and underlays.

Nevertheless it is important to understand the limitations of the model: only water vapour transfer by diffusion and convection is taken into account, while liquid moisture transfer is neglected. Therefore the method is not reliable to evaluate condensation in roofs with capillary materials (such as roofs with fibre cement underlay boards).


- 99 -

Performance criteria for air barriers and underlays

0.01

0.10

1.00

10.00

0.00E+00 1.00E-04 2.00E-04 3.00E-04 4.00E-04 5.00E-04


µd o

nder

dak

(m)

KK1

KK2

KK30.01

0.10

1.00

10.00

0.00E+00 1.00E-04 2.00E-04 3.00E-04 4.00E-04 5.00E-04


µd o

nder

dak

(m)

KK1

KK2

KK3

Figure 59: Allowable air leakage rates to limit condensation amounts during a cold winter week at 0.2 kg/m², as a function of the diffusion thickness of the underlay. Results of the 1D-diffusion convection method for climate class 3 (full line), 2 (dashed line) and 1 (dotted line).

In the following the influence of the vapour transfer properties of the underlay on the occurrence of interstitial condensation due to air leakage is investigated more in detail. The diffusion convection method is used to define which air leakage rates may be allowed before the condensation amounts during a cold winter week amount to 0.2 kg/m² (limiting value for dripping of condensate). The calculations have been performed for the same roof lay-out described in the previous paragraph, assuming different values of the underlay diffusion thickness. The boundary conditions for the calculations have been the following: – Outside: see Table 13 – Inside: θi = 20°C, αi = 8 W/m²K

– pi = 667 Pa (Borderline climate class 1 and 2) – pi = 974 Pa (Borderline climate class 2 and 3) – pi = 1251 Pa (Borderline climate class 3 and 4)

The results of the calculations are shown in Figure 59. The figure should be read as follows: starting from a given value of the diffusion thickness of the underlay (ordinate), the curves communicate the air leakage rate resulting in a condensation amount of 0.2 kg/m² during a cold winter week, and this for a humid indoor climate (3), an average indoor climate (2) and a dry indoor climate (1). Larger air leakage rates may cause moisture damage.

The results show the following:


- 100 -

– In buildings with a dry indoor climate (climate class 1) more air leakage may be tolerated without the risk of condensation problems than in buildings with a humid indoor climate.

– In roofs with a more vapour permeable underlay more air lekage may be tolerated without the risk of condensation problems than in roofs with a vapour retarding underlay.

– The benefits of a vapour permeable underlay are only significant at very small values of the underlay diffusion thickness, i.e. smaller than 0.05 m. At larger values the predicted condensation amounts are almost insensitive to the vapour resistance of the underlay. For example a decrease of the diffusion thickness of the underlay from 10 m to 0.1 m, results only in a 20% increase of the allowable air leakage rate.

Some European guidelines already contain criteria to differentiate between vapour permeable, so called ‘breathable’ membranes and the more vapour retarding membranes The UEAtc technical guide for the assessment of discontinuous roofing underlay systems (2003) defines a membrane with μd-value ≤ 0.05 m as being vapour permeable. The calculation results presented in this section show that this class of materials may contribute to the reduction of condensation problems due to air leakage.

The allowable air leakage rates found in the previous section may be converted into a set of upper limits for the air permeance of a component, by taking a value for the air pressure difference across the component into account. Air pressure measurements on site show that a value of 5 Pa is statistically representative for the air pressure differences occurring across sloped roofs in dwellings due to the effects of wind and temperature (Heijmans et al. 2004)102.

Table 16 lists the air tightness criteria as a function of the indoor climate and the underlay material. The listed values represent the air permeance that should be minimally achieved in order to prevent condensation problems due to air leakage. The air permeance is the linearised value at a reference pressure difference of 4 Pa.

Table 16: Air tightness criteria to prevent interstitial condensation in sloped roofs (Ukkel, Belgium, Δpa = 5 Pa)

Maximum allowable air permeance (10-4 m3/(m²sPa)) Indoor climate Underlay

μd = 2.0 m Underlay μd = 0.02 m

KK1-2: Δpv = 159 –10.θe < 0.6 < 1.1 KK2-3: Δpv = 436 –22.θe < 0.2 < 0.3 KK3-4: Δpv = 713 –22.θe < 0.1 < 0.2

The values found in Table 16 are important to define future performance criteria for the development and quality assurance of air barrier systems. The criteria may be put into perspective by comparison to the measured values of the effectively achieved air permeance of lightweight components (Table 12). Following conclusions may be drawn:

102 Heijmans, N., P. Wouters, F. Dobbels, G. Houvenaghel, B. Vandermarcke. 2004. Setting up a database of indoorclimate

measurements in recently built Belgian dwellings. AIVC-conference, Praha.


- 101 -

– Except for dry indoor climates (climate class 1), the air permeance criteria in Table 16 are smaller than the air permance values typically achieved in practice (order of magnitude 1. 10-4 m3/(m²sPa)). This means that in general the air tightness achieved in current roof construction practice is insufficient to eliminate the risk of condensation problems in the Belgian climate.

– This conclusion applies both to roofs with vapour retarding and vapour permeable underlays. However the use of a vapour permeable underlay contributes to a higher tolerance for air leakage. In comparison to a roof with vapour permeable underlay, the airtightness of a roof with vapour retarding underlay should be twice as good in order to eliminate condensation problems.

– For more humid indoor climates (border line 3 to 4) the criteria for airtightness shift to values which are difficult to achieve with interior air barriers in current construction practice. In case of a humid indoor climate and a vapour retarding underlay, the required air tightness is only reliably achievable by applying an exterior air barrier.

Based on these observations the air tightness criteria in Table 5 may be applied in a pragmatic way. This results in recommendations for sloped roof design and for adequate air barrier and underlay materials, considering the moisture load of the building, and the achievable air permeance with a certain type of air barrier. An example of such recommendations is given in Table 6.

Table 17: Recommendations for choice of air barrier and underlay in sloped insulated roofs

Indoor climate class Air barrier (μd ≥ 2 m) Aanbevolen onderdak ICC1: workshop, sports hall,.. Interior air barrier μd ≤ 2.0 m ICC2: office, large dwelling,… Interior air barrier μd ≤ 2.0 m ICC3: social housing,... Interior air barrier μd ≤ 0.05 m ICC4: swimming pool,... Exterior bituminous membrane

(warm roof construction) According to EN ISO 13788

Conclusions

The investigation on air tightness criteria for sloped roofs has produced the following results:

– A simplified analytical calculation method to assess interstitial condensation due to air leakage, and a design methodology to derive air tightness criteria using the calculation method. A model comparison showed that the calculation method is suitable to estimate the maximal impact (worst case) of air leakage on the steady state moisture performance of building components.

– A classification of underlay materials based on the diffusion resistance of the underlay. The class of vapour permeable underlays (μd ≤ 0.05m) contributes to a reduction of the sensitivity of the roof to condensation due to air leakage.


- 102 -

– Air tightness criteria for sloped roofs, as a function of the indoor climate class of the building and the vapour resistance of the underlay.

– Pragmatic recommendations for sloped roof design and for the choice of air barrier systems and underlay materials.

On following aspects additional research is needed:

– The calculation of moisture transfer in capillary and hygroscopic underlay materials in response to convective vapour transfer.

– Testing and classification of air barrier systems.

5.1.2 Controlled Ventillation of cold attics

Carl-Eric Hagentoft and Angela Sasic (Chalmers):

https://www.kuleuven.be/bwf/projects/annex41/protected/data/CTH%20Apr%202007%20Prese%20A41-T4-S-07-1.pdf


- 103 -

6 Influence of Moisture Buffering Materials on the Energy Performance of a Building

6.1 Impact of moisture buffering on energy performance of cooling ceilings

T. Catalina, M. Woloszyn, J. Virgone

https://www.kuleuven.be/bwf/projects/annex41/protected/data/CETHIL%20Oct%202006%20Paper%20A41-T4-F-06-6.pdf

6.2 Indirect Evaporative Cooling: Interaction between thermal performance and Room Moisture Balance

M. Steeman1, A. Janssens1, M. De Paepe2

https://www.kuleuven.be/bwf/projects/annex41/protected/data/UG%20Oct%202006%20Paper%20A41-T4-B-06-9.pdf

6.3 Advantages of moisture buffering materials in hot and humid climate

Let’s assume that our test rooms used in IEA Annex 41 Subtask 1 Common Exercise 3 (CE3) are transferred to Bangkok and to Miami where they serve as hotel rooms which are occupied during the night from 8:00 p.m. to 8:00 a.m. and unoccupied the rest of the day. The ventilation system provides constantly 0.5 ACH but the air-conditioning is only working as long as the room is occupied. When the room is unoccupied there is an interior heat and moisture production rate of 100 W res. 0.5 g/m³h (caused by moisture release from plants or furniture). During night-time occupation the interior heat production rises to 500 W and the moisture production to 2.0 g/m³h. The set-points for the intermittently working temperature and humidity controlled air-conditioning system are 23°C and 50% RH. The partition walls to the adjacent rooms are adiabatic while the wall to the corridor and the ceiling face a conditioned space which is kept round the clock at 23°C and 50% RH.


- 104 -

0°° 12°° 0°° 12°° 0°° 12°° 0°°10

20

30

40

50

20

40

60

80

100

Temp.

RH Relative H

umidity [%

]

Outdoor Climate

Air

Tem

pera

ture

[°C

]

Figure 60: Outdoor air temperature and relative humidity recorded for Bangkok in March (3 day observation period).

The simulation results describing the hygrothermal behaviour of the test rooms with and without moisture buffering capacity are presented for a period of three days end of March. The outdoor temperature and humidity conditions for that period are plotted in Figure 60. The evolutions of the indoor air conditions are shown in Figure 61. After shutting off the air-conditioning the indoor temperature rises almost independently of the interior lining of the rooms from the set-point of 23°C to a maximum of 25°C during the first day of the simulated period. When the air-conditioning is turned on again the set temperature is regained within less than one hour.

The evolution of the indoor humidity is, however, greatly affected by the interior lining of the rooms. Due to the more humid outdoor air conditions (absolute humidity) during the third day of the simulated time period the increase in indoor RH is highest in the evening just before the end of the AC shut-off period. While the indoor humidity in the plastered room stays well below 80% RH even under the unfavourable conditions of the third day, it clearly exceeds 90% RH in the room with aluminium lining. According to chapter 3.3.1.1 the risk of mold formation increases dramatically when the relative humidity in a room goes beyond 80% several times a week. Therefore a sufficient humidity buffering capacity of the walls and ceiling of a room can help to avoid fungal contamination in a situation where intermittent air-conditioning is employed for energy savings.


- 105 -

22

23

24

25

0°° 12°° 0°° 12°° 0°° 12°° 0°°40

50

60

70

80

90

100

Tem

pera

ture

[°C

]

Surface Lining Aluminium Plaster

Rel

ativ

e H

umid

ity [%

]

Figure 61: Simulated indoor temperature and relative humidity during the observation period in March (Bangkok climate).

The lower indoor humidity peaks in the plastered room are a result of the vapour sorption of the plaster which is charged with moisture during the AC shut-off period. This additional moisture has to be removed when the AC-system turns back on again. This effect can also be detected looking at the required cooling power of the AC-system in Figure 62. While the sensible heat removal is nearly the same for both rooms, the energy consumption to remove the latent heat from the plastered room is slightly higher because the plaster releases the vapour it has stored during the AC shut-off period


- 106 -

0.0

0.4

0.8

1.2

0°° 12°° 0°° 12°° 0°° 12°° 0°°0.0

0.4

0.8

1.2

Sensible Heat

Coo

ling

Pow

er [k

W]

Latent Heat Surface Lining Aluminium Plaster

Coo

ling

Pow

er [k

W]

Figure 62: Sensible and latent cooling load to be removed in the different rooms by the AC-system during the observation period.

0°° 12°° 0°° 12°° 0°° 12°° 0°°10

20

30

40

40

60

80

100

Temp.

RH

Relative H

umidity [%

]Outdoor Climate

Air

Tem

pera

ture

[°C

]

Figure 63: Outdoor air temperature and relative humidity recorded for Miami in June (3 day observation period).

The tropical climate of Bangkok may not be representative for cooling climates in other part of the world. Therefore the hygrothermal simulation is repeated for Miami in Florida. Again three days during the hottest months of the year are chosen (Figure 63). The simulation results describing the hygrothermal behaviour of the test rooms with and without moisture buffering capacity are presented for a period of three days at the end of March. The evolutions of the indoor air conditions are shown in Figure 64. After shutting off the air-conditioning the indoor temperature rises again independently of the interior lining of the rooms from the set-point of 23°C to a maximum of 24.5°C during


- 107 -

the first day of the observed period. When the air-conditioning restarts the set temperature is rapidly regained.

The evolution of the indoor humidity is again affected by the interior lining of the rooms. While the indoor humidity in the plastered room does not exceed 70% RH, the humidity in the aluminium coated room reaches 80% RH. The conditions in the room without moisture buffering capacity are not as bad as in Bangkok but since the humidity limit for mold growth is reached, the risk of microbial growth is much greater than in the plastered room.

22

23

24

25

0°° 12°° 0°° 12°° 0°° 12°° 0°°40

50

60

70

80

90

Tem

pera

ture

[°C

]

Surface Lining: Aluminium Plaster

Rel

ativ

e H

umid

ity [%

]

Figure 64: Simulated indoor temperature and relative humidity during the observation period in June (Miami climate).


- 108 -

7 Impact of the boundary conditions on the results of whole building simulations

7.1 Impact of different weather years

The hygrothermal behaviour of the building envelope affects the overall performance of a building. In this paper the combined model WUFI-Plus , that takes into account moisture sources and sinks inside a room, input from the envelope due to capillary action, diffusion and vapour ab- and desorption as a response to the exterior and interior climate conditions as well as the well-known thermal parameters will be used. As boundary conditions hourly measured weather data from 1990 till 2006 were usesd and its influence on the internal climate and the hygrothermal performance of the envelope was studied.

Description of the Building

In the following the hygrothermal behaviour of a 1.5 story high building with a total living area of 160 m² and 422 m³ will be studied. The building is designed without a basement. The pitched roofs with an inclination of 50° are orientated to the north and to the south. The distance between the rafters of 160 mm will be embanked with cellulose fibre (assumed bulk density of 60 kg/m3). A vapour-sealed bituminous sheet is resting on the wooden roof sheeting. short-wave absorption coefficient of the bituminous felt is 0.6. The insulating material has a thermal conductivity of 0.04 W/m²K. The corresponding U-value is 0.226 W/m²K. On the inside a smart vapour barrier is applied. The roof is closed to the room with a gypsum board with a corresponding sd-value of 0.15 m. The façade elements are made out of a 1 cm thick exterior mineral plaster (short-wave absorption coefficient of 0.4) applied on 5 cm wood fibre board followed by a OSB panel and 20 cm hydrophobic mineral wool insulation. An airtight system of chipboard and gypsum board serves as an internal shell. In each facade several double-glazed windows are integrated. For all other parts of the building Table 18 indicates all construction input data relevant for the calculation. All material properties are taken from the WUFI data base.

The starting point for all simulations is the beginning of April with an initial moisture content corresponding to 80 % relative humidity. The hygrothermal behaviour is simulated over a period of two years. Hourly weather data measured in a typical year in Holzkirchen (Germany) represent the climatic conditions. Rainwater absorption is neglected only for the roof. For the facades the wind driven rain load is calculated according to their orientation. The heat transfer resistance at the external surface is 0.0526 m²K/W for the roof and 0.0588 m²K/W for the facades. For all internal surfaces this resistance is set to 0.0125 m²K/W. The heating system of the house with 15 kW should be able maintain a minimum room temperature of 20°C.


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Table 18: construction input data relevant for the WUFI calculations

Area Assembly Exterior facades (0.152 W/m²K)

South: 15 m² North: 15 m² West: 34 m² East: 34 m²

10 mm mineral stucco 50 mm wood fibre board 20 mm OSB 200 mm mineral wool 16 mm chip board 12.5 mm gypsum board

Windows South: 3 m² North: 3 m² West: 6 m² East: 6 m²

Type: reflective double glazing u-value: 2.73 W/mK SHGC: 0.11 frame: 30 %

Pitched roof (0.226 W/m²K)

North: 30 m² South: 30 m²

bituminous sheet (sd-value: 1000 m) 16 mm wooden sheeting 160 mm cellulose fibre Smart vapour retarder 15 mm gypsum board

Floor to ground 50 m² 100 mm cellular glass PE membrane 220 mm concrete

Ceiling 50 m² 140 mm soft wood 20 mm mineral insulation board 50 mm screed 10 mm hard wood

The starting point for all simulations is the beginning of April with an initial moisture content corresponding to 80 % relative humidity. The hygrothermal behaviour is simulated over a period of two years, but in the following evaluations only the second year will analysed.

Hourly weather data measured in a in Holzkirchen (Germany) during the years 1990 till 2006 represent the climatic conditions. Rainwater absorption is neglected only for the roof. For the facades the wind driven rain load is calculated according to their orientation. The heat transfer resistance at the external surface is 0.0526 m²K/W for the roof and 0.0588 m²K/W for the facades. For all internal surfaces this resistance is set to 0.0125 m²K/W.


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Figure 65: Influence of weather years




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7.2 Impact of wind-driven rain on historical buildings with brick walls on indoor climate, mould growth and energy consumption

by:

M. Abuku, S. Roels (Belgium) and H. Janssen (Denmark)

The whole building performance analysis is a comprehensive investigation with respect to several points: the hygrothermal response of the walls, durability of building facades, algae formation at exterior wall surfaces, mould growth, indoor climate, energy consumption, etc (see chapter 3). The answers to these questions depend on the building configuration, material of the wall, moisture buffering capacity of the interior, outside climate, heat and moisture source in the building, ventilation rate, etc. Some of these points are not of great concern for some recent wall configurations such as well-insulated walls with air cavity and vapour retarder inside, walls with impermeable siding or sheathing, etc. On the other hand, in historic buildings in Europe, brick was often used without the installation of an adequate air space, insulation or vapour retarder inside, which may result in mould growth at inside wall surfaces and/or increased indoor humidity etc. For such buildings it may be relavant to resort the whole building analysis. An onset to this kind of whole building simulation is given in e.g. Nakhi (1995)103, Mendes et al. (2003)104,

103 A.E. Nakhi. 1995. Adaptive construction modelling within whole building dynamic simulation. PhD thesis, University of Strathclyde. 104 N. Mendes, R. Oliveira, G. Henrique dos Santos. 2003. Domus 2.0: a whole-building simulation program. Eighth International

IBPSA Conference, Eindhoven, The Netherlands, August 11-14, 2003, 863-870.


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Holm et al. (2003)105 and Rode et al. (2003)106. In this part a whole building performance analysis is shown by investigating the impact of wind-driven rain (WDR) (Blocken and Carmeliet, 2004)107 on indoor climate, energy consumption and mould growth at interior brick wall surfaces of historical buildings in Europe.

Methodology of whole building performance analyses

In the whole building performance analysis as a moisture-engineering application, the heat and moisture transfer in building components (Janssen et al. 2007) and the indoor temperature and humidity are analysed simultaneously. In the current study, the heat and moisture transfer in a 2-dimensional brick wall model of 4×4 m2 with a wall thickness of 29 cm and the indoor humidity were numerically analysed, taking into account a constant indoor temperature of 20 °C, a constant ventilation rate of 0.5 ACH, absorption/evaporation at the wall surfaces, and radiative heat transfer. Also the climate data record of Essen, Germany, was used here. For the detailed information of the methodology and conditions of the numerical simulations the reader is refferd to Abuku et al. (2007)108. As a second step of the whole building performance analysis, the results obtained here are elaborated from several points of view, which are described in the following three parts.

Impact on indoor humidity

Figure 71 shows the evolution with time of the indoor relative humidity (RH) over the year with comparison to the cumulative WDR over the entire facade, in which results of the simulation with WDR load are compared to results for the same configuration and boundary conditions but without WDR load. In addition the case is also shown in which the indoor RH is only affected by the ventilation and not linked to heat and moisture transfer through the walls. Comparing the results without WDR and the results only with ventilation shows that the absorption and evaporation at wall surfaces cause a few days delay of indoor RH change. Comparison of the results with WDR load to the results without WDR load shows that WDR load causes an increase of indoor relative humidity of up to 51 %, which is seen at 19th of December. The differences between the results with WDR load and the results without WDR are significant in winter and summer; it is less significant in spring and autumn.

105 A. Holm, H. Kunzel, K. Seldbauer. 2003. The hygrothermal behabiour of romms: combining thermal building simulation and

hygrothermal envelope calculation. Eighth International IBPSA Conference, Eindhoven, The Netherlands, August 11-14, 2003, 499-505.

106 C. Rode, M. Salonvaara, T. Ojanen, C. Simonson, K. Grau. 2003. Integrated hygrothermal analysis of ecological buildings. Research in Building Physics, 2nd International Conference on Building Physics, Leuven, Belgium, September 14-18, 2003, 859-868.

107 B. Blocken, J. Carmeliet. 2004. A review of wind-driven rain research in building science, Journal of Wind Engineering and Industrial Aerodynamics 92 (13) 1079-1130.

108 M. Abuku, H. Janssen, S. Roels. 2007. Impact of Wind-Driven Rain on Mould Growth and Indoor Climate, Contribution to the IEA Annex 41 Whole Building Heat, Air and Moisture Response, Subtask 4 – Moisture-Engineering Application, International Report A41-T4-B-07-2.


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0

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Figure 71: Time evolution of indoor RH with comparison to cumulative WDR over the entire facade

Impact on energy consumption

Seasonal and annual energy consumptions are given in Table 19, comparing the simulation results with WDR load to the results without WDR load and the results only with ventilation. Comparing the results without WDR to the results with only ventilation shows that the energy consumption is mainly in-fluenced by the heat flow through the walls and that ventilation is less important for energy con-sumption in the current conditions. When the results with WDR load are compared to the results without WDR load, the seasonal impact of WDR on energy consumption under such conditions is estimated as 11.1 % in winter (December, January and February); 1.8 % in spring (March, April and May); 12.3 % in summer (June, July and August); and 2.8 % in autumn (September, October and November). The annual impact is estimated as 6.9 %. The impact of WDR is considered to be smaller when the ventilation rate is more important for energy consumption. Note that the energy consumption for cooling in summer decreases due to rain load in hot climate (Hokoi 1986)109, while rain load may still increase the energy consumption in winter. In such climate, it is much more com-plicated to optimise the facade design taking into account WDR load from an energy point of view.

Table 19: Seasonal and annual energy consumption

109 S. Hokoi. 1986. Fundamental study of thermal characteristics of wet building walls. PhD thesis, Kyoto University


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a.

only ventilation b. ventilation + evaporation/absorption without WDR load

c. ventilation + evaporation/absorption with WDR loads

(c-b)/b×100 (impact of WDR)

winter 78.1 kW 852.9 kW 948.0 kW 11.1 %spring 49.7 kW 516.4 kW 525.5 kW 1.8 %

summer 20.1 kW 165.0 kW 185.2 kW 12.3 %autumn 43.9 kW 464.7 kW 477.8 kW 2.8 %annual 191.8 kW 1999.0 kW 2136.5 kW 6.9 %

Impact on mould growth

Figure 72 shows the daily averaged temperature and relative humidity at the edge (facing south-west) and centre (facing south) of the inside wall surfaces on the graphs of generalised iso-pleths of the spore germination time of the fungus mould for substrate category I (Sedlbauer 2001) [67]. Figure 72 (a) and Figure 72 (b) show the results of the simulation without WDR load and Figure 72 (c) and Figure 72 (d) show the results with WDR load. The same plots can be applied to the mycelium growth rate and similar discussion can be done as shown below. Though brick is considered to be in the substrate category II, the isopleths for the substrate category I are adopted here as worst case scenario. If the relative humidity for a given temperature is below the line of ∞ days, no biological activity is expected. Each figure compares seasonal risks. In both Figure 72 (a) and Figure 72 (b), no mould growth is expected; but, when looking at Figure 72 (c) and Figure 72 (d), mould growth is expected, mainly in summer and winter, with a more serious risk in summer than in winter. The comparison of the results of the simulation with WDR and those without WDR shows that the impact of WDR on the mould growth at the inside wall surfaces is significant. Comparing Figure 72 (c) and Figure 72 (d), a wider variation of surface temperature is seen at the edge than at the centre and the surface temperature is averagely lower at the edge than at the centre, because the surface temperature is more influenced by the outdoor temperature at the edge than at the centre, which can even result in a lower risk at the edge than at the centre. For our configuration though, the criterion of relative humidity for mould growth is more severe than that of temperature.

Conclusion and remarks

An onset was given to the whole building performance analysis which investigates the impact of WDR loads on indoor climate, energy consumption and mould growth at interior wall surfaces. The hygrothermal response of the solid brick walls of a cubic building and the indoor humidity were numerically analysed with/without WDR loads. The results showed that WDR causes an increase of indoor relative humidity of up to 51 % and an increase of energy consumptions for heating by 11 % in winter and by 12 % in summer respectively; while a much smaller impact on indoor humidity and a very small impact on energy consumption were seen in spring and autumn. Furthermore the obtained relative humidity and temperature at the interior wall surfaces were combined with isopleths of generalised spore germination time of fungus mould. The results showed that WDR loads can have a significant impact on mould growth especially at the edge of the wall.


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The results obtained here are considered to strongly depend on the material properties, climate, etc. Yet some of the buildings and climate in Europe and some other countries are indeed not far from the ones used for the current study. Therefore the results adequately indicated that a similar analysis of WDR load impact for different climates and other building materials can be worth the effort. It should be also pronounced that the whole building performance analysis can provide a recommendation for building and environment design with a comprehensive point of view (e.g. roof overhang design, building façade design, etc.).


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Figure 72: Daily averaged temperature and humidity at the inside wall surfaces on the graphs of generalised isopleths of the spore germination time (black solid lines) of the fungus mould for the substrate category I.

without WDR without WDR

with WDR with WDR


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7.3 The influence of internal boundary conditions on the hygrothermal performance of construction assemblies - Simplified approaches versus hygrothermal building simulations

The hygrothermal behaviour of building components is very important, due to the effects of weathering and ground moisture as well as increased air-tight construction and higher indoor relative humidity. Thus, it is significant to assess this behaviour as concerns the construction of new buildings or in case of renovation measures. The indoor air temperature and relative humidity must be known parameters besides outdoor climate boundary conditions to carry out hygrothermal computation of building components. The objective is to investigate the impact of standard boundary conditions or hourly measured indoor climate simulation values on the behaviour of the building components of a wall or roof construction.

7.3.1 Simplified approaches

Indoor boundary conditions for hygrothermal building component simulations are differently handled at present. Various standard boundary conditions for hygrothermal simulations are described in the following text and differences are discussed. The standard EN-ISO 13788 110 has been especially developed for steady computations according to Glaser, and the standard EN-ISO 15026 111for non-stationary building component simulations. In addition, a data sheet for hygrothermal building component simulations was established by the International Association for Science and Technology of Building Maintenance and Monument Preservation (WTA) 112, which is valid especially for Central European countries.

EN ISO 13788

Indoor air humidity in buildings with natural ventilation is calculated from the monthly average value of outdoor air temperature, indoor air humidity and moisture load according to the European standard EN-ISO 13788. The correlation of outdoor air temperature and indoor moisture load, as well as the resulting relative humidities are indicated in Figure 73 for a building in Holzkirchen with varying utilisation. Humidities are defined in 5 different classes according to utilisation:

1. storehouse

2. offices, shops

3. apartments with low occupation

4. apartments with high occupation, sports halls, kitchens, staff canteens, buildings with gas thermes

5. specific buildings, i.e. breweries, indoor swimming pools, etc.

110 ISO 13788:2001 Hygrothermal performance of building components and building elements -- Internal surface temperature to

avoid critical surface humidity and interstitial condensation -- Calculation methods. 111 Hygrothermal performance of building components and building elements. Assessment of moisture transfer by numerical

simulation 112 WTA-6-2-01: Simulation of Heat and Moisture Transfer


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Relative indoor humidity is determined by the absolute humidity of indoor air at the constant annual indoor air temperature of 20° C. The result for the absolute humidity ci is ci= ce + Δc, with ce as outdoor absolute humidity and Δc as moisture load, resulting from the monthly outdoor air temperature and the respective classification in 1 to 5.

The approach to compute indoor environment conditions described in this standard is based primarily on Scandinavian findings. The transformation of this approach to non-stationary conditions is highly dependent on the type of building and ventilation conditions. Another question is, whether this approach can be transformed to other European climate regions, especially southern European countries. The progression of moisture generation at temperatures below 0° C and classification in moisture classes is still problematic.

. Figure 73: left: moisture load ranges in heated rooms without air-conditioning in dependence of the

monthly average outdoor air temperature. Utilisation is defined as a class: 1: storehouse 2: offices, shops 3: apartments with low occupation 4: apartments with high occupation, sports halls, kitchens, staff canteens, buildings with gas thermes 5: specific buildings, i.e. breweries, indoor swimming pools, etc. right: progression of calculated relative indoor air humidity for the Holzkirchen location according to 5 moisture classes.

EN ISO 15026

The method described in EN-ISO 15026 is based on measurements, carried out in different buildings in Germany. The progression of indoor temperature and relative humidity can be determined by means of transfer functions from outdoor air temperature. In this case and in contrast to EN-ISO 13788 a 24-hour variable average value is used for outdoor temperature with the intention to simulate daily variations. The transfer functions for indoor air temperature and relative humidity in dependence of the 24-hour variable average value of outdoor air temperature as well as the respective indoor environment conditions for the Holzkirchen location are indicated in Figure 74. The progression of temperature seems to be more realistic in this standard in comparison to EN-ISO 13788, especially as concerns the fact that indoor temperatures in summer rise to values between 20° C and 25° C. There is


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generally a differentiation between only two moisture classes, a high and a low moisture load.

Figure 74: left: Transfer functions for indoor air temperature and relative humidity in dependence of the 24-hour variable average value of outdoor air temperature right: progression of the computed relative indoor air humidity for the Holzkirchen location according to 2 moisture classes.

WTA - 6-2-01/E

Indoor environment conditions are reproduced in a simplified way by a sinusoidal smoothed annual progression in the German WTA-Directive 6-2-01. This approach is exclusively suited for Central European climate conditions. In contrast to EN-ISO 13788, however, there are only three moisture classes. If we compute the respective moisture load for a room, we learn that the limits of the respective classes intensely diverge in both standards. For example, the maximum moisture load for a living room with usual occupation at a monthly average outdoor air temperature of 10 °C results in 2 g/m3, and according to DIN EN ISO 13788 in 3 g/m3 (corresponds to a high moisture load in the WTA Directive). A possible explanation for this fact may be the differences in construction and climate-related ventilation behaviour, and could be justified as future approach on the basis of a secure interpretation of the moisture computation (higher indoor air humidity generally means higher humidity in the construction). Figure 75 clearly shows that the days with maximum values for relative humidity and temperature diverge. The maximum value is already reached in June in contrast to the observed progression of indoor air humidity.


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Figure 75: top: Transfer functions for indoor air temperature and relative humidity in dependence of the 24-hour variable average value of outdoor air temperature bottom: progression of the computed relative indoor air humidity for the Holzkirchen location of 2 moisture classes.

7.3.2 Hygrothermal Performance of a whole Building

Description of the studied cases

The building studied here is the same as described in chapter 7.1 (see page 108 ff). In Figure 76 an overview of all simulated scenarios for the daily profiles for ventilation, heat and moisture loads is given. First the hygrothermal performance of an empty building will be studied. In cases 1 the building in only ventilated by infiltration. The hourly air change rate is due to tight windows 0.2 h-1. In the following cases the average daily air change rate is according to the minimum ventilation requirements 0.5 h-1. In case 3 to case 5 the building is occupied 24 hours a day with 4 persons. In order to simulate the user behaviour, two daily short intensive ventilation scenarios will be examined. In case 4 the windows are slightly open (ACH = 4 h-1) for one hour at a 8 a.m. and at 6 p.m.. In cases 5, 6 and 7 a shorter (30 min) but more intensive ventilation (ACH = 8 h-1) will be simulated. In order to study the influence of a transient user behaviour over a day in case 6 and 7 it is assumed, that the building is only full occupied from 6 p.m. to 8 a.m. During daytime the heat and moisture profile varies from hour to hour. But the daily sum, especially of the moisture load is kept constant to almost 12 kg/day. The hygrothermal computations started in April with an initial water content equivalent to 80 % relative humidity. The computations were carried out over a period of 2 years. Hourly climate boundary conditions for the Holzkirchen location are applied.


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Impacts of driving rain are only considered for façades and according to orientation. The external heat transmission resistance is 0.0526 for the roof and 0.0588 m2K/W for the façades. The heat transmission resistance for all internal surfaces is 0.0125m2K/W. The minimum indoor temperature of 20° C should be maintained by means of a heating system of 15 KW.

Resulting indoor climate conditions

The results for indoor air temperature and relative humidity computed for the cases 1 to 7 by means of WUFI-Plus are indicated in Figure 77. These clearly demonstrate the impact of the internal thermal and moisture load as well as the non-stationary ventilation behaviour. If there are no additional thermal and moisture loads in the building, relative humidity varies between 15 % in winter and 70 % in summer. The difference of an air exchange of 0.2 and 0.5 is almost negligible. In an occupied building with a constant air exchange rate of 0.5 relative humidity varies between 40 % and 70 %. The minimum temperature of 20° C can be maintained. The temperature in summer rises to maximum values of almost 30° C. The reason is that no additional ventilation was assumed in summer and no additional measures for solar control were adopted. Most obvious is the impact of ventilation behaviour. In contrast to continuously constant ventilation short-term ventilation by opening the window modifies daily variations of relative humidity by almost -10 %. Moreover, it is impossible to maintain the indoor air temperature of 20° C in winter because of the insufficient heating systems during the ventilation phase. This corresponds to the measurements.


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Figure 76: Overview of the simulated daily profiles for ventilation, heat and moisture loads.


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Figure 77: Indoor temperature (top) and relative humidity (bottom) for case 1 to 7 resulting from WUFI Plus.

7.3.3 Comparison of the resulting indoor conditions

The resulting indoor air temperatures and humidities must be statistically analysed. Figure 78 gives the so-called box-plot representation. It is possible to discern the position and diffusion of distribution. Besides the average values the median, the value range of 25 % and 75 % as well as the corresponding standard deviations are reproduced. The direct comparison of standard boundary conditions and progressions computed by WUFI-Plus shows clear-cut differences. This may be demonstrated by the example of an apartment with low and normal occupation. In this case and in accordance to EN 13788 the humidity class HC 3 is to be applied. The average value of relative humidity is approx. 60 % with fluctuations of approx. ± 10 %. The usual moisture load can be applied for such a building according to EN 15026. In this case, the average value of the relative humidity is approx. 45 % with fluctuations of approx. ± 10 %. A similar progression of the relative humidity is achieved by applying the standard boundary conditions according to the WTA Directive with an average value of approx. 50 % with fluctuations of approx. ± 10 %. If we compute the progression of relative humidity by means of WUFI-Plus, the result for the two first cases without any additional thermal and moisture sources is a relative humidity of a value slightly below 40 % with fluctuations of slightly below 20 % to 60 %. The impact of the users (cases 3 to 7) is clear-cut. The average relative humidity is increased by 45 % to 50 % for the respective case only by the real utilisation of the building simulated by thermal and moisture load. Indoor relative humidity is increased by a further 5 % by short-term ventilation by opening the window despite the same amount of thermal and moisture load.


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Figure 78: Statistical analysis of the temperature and relative humidity derived from the methods in the standards and simulated with WUFI Plus. The boxes show the average (dot), median and 25 resp. 75% boundaries. The whiskers the 5 and 95 range. The minimum and maximum values are also indicated.

7.3.4 Influence on the hygrothermal performance

It is investigated in the following, how the various standard boundary conditions according to the 3 standards and different user behaviour computed by means of WUFI-Plus influence the hygrothermal behaviour of a roof orientated to the north, or of a façade orientated to the west. For comparison, the water contents in the timber shell of a roof with a 50-degree inclination to the north was investigated as well as the total water contents in the construction. The fulfillment of the total annual balance and the moisture content in the wooden sheeting during the period of observation apply as evaluation criteria.


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Figure 79: Simulated water content of the wooden sheeting (top) and the total water content in the construction (bottom) based on various indoor climate data.

Figure 79 shows the results for the investigated roof in a diagram. Computations by means of WUFI of the hygrothermal behaviour according to the boundary conditions of EN 13788 show that this roof is no longer hygrothermally “secure” from moisture class 2. The assessment of this construction according to EN 15026, however, rates it as hygrothermally harmless according to the respective usual boundary conditions. According to WTA, a slight increase in the total water content occurs in the course of the year at a standard moisture load. Computations by means of WUFI-Plus also provide clear-cut differences in the hygrothermal behaviour. Whereas the construction is rated to be hygrothermally harmless for the cases C1 and C2, it is assessed to be just hygrothermally harmless for case C3 (continuous occupation, constant ventilation). In case of non-stationary ventilation or non-stationary user behaviour with the same thermal and moisture load, however, the total water content as well as the water content in the timber shell clearly increase. If the results are compared to the respective standard boundary conditions, it is obvious that the moisture class is modified by one category by non-stationary behaviour alone. This means that the computations according to EN 15026 in case of non-stationary behaviour provide more realistic values for the moisture class. Similar results are achieved for the west façade (Figure 80). If we compare the computed results by means of WUFI-Plus with those of the standard boundary conditions, we see that the standard boundary conditions have various effects on the hygrothermal behaviour, and that there is a clear impact of the user behaviour.


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Figure 80: Simulated water content of the OSB (top) and the total water content in the construction (bottom) based on various indoor climate data.

The comparisons of the results from indoor climate simulation show that WTA boundary conditions for a standard moisture load should be usually applied for residential buildings or the respective boundary conditions according to EN ISO 15026. Applying these boundary conditions means the choice for the safe side. It seems to be reasonable to apply high moisture loads in buildings, where particularly unfavourable indoor humidity conditions must be expected.

The moisture classes according to EN ISO 13788 provide too high values in any case, and must thus be selected with caution.


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8 Benefits


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9 Conclusion

The hygrothermal behaviour of the building envelope has an important effect on the overall performance of a construction. Therefore, a combined tool for hygrothermal envelope calculations and whole building simulations has been developed and the first steps to validate the model have been successfully completed. The results are promising but many more validation examples are necessary in order to gain confidence in the new model and enhance its performance. Since there are some building materials that react differently when exposed to transient instead of steady state conditions more appropriate hygrothermal parameters may have to be determined for hygrothermal building performance calculations.

Models like those presented here will help to improve energy simulations because latent heat loads and their temporal pattern can be calculated more accurately. At the same time the determination of indoor air and surface conditions in a building becomes more reliable. This is very important to assess indoor air comfort and hygiene. Post processing models for the determination of mold growth or corrosion risks rely on accurate results of the transient temperature and humidity conditions. The same holds for the design of HVAC systems in heritage buildings or museums113 where the humidity buffering capacity of the envelope and furniture helps to control temperature and humidity fluctuations.

113 Harriman, L., Brundrett, G. & Kittler, R. 2001. Humidity Control Design Guide for Commercial and Institutional Buildings. Atlanta:

ASHRAE publication.


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10 Outlook


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11 Unclear Papers

SOLAR DESSICANT COOLING EXAMPLE OF APPLICATION TO A LOW ENERGY BUILDING

Thibaut Vitte, Monika Woloszyn, Jean Brau

https://www.kuleuven.be/bwf/projects/annex41/protected/data/CETHIL%20May%202005%20Paper%20A41-T4-F-05-1.pdf

PARAMETRIC ANALYSIS OF A DESICCANT COOLING SYSTEM: EFFECT OF HYGROTHERMAL INTERACTIONS WITH BUILDING ENVELOPE –PART 1

C. Maalouf 1*, E. Wurtz2, K. C. Mendonça3, L. Mora 1

https://www.kuleuven.be/bwf/projects/annex41/protected/data/ULR%20Oct%202005%20Paper%20A41-T4-F-05-2.pdf

MOISTURE PERFORMANCE CRITERIA FOR UK DWELLINGS

Hector Altamirano-Medina, Mike Davies, Ian Ridley, Dejan Mumovic, Tadj Oreszczyn and Marcella Ucci

https://www.kuleuven.be/bwf/projects/annex41/protected/data/UCL%20Apr%202006%20Paper%20A41-T4-UK-06-3.pdf

House dust mites in beds and bedrooms

Kaisa Svennberg, Building Physics, Lund University, SwedenLars Wadsö, Building Materials, Lund University, Sweden

https://www.kuleuven.be/bwf/projects/annex41/protected/data/LTH%20Apr%202006%20Paper%20A41-T4-S-06-2.pdf

Moisture Buffer Value of Building Materials

Carsten Rode, Ph.D.1, Ruut Peuhkuri, Ph.D.2 , Berit Time, dr.ing. 3, Kaisa Svennberg, Tekn.Lic.4, and Tuomo Ojanen, M.Sc. (Tech) 5

https://www.kuleuven.be/bwf/projects/annex41/protected/data/DTU%20Apr%202006%20BGinf%20A41-T4-Dk-06-1.pdf

Durability of Roof Membranes: Long Term Reflective Performance of Roof Membranes-Thermal and Moisture Performance


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William A. Miller, Oak Ridge National Laboratory (Leading Scientist/Manager), and Achilles Karagiozis, Oak Ridge National Laboratory

https://www.kuleuven.be/bwf/projects/annex41/protected/data/ORNL%20Oct%202006%20Paper%20A41-T4-US-06-1.pdf

MOISTURE SUPPLY IN HEAVYWEIGHT DETACHED HOUSES

Kati Salminen, M. Sc. Student, Mikko Salminen, M. Sc. Student, Minna Korpi, M. Sc Juha Vinha, Lic.Tech., Jarek Kurnitski, Ph.D., Targo Kalamees, M. Sc.2)

https://www.kuleuven.be/bwf/projects/annex41/protected/data/TTY%20Oct%202006%20Paper%20A41-T4-Fin-06-2.pdf

PARAMETRIC ANALYSIS OF A DESICCANT COOLING SYSTEM: EFFECT OF HUMIDITY ON SYSTEM PERFORMANCE

Chadi Maalouf*, Etienne Wurtz, K.C. Mendonça, Laurent Mora

https://www.kuleuven.be/bwf/projects/annex41/protected/data/ULR%20Oct%202006%20Paper%20A41-T4-F-06-7.pdf

THE HYGRIC INERTIA OF BUILDING ZONES: CHARACTERISATION AND APPLICATION

HANS JANSSEN1 AND STAF ROELS2

https://www.kuleuven.be/bwf/projects/annex41/protected/data/DTU%20Apr%202007%20Paper%20A41-T4-Dk-07-1.pdf

A Study of Air Movement in the Cavity of a Brick Veneer Wall

Christopher Hannan, Dominique Derome,

https://www.kuleuven.be/bwf/projects/annex41/protected/data/CON%20Apr%202007%20Paper%20A41-T4-C-07-1.pdf

AIRBORNE MOISTURE DISTRIBUTION IN A SINGLE ROOM

Sergio Vera, Jiwu Rao, Paul Fazio

https://www.kuleuven.be/bwf/projects/annex41/protected/data/CON%20Apr%202007%20Paper%20A41-T4-C-07-2.pdf

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