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THE DYNAMIC THERMAL PERFORMANCE OF MASONRY WALL ASSEMBLIES OF SIMILAR THERMAL RESIST ANCE

Prof. Dr. Ossama A. 'Abdou Architectural Engineering Prograrn

Drexel University, Philadelphia, Pennsylvania 19104, USA

ABSTRACT

The dynamic thermal performance of three types of masonry walls -- insulated hollow block and face brick, insulated common brick, and insulated common brick and face brick -­exhibiting similar thermal resistance is investigated by means of computer simulation of a typical residential space with a southem exposure using representative weather data of both the hot, dry climate and the moderate climate of the United States. Three thermalload modes were investigated to accurately predict thermal performance of the masonry walls: controlled ventilation, no ventilation, and loads occurring in the absence of windows. Results are given, which show that the thermal performance of the walls does not vary significantly , and that increasing thermal mass in the wall decreases cooling requirements and the occurrences of extreme temperatures in summer, and tends to increase the prevalence of warm conditions in the space in winter. Further, it was concluded that ventilation and the incorporation of windows are essential to balance thermalloads in the building space.

INTRODUCTION

In the past two decades energy consumption has become a prime concem in residential building design and construction. Nearly one-eighth of the energy consumed in the United States serves to maintain cornfort in residential buildings [1]. Energy conservation has become a significant performance factor in the design and operation of buildings. One of the most profound aspects of energy conservation in buildings is the enhancement of the thermaL performance of the building envelope, especially that of the exterior walls particularly in multi­story applications. There has been an increased use of masonry in exterior wall assemblies in an effort to provide greater potentials for energy savings.

This research presents resuIts of a computer modeling and simulation study in which the dynamic thermal performance of masonry walI assemblies of similar thermal resistance (R) -­but varying in other thermo-physical properties -- was investigated; the total resistance to heat flow through a building section being equal to the sum of the R-values of the various components of the building section. Resistance, the reciprocaI value of thermal transmittance (U-Factor) of a building element, represents the ability of a material to retard heat flow, taking into account the resistance of both the inside and outside airfilms. Thermal conductance describes the quantity of heat which flows through a composite material of a particular thickness before airfilm resistance is accounted for, and is of particular relevance in this research.

SIMULA TION PROCEDURE

Computer simulation has been performed to examine three particular types of exterior masonry wall construction taking into account alI pertinent physical characteristics and the therrna1 mass of alI building materiais used in the assembly: insulated lightweight hollow block

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and face brick (referred to as HB-FB in the figures); insulated, stuccoed common brick (CB); and insulated common brick and face brick (CB-FB). The wall types selected are representative of prevalent construction practices in North America for many residential and small commercial structures. The thermo-physical properties of the walls are depicted in Table 1. As shown, ali of the above wall systems have similar thermal conductance values to allow for accurate comparison of therrnal behavior.

Table 1. Thermo-physical properties of simulated exterior masonry walls

Construction Description Thiclrness [m]

Conductivity [W/m+K]

Density [kg/m3j

Specific Heat Conductance V-Factar [W/m2+K] [KJ/(kg+K)] [W/m2+K]

Exterior Walll: Insulated Hollow B10ck and Face Brick (HB-FB)

Dense Face Brick .10 Insulation .05 Lightweight Concrete Block .10 Plaster or Gyp Board .02

Outer ThermaI Absorptance = 0.90 Inner ThermaI Absorptance = 0.90

1.245 0.430 0.381 0.726

2082 32

609 1602

Outer Solar Absorptance = 0.93 Inner Solar Absorptance = 0.92

Exterior Wall 2: Insulated, Stuccoed Common Brick (CB)

Stucco Insulation Common Brick Plaster or Gyp Board

.025

.05

.20 .02

Outer ThermaI Absorptance = 0.90 Inner ThermaI Absorptance = 0.90

0.692 0.430 0.726 0.726

1858 32

1922 1602

Outer Solar Absorptance = 0.92 Inner Solar Absorptance = 0.92

Exterior Wall 3: Insulated Common Brick and Face Brick (CB-FB)

Dense Face Brick Insulation Common Brick Plaster or Gyp Board

.10

.05

.20 .02

Outer TherrnaI Absorptance = 0.90 Inner ThermaI Absorptance = 0.90

1.245 0.430 0.726 0.726

2082 32

1922 1602

Outer Solar Absorptance = 0.93 Inner Solar Absorptance = 0.92

.920

.837

.837

.837

.644 .588

Ou ter Surface Roughness: Rough

.837 .837 .837 .837

.658 .599

Outer Surface Roughness: Smooth

.920

.837

.837

.837

.639 .583

Outer Surface Roughness: Rough

The thermal performance of these masonry construction types were evaluated in terms of the effect on indoor temperature conditions in response to varying outdoor conditions. The evaluation encompassed assessment of winter and summer conditions of unacceptable temperature leveIs in the simulation zone when the building is unconditioned. The unconditioned mode of operation is particularly important here, since the majority of residential units in the world does not have air-conditioning.

D.ynamic Response Concept

Traditiona11y, thermalload calculations are made on the basis of steady-state conditions. Such an approach is misleading, because steady-state conditions do not adequately reflect actual performance; they do not reflect the effects of natural daily variations in climatic variables.

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Diurnal variations produce an approximately repeti tive 24-hour cycle of increasing and decreasing temperatures. The effect of this on a building is that in the hot period heat flows from the environment into the building, where some of it is stored, and at night during the cool period the heat flow is reversed: from the building to the environment. Hence, the true thermal performance of a wall construction subjected to daily cycles of temperature change through some range of values from a minimum temperature at night to a maximum during the day depends on two things: (1) The U-Factor which describes how fast heat can flow through the wall, and (2) the thermal inertia of the wall, a dynamic condition which describes how fast the wall can heat up or cool down, the latter being a measure of the heat storage capacity of the wall.

If external and internaI temperatures remain steady, the heat flow through walls with the same U-Factor would be the same, despite the fact that one wall may be a "passive" structure with a large heat capacity and the other a lightweight wall. But it is a different story when fluctuating temperatures are considered. Therefore, during a 24 hour cycle, walls with equal U­Factors, but different mass produce different peak loads. Heavy walls absorb heat energy, or release it depending on externaI temperature fluctuations, acting as dampening storage tanks to changes within the enclosure.

Computer Model

To place the role of the building envelope in its proper perspective, a computer model utilizing the dynamic response concept was established using the Building Loads Analysis & System Thermodynamics (BLAST) microcomputer software [2]. The program utilizes response factors and conduction transfer functions in its calculations which permit the careful and complete analysis of transient heat conduction through walls and of heat storage in rooms. In this respect, the effects of surface roughness and hourly variations in wind speed on outside wall convective heat transfer coefficients (air mm resistance) are accounted for.

The thermal performance of the three wall types described above was examined in the context of a typical2.7 m high residential space of 17.28 m2 (3.6 m wide and 4.8 m long) with exposed exterior masonry wall area of 12.96 m2 perforated by a single 1.44 m2 square, double­glazed and shaded window in the middle of the wall. The window sill was 0.90 m high. Floor/ceiling construction was of high thermal mass (10 cm reinforced concrete slab) and the interior partitions were made of 20 cm lightweight hollow masonry. All construction was constant in alI simulation runs with the external wall as the variable. No heat exchange was allowed across partition walls, floor slab or ceiling.

The physical system as modelled here encapsulated the residential space and was coupled to ambient weather and subject to solar energy absorption. The direct coupling between the externaI ambience and the simulated space was represented by the heat transfer from ventilation/infiltration and other heat flow paths where there was no significant thermal capacitance. In this model, this was represented by the window.

Climatic Conditions

The performance of the above physical system during heating and cooling modes in two distinct climates were assessed. To establish representative performance, real weather data with actual transients were used. To meet these criteria Typical Meteorological Year (TMY) weather files were used for the city of Phoenix, Arizona, being representative of the hot, dry climate, and Philadelphia, Pennsylvania, a representative candidate for the moderate climate of the United States. They differ mainly in the fact that Phoenix possesses a much greater diurnal amplitude in both solar radiation and ambient temperature than does Philadelphia. This range of weather conditions allowed the model to be exercised over a wider domain.

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Temperature Cycles: For each climatic zone the outdoor temperature cycIe was changed twice to analyze the effect of cIimatic changes on the thermal performance of each of the wall systems. The first outdoor air temperature cycIe used was the 24-hour cycIe for January 21 designating the winter design day. CycIe 2 was a typical July 21, representing the summer design day. For all runs of the program using outside temperature cycIes 1 and 2, the indoor temperature was not controlled, but rather was allowed to "float" according to the response of the building space to the varying outdoor conditions, so that the response of the different wall sections to changing outdoor conditions could be analyzed.

VentilationlSolar Radiation Strategies: A common criticism of heavyweight construction (such as masonry) is its tendency to maintain uncornfortably high temperatures on hot summer nights, when the quicker response of a lightweight structure might be expected to take better advantages of cooler ambient air temperatures, and so improve indoor conditions. To examine this effect, the occurrences of temperatures greater than the upper comfort bound were evaluated. A ventilation strategy has been implemented in which no outdoor air was allowed in the building if the indoor temperature exceeded the neutral temperature leveI of 2S°C. And, further, to ensure that only ventilation cooling occurred, it was specified that the outside temperature be O.SoC cooler than the thermal zone's air before ventilation occurred. To assess the effect of ventilation, simulation was also performed with a "no ventilation" mode. And, further, to negate the effect of instantaneous solar radiation into the building, a "no window" mode was employed in separate runs, a case which is becoming increasingly common in commercial structures.

ANALYSIS AND DISCUSSION OF RESULTS

The three wall systems were run once for each of the 2 temperature cycIes representing winter and summer conditions for the three ventilation/radiation strategies described above producing a total of 18 output files for each climatic zone. The analysis is broken down into two groups, each of which represents one cycIe. Note that comparisons between different cIimatic conditions should be made with caution, because of the different extent of the thermal cornfort zone in each cIimate. For the hot dry cIimate, the range is taken to be between 18°C and 30°C [3]. For the moderate cIimate the upper bound of the thermal comfort zone is assumed to be 28°C and the lower bound 20°C [4]. Figures 1 and 2 show the diurnal variations of externai and internai temperatures of a free response test in a periodically changing thermal regime.

Winter Conditions

Hot Dry CLimate: The indoor temperature profiles for the two modes: controlled ventilation, and no ventilation, were identical for the respective wall types. This comes as no surprise, since the outdoor temperature never reached the neutral temperature leveI. However, it is of interest to note that there was a slight variation in the temperature magnitudes between the wall types. Wall (CB) showed a relatively better performance (i.e., higher leveis) followed by Wall (CB-FB) and then wall (HB-FB), with a time lag of only two hours for all wall systems. The maximum range of the inside temperature for this group was only approximately 4°C even though the outside temperature ranged through 11°C over the 24-hour period. A considerable difference in performance occurred in the case of "no windows". This describes a condition in which no direct solar radiation was allowed to enter the space. Therefore, maximum temperatures reached inside the space were less than in the first two modes, with a more pronounced magnitude difference of approximately 2.SoC in the afternoon, as compared to a difference of only 1°C in the early morning hours. Here, the maximum range of the inside temperature got even smaller (approximately 2.7°C). Again, Wall (CB) performed best among the three wall types in the absence of direct solar radiation. It is of particular interest to note, that the time lag here increased to 6 hours, indicating a relatively high heat storage capacity of the masonry walls. Although higher temperature leveIs could be attained in the simulated zone as

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<1.> L

.3 ro L <1.> o. E <1.> I-

.~. .~.~ .~.

I ----------O · I 1 1 I 1 I I 1 1

25

20

15

lO

5

2 3 4 5 6 7 8 9 l O I I 12 13 14 15 16 17 18 19 2021 22 23 24

Hours

January 21

" " ~;: « :,"':-';:

2 3 4 5 6 7 8 9 lO I I 12 13 14 15 16 17 18 19 20 21 22 23 24

Hours

Ju)y 21

Legend

-.- Outdoor Temp.

Controlled Ventllation

-o- Wall (HB-FB l -•. Wall (CB l -<>- Wall (C B- FB l

No Ventilation

-.- Wall (HB-FBl -ir Wall (CBl -x· Wall (CB-FB l

No Windows

-x· Wal l (HB- FBl - Wall (CBl - Wall (CB -F Bl

Figure 1. Winter and summer design day temperature profiles for the hot dry climate

u

<1.> L :J ~

ro L <1.> O-

E <1.> I-

45

40

35

30

25 ~~~~~~~~-Q~~

---.--.--::~.~~.~.--. ."...""'" 20

15

lO

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5

O +-;--r-+~~~~+-+-;--r-+-+~~~~+-+-~-r-+~~~

2 3 4 5 6 7 8 9 10 I I 12 13 I 4 15 16 17 18 19 20 2 I 22 23 24 Hours

July 21

Legend

-8- Outdoor Temp. Thermal Com fort Zone

Contro lled Vent 11 at 1 on

-o- Wall (HB-FB) -. - Wall (CB) ~ Wall (CB-FB)

No Ventl1a t lon

-.0.- Wall (HB-FB) -ir Wa ll (CB) -x Wall (CB-FB)

No Windows

-)I(. Wal l (HB- FB) - Wal l (CB) - Wall (CB - FB)

Figure 2. Winter and summer design day temperature profiles for the moderate cIimate

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compared to outdoor leveIs, they were still considerably lower than the lower bound of the cornfort zone.

Moderate C/imate: In terms of wall performance, no significant change in results can be reported here for the winter conditions, except that a considerable shift in temperature leveIs was noticed, mainly due to the more severe climatic conditions as compared to the conditions in the hot, dry c1imate. Ali temperature leveIs were below freezing. Furthermore, it was noticed that the indoor leveIs, in general, were c10ser to outdoor leveIs . Nearly the same maximum range of indoor leveIs was attained as in the hot, dry climate, although the maximum range of outdoor temperatures here was on1y approximately 7"C.

Summer Conditions

Hot Dry Climate: The results of this group of tests are quite contradictory to the results obtained and discussed in the group above. Therefore, it can be concluded on the basis of this test that there is quite a difference in the seasonal performance of the wall systems. Again, no significant difference <± 1 C) in the thermal behavior among the different wall assemblies was present. With controlled ventilation, the walls exhibited almost identical temperature trajectories with a time lag of approximately 3 hours. The highest outdoor temperature leveI was 41.7°C, whereas the highest indoor temperature reached only approximately 37SC, well above the upper bound of the cornfort zone. The profiles show that within the first 8 hours of the day, thermal cornfort could be achieved passively. In the other two modes, indoor temperatures increased considerably reaching a maximum of 42°C (no window mode) with a time lag of approximately 6 hours. The significance of this result lies primarily in the fact that with the elimination of windows from the exterior walls, the thermal mass of the envelope is increased and the direct transfer of "night coolth" from the ambience to the inside was eliminated, thus depriving the thermal mass, which has been charged with heat (due to the combination of air temperature and solar radiation) of cooling down. Because glass has a relatively low mass, its response to c1imatic changes was almost instantaneous. Moreover, it is interesting to note that the walls although possessing virtually the same thermal conductance, in the "no ventilation" and "no window" modes they exhibited on the average smaller temperature swings (3.9'C and 4SC, respectively) than the case with controlled ventilation (9.6°C). The outdoor temperature range was approximately 16°C.

Moderate Climate: Here, the indoor temperature trajectories tracked the general behavior of the hot, dry c1irnate, with the exception that cornfort could be attained for nearly the entire 24-hour period when ventilation was controlled. In the other two modes, the indoor temperature hovered above the upper comfort limit, with minor differences between the walls, Wall (CB­FB) performing best in ali cases. Wall (HB-FB) although possessing virtually the same thermal resistance as the other walls, exhibited a larger temperature swing. Ali walls exhibited a six­hour time lag compared with the ambient (for which the maximum occurred about 3 pm).

CONCLUSIONS

In this paper the thermal performance of three types of common masonry wall construction of similar thermal resistance was explored: insulated lightweight hollow block and face brick; insulated, stuccoed common brick ; and insulated common brick and face brick. Results for computer simulation of a model residential space have been obtained using climatic data from two locations in the United States. Thermal performance has been evaluated in terms of 24-hour temperature profiles for summer and winter conditions. A number of conc1usions may be drawn from these results. First1y, for the cases under consideration, indoor temperature leveIs are not very dependent on the wall type for each thermal load mode modelled. Specifically, the wall type does not great1y affect cooling requirements, but does slight1y reduce heating requirements in the hot, dry clima te. In the moderate climate, the variation among the

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wall types was even less pronounced. AlI of them reduced the peak temperatures and provided for cornfort most of the time in the summer. In the winter, however, alI indoor temperature leveIs, although higher than outdoor leveIs, remained below freezing point, indicating a poor thermal performance in such aclimate. Given the above, it is quite clear that decisions concerning selection of any of the wall types studied here should be based on aspects other than the thermal performance.

The second conclusion that may be drawn from the figures is that both ventilation and the inclusion of windows in external masonry walIs are essential. Ventilation presents a mass pre-cooling in the summer, and helps reduce the occurrence of extreme temperatures in the overheated periods. It may also lead to long-term reductions in cooling requirements by reducing the need for air conditioning.

An interesting physical phenomenon was identified when the time lag shifted from being controlled by the heat transfer through the wall, to being controlled by the direct heat transfer from the ambient through the windows. This occurred because the amplitude of the thermal wave penetrating the wall was significant1y reduced ?? through increased thermal mass to be sma1ler than the amplitude caused by the heat flow from the ambient (window effect ??).

Results indicate the presence of a thermal mass effect due to non-linear interaction between the thermal wave penetrating the wall and the indoor temperature gradients. Masonry walls in themselves should not be looked at alone, but other parameters such as glass and ventilation have a significant impact on the way the masonry walI performs. Ventilation presented a mass pre-cooling in the summer. In winter, the effect was reversed. Because the outside temperature was always below that of comfort no ventilation occurred. The wall mass, however, helped improve the indoor thermal conditions and moved the temperature profile closer to the cornfort zone.

REFERENCES

(1) Burch, D. M., Kritz, D.F. and Spain, R. S., "The Effect of Wall Mass on Winter Heating Loads and Indoor Cornfort - An Experimental Study", ASHRAE Transactions, 90, Part I, pp. 94-121 (1984).

(2) The Bui/ding Loads Analysis & System Thermodynamics (BLAST) Computer Program, B1ast Support Office, Urbana, llIinois 61801.

(3) Abdou, O. A., The lmpact of Passive Solar Energy Utilization on Multi-story Apartment Houses in Hot Dry Climates, Doctoral Dissertation, University of Michigan, 1987.

(4) Olgyay, A. and Olgyay, V., SolarControl & Shading Devices, Princeton University Press, Princeton, (1957).