Thermal insulation of buildings

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1975 Physics in Technology 6 164

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As energy becomes more and more expensive, so it becomes necessary to use it more sparingly and to conserve it more carefully. But even today as the media are screaming energy crisis and fuel bills rocket, householders still use (and therefore pay for) more fuel than they need to harm their homes. This is despite government campaigns such as Save it which encourage us to keep the heat we have paid for so dearly.

Insulation materials and techniques have not previously received the attention they deserve; increasing fuel costs will create pressures to correct this. Insulation is all too often skimped in building design and left out of household budgets.

This article looks at the main ways heat is lost from our homes, methods of stopping this loss and perhaps most important, the economics of it all.

Reasons for improving thermal insulation Improved thermal insulation of buildings is directed at reducing the overall rate of flow of heat from them, thereby conserving fuel in cool weather and securing better comfort when ambient temperatures are

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THERMAL INSULATION OF BUILDINGS E A Raynham Fibre Building Board Development Organization, London WC2

high. It is also directed at improving comfort at reduced cost by reducing draughts which may originate from admission of cold air through direct openings to the outside or from local regions of high thermal loss, such as windows.

Causes of heat loss from buildings A hermetically sealed building is ideal from a heat loss point of view. Clearly if no cold air is coming in, being heated, and then flowing out, one source of heat loss is eliminated. However this is not a practical proposition since sufficient air must be allowed to flow through a building to permit the necessary combustion processes to proceed with safety and comfort. These are the burning of fuel for cooking, heating, to a lesser extent lighting and, most important of all, breathing. The guide book (1970) of the Institution of Heating and Ventilating Engineers (IHVE) suggests, for example, half an air change an hour as that which is necessary for bedrooms, one per hour for living rooms, one and a half per hour for lavatories, two for bathrooms and five for cells in police stations!

Physics in Technology Ju ly 1975

Figure 1 Comparison between conduction and air infiltration heat losses through windows (a) is air infiltration through a weather-stripped window, and (b) is the equivalent heat loss, (c) is heat loss by conduction through a double glazed metal window, (d) is air infiltration through a window without weather-stripping window, and (f) is the equivalent heat loss, (e) is the heat loss by conduction through a double glazed metal window

In fact, one and a half air changes per hour is considered adequate for most situations in dwellings, offices, shops, factories, hospitals, etc.

Excessive ventilation Tests conducted some years ago by the Building Research Station on brick built houses with standard factory-made joinery indicated that they suffered some two and a half air changes per hour. Such a level of ventilation would result in a heat loss from a pair of typical semidetakhed houses of approximately 470 W/K temperature difference between the air inside and outside the dwellings. This is 50 % worse than conducted heat loss through cavity brick walls of the same two dwellings, and nearly twice as much as the heat loss through their uninsulated roofs. Clearly then, to reduce heat loss from buildings the first priority is to control ventilation. This can be done by weather stripping doors and windows, either with metal strips or with self-adhesive plastic foam. These measures are not perfect and are most unlikely to reduce the air change below the one and a half referred to above as the acceptaMe minimum.

Windows It would perhaps be convenient here to compare heat lost by ventilation with conducted heat losses attributable to windows. It is commonly assumed that double glazing reduces heat loss through windows by 50 %. In fact the IHVE guide book shows that the conduction heat-loss through a double glazed wooden window is 2.5 W m-2 K-l compared with 4.3 W m-2 K-l through a similar single glazed window. The corresponding figures for metal windows are 3.2 and 5.6 Wm-2 K-l respectively.

It should also be realized that in the case of the

Physics in Technology July 1975

Figure 2 Use of dense linings to form an insulating cavity with low emissivity

pair of houses already referred to above, conduction heat losses through windows amounted to some 130 W/K temperature difference between inside and outside air. Double glazing would reduce this by not much more than 50 W K-l.

Two Building Research Station reports (Thomas D A and Dick J B 1953, Dick J B 1950) permit a comparison to be made between the effects of weather stripping, and double glazing. This com- parison is summarized in figure 1 and shows that weather stripping does far more to reduce heat loss than double glazing. Indeed, unless double glazing is fitted in such a manner as to reduce ventilation losses as well, it will not be effective or economical. It must be remembered, however, that double glazing does provide other benefits, such as reducing down draughts, reducing condensation and im- proving security.

Walls and roofs Any material or technique used to reduce heat flow through walls or roofs does so by incorporating air or other poorly conducting gases into the structure. This is often done by using lightweight materials which themselves contain trapped air. The lightweight materials that are available include mineral wool, quilts and slabs, expanded poly- styrene slabs, foamed polyurethane slabs, low- density wood-fibre insulating boards, cement- bonded slabs and strawboard. Heavier materials, which nevertheless make a contribution to thermal

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insulation, include lightweight aggregate concrete blocks and aerated concrete blocks. Apart from the use of such materials, air can also be incorpora- ted into a structure by forming a cavity, ie the conventional cavity wall used for house building. The insulation performance of such walls may now be further improved by injecting in situ foamed plastic cavity fills.

Whilst some of the above materials are best installed during initial construction (lightweight concrete blocks for example), others may be added at any time (mineral wool quilts in loft spaces, providing loft access is available). Fortunately, there are further techniques and materials which may be employed to effect thermal insulation improvements to existing walls and roofs. An additional wall lining fixed to timber battens creates an insulating cavity. Moreover, since half the heat flowing across such cavities is radiated, the insu- lation performance of the cavity may be almost doubled by including a low emissivity surface within the cavity. This is done, for example, by using aluminium foil backed plasterboard. An alternative, used by the author in his dining room, is to fix the foil under the battens and then use a predecorated rigid form of medium density hard- board (see figure 2).

Since the effectiveness of the above materials is dependent on thickness it cannot be compared with, say, the effectiveness of a cavity by referring to thermal conductivity figures. It is usual therefore to use thermal resistance as a parameter. This is the product of thermal resistivity, k-' , (where k is the coefficient of thermal conductivity) and thickness, and is expressed as m z K W- I.

Insulation performance of structures The ability of a wall, floor or roof to conduct heat, ie its thermal transmittance, is called its U value. It has the same unit as thermal conductivity, except that since a U value refers to a given con- struction, the thickness of whichis takeninto account, it has the unit W nir2 K-l. U values are computed according to the formula

U=(R, ,+ Re ,+R, , ,+k , - l+k , - '+) - l . . . . where Ris , R e , are the thermal resistances of internal and external surfaces respectively; R,,, is the sum of thermal resistances of any cavities; k1-l , k2--' . . . are the thermal resistances of each material used. Ri, and R e , are in fact practical expressions of coefficients of convective and radiative heat trans- fers from and to the air on either side of the wall or roof. Naturally because of wind effects and so on which facilitate convective transfer on the outside R e , is a lot less than Ri,.

Legislative standards At the time of writing (December 1974) building regulations may only prescribe maximum U values for floors, walls and roofs of dwellings and other residential buildings. The principal provisions are

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indicatcd in table 1. However, it is known that these will be improved before the end of January 1975 as the table also shows. The figure of 1.7 W m-2 K-l shown for roofs of industrial buildings is that required by the Thermal Insulation (Indirstrial Buildings) Act 1957. The Health and Safety at Work Ac t 1974 now confers powers for thermal insulation regulations to be applied to walls, floors and roofs of all new buildings, and table I indicates the levels expected by the insulation industry.

Current regulations for floors next to the ground in Scotland call for a U value no more than 1 .O W m-2 K-l, and in England and Wales 1.42. The latter

figure is expected to be decreased to that in Scotland. In fact the common floor constructions used in housing do not always comply with these regulations, and it therefore follows that as insulation standards are improved generally, to ignore the floor is to leave a proportionately larger gap unplugged.

Improvements can be made to concrete ground floors by using water impermeable insulating layers beneath and at the edge of the slab. Alternatively an insulating membrane can be installed above the slab. This can also be done with suspended timber floors.

Limitations of the U value The primary purpose of the U value was, and still is, to assist heating engineers to determine the size of heating installation required in a building. I t was first used as a legislative instrument for factory roofs in 1957 and then extended to residential buildings in Scotland in 1964 and to dwellings in England and Wales in 1966.

The limitation of the use of the U value even for its initial purpose will be immediately obvious to the physicist. It is computed from thermal conductivity data for materials of a given moisture content measured under steady state conditions. However, steady state heat flow conditions across walls and roofs seldom, if ever, occur in the UK. Even if the internal air temperature is constant, the external air temperature is not. The engineer

Table 1 Existing and expected thermal insulation requirements for roofs and walls ( U values in W m K-l). Column A dwellings, B other residen- tial buildings, C industrial buildings, D other residential buildings

"Other than hospitals, sanatoria, etc.

Roofs A B C D England and Wales 1.4 1.7 Scotland 1.1 1.1" 1.7 Expected 0.60 0.60 1.0 1.0 Walls England and Wales 1.7 Scotland 1.7 1.7" Expected 1.0 1.0 1.2 1.2

Physics in Technology July 1975

realizes this and makes allowances for intermittency. Moreover it is usual to assume an average external air temperature for the whole heating season in the UK of 5.5 "C. Thus the U value may be used for long term calculations, but even then only with care. It is only comparatively recently that the calculation has been refined to account for the moisture content of masonry walls.

In legislative use it is even more of a blunt instru- ment. The new Health arid Safety at Work Act enables regulations to be made in the interests not only of health, but also of energy conservation and condensation control. If provision is to be made for energy conservation on an equitable basis, taking into account the geometry of a building, a more sophisticated approach than just a U value will be required, even within a given building type. For example, an examination (Raynham 1971) has been made of the conductive heat loss charac- teristics of a range of dwellings constructed by a single large contractor, both for local authorities and for private sale.

The value of total conducted heat loss was derived for each of nine different types of dwellings and three levels of insulation: A, wall U value of 1.7, roof value of 1.4, with single glazing of U value of 5.7, B, wall U value of 1.0, roof U value of 0.5, and with single glazing: C, wall U value of 0.5, roof U value of 0.5 and with double glazing. A is the requirements of the Building Regulations for England and Wales. Floor U values, when relevant, were taken to be 1.0 W m '' K - ' . These assumed values were then divided by the occupied volume to give the rate of heat loss per l o o m 3 of occupied volume. The symbol I was assigned to the resulting value. A summary of these calculations for the three levels of insulation is given in table 2.

The first column of results in table 1 is broadly related to the insulation levels set for dwellings since 1966 by the Building Regulations for England and Wales. It demonstrates that the specification of uniform standards for thermal transmittance of each component of the boundary surfaces of the different types of dwelling does not result in uniform standards of insulation. (For example, if you want to be warm economically the regulations encourage you to live in a ground floor flat rather than a detached house.) This is no more than is to be expected since the I value takes into account those aspects of the geometry of the duelling which are significant heat loss considerations -. the ratio of external wall, roof and floor area, to the volume they enclose, and to each other. The results suggest that such considerations are necessary if a realistic performance specification is to be derived for ther- mal insulation of any building, particularly dwellings. The point is well illustrated for example by compar- ing the values for a local authority flat, a detached house and a bungalow built for private sale.

It is suggested that one way to exert sensible control on heat loss from dwellings would be to specify a maximum value for I . It can be seen from

Physics in Technology July 1975

Local Airthority Contract A B C Terraced 120 86 57 Semidetached 155 110 69 Top flat 167 110 87 Intermediate flat 61 47 23 Ground flat 105 91 67 Private sale Bungalow 250 180 120 Semidetached 160 122 75 Detached 178 128 80 Ground flat 137 125 85

Table 2 Summary of the calculations of heat loss (W K-' 100 m-3) for three levels of insulation A, B, C (see text)

table 1 that middle flats enjoy the lowest rate of heat loss at any given level of insulation. It will also be realized that to consider a block of flats as a whole rather than as the individual dwellings in it, will indicate that flats in general have the lowest heat loss characteristics. Indeed, if all dwellings were insulated to give a value for I not greater than, say 100 W K - l 100rr3 (approximating to the value for ground floor local authority flats derived from current legislation requirements), this would ensure a uniform standard for all dwellings irrespec- tive of their geometry. If desired, this figure could be subject to refinements such as relating it to the cost of fuel used, and to preserving a reasonable balance of heat loss between each of the separate components. It could also be applied to other building types such as schools, offices, shops and factories, taking into account their intermittent use and other relevant factors.

Economics and physics The provision of higher standards of thermal insulation is frequently a matter of economics; of spending money on materials to save money by reduced heat losses thereafter. The financial return for a given expenditure is clearly greater if the building is occupied and heated continuously (a house for example), than if it is only occupied for eight hours a day, five days a week (a school, shop or office for example). Factories may fall into either of these categories. Thus the question of intermittency has to be considered when determining the amounts of insulation to be used, always accep- ting that adequate comfort levels must be achieved throughout the period of occupancy.

Thermal mass and thermal diffusivity When buildings such as schools are intended for intermittent occupation there is a n advantage to be derived from using lightweight walls of low thermal capacity, because they will warm up quickly. In so doing they will achieve a higher surface temperature a t the early stages of the heating cycle than would a more massive wall of high

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thermal capacity (see figure 3), and higher surround- ing surface temperatures lead to improved comfort conditions. This effect gives an added reason for using lightweight concrete blocks for the inner leaf of a cavity wall rather than a brick inner skin.

Partition walls of high thermal capacity are, however, an advantage when off-peak heating equipment is used, since they can absorb this heat during the night charging period, and slowly release it during the subsequent day.

Finally it is worth considering that for fuel conservation and economic reasons i t may in future no longer be possible to heat whole buildings throughout a heating season. Some of the rooms may only be occupied either for a fairly short time, or not very often. The need hould then be for an insulating lining which would raise the air tempera- ture and the radiant temperature of the boundary surfaces as quickly as possible for any given heat input. This subject was investigated by the Building Research Station soon after the war ( I H V E Joirrnal 1974). They investigated the use of 13 mm wood-fibre insulating board on 13 mm battens, and a number of different appliances, both radiative and convective. The rate of temperature rise without the lining was 1.7 "C h-l and with it was 2.8 ' C h-I. Moreover it was found that after about an hour's heating of the lined room it was possible to reduce the rate of fuel consumption and still maintain a comfortable temperature. The fuel savings were calculated to be 54 "/, in the ventilated room heated by tubular heaters, and 32 O 0 in that with a gas fire.

Shorter heating seasons It is possible to use such large amounts of thermal insulation that the heat created within a building may almost suffice for maintaining comfort tempera- tures, although this may not be a very practical solution. However, it has been pointed out on a number of occasions, most recently by Billington 1974, that provision of adequate insulation makes heating unnecessary until the daily mean outdoor temperature falls below about 14 'C. In other words the heating season for most of the U K could be reduced by almost two months, to run from late October to mid April, instead of from early October to mid May.

Fuel costs The sensible level of expenditure on insulation is directly related to the cost of the heat lost, and to the length of time required to produce an appreciable return in terms of fuel saving. Since fuel costs differ so widely, it is not possible to generalize on amounts of insulation which should be used. Until recently it has been thought reasonable to assume a two or three year period for the insulation cost to be compensated by the accumulated fuel bill reductions. However since fuel costs are likely to go on rising in the foreseeable future it would seem prudent to provide more than this amount of insulation.

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Figure 3 Effect of thermal capacity on wall surface temperature. A is air temperature, B is for a light- weight wall, and C for a 280 mm cavity brick wall

(A house extension currently being built for the author will have 150" of mineral wool roof insulation.)

The significance of the cost of different fuels can be understood when it is realized that the cost of heat lost annually through a 275 mm thick 6 m long, 2.5 m high cavity brick wall is f 9.80 using coal, f 5.95 burning gas, f 11.80 using oil and f 6.60 using off peak electricity. The corres- ponding figures when the wall is lined with 12 mm hardboard on an aluminium lined cavity are E 6.00, f 3.60, f 7.20, f 5.70 respectively.

Other benefits of insulation Most readers will have seen the pattern in a lath and plaster ceiling, or something similar. This is caused by smoke and dust deposition on surfaces of structures having unequal thermal transmittance through adjacent parts. The dust is directed against the cooler surfaces having higher thermal trans- mittance. Adequate extra insulation eliminates this phenomenon, because it reduces the differential heat transmission between adjacent parts.

Temperature stress reduction Regardless of any insulation placed beneath roof decks, especially concrete ones, insulation is also required above the deck to limit thermal expansion during hot weather. This point is perhaps best made by quoting the results (table 3) of temperature measurements taken over a 24 h period in a 150 mm roof deck in London, covered with asphalt, but only partly provided with insulating board between the deck and asphalt. The maximum and minimum air temperatures in the shade were 25.5 and 14 "C respectively.

Physics in Technology July 1975

Figure 4 Double glazing is an effective way of preventing heat loss (F Squires, London)

These figures demonstrate how the insulation not only reduced temperature differentials through the day, but throughout the thickness of the slab. Clearly this reduction in temperature differentials implies a reduced expansion in the insulated slab.

Condensation To avoid condensation on the surface of walls, etc, it is necessary to have sufficient ventilation, and adequate insulation and heat input. It is also neces- sary to consider the question of whether conden- sation is likely to occur within a structure. Since most structures experience a falling dew point as well as temperature gradient from inside to out, it is possible (especially where insulating internal linings are provided) for the temperature on the cold side of the insulation to fall below the dew point, causing interstitial condensation. This can be controlled by providing water vapour barriers on the warm side of the insulation. This may take the form of polythene film or certain types of paint treatment, such as chlorinated rubber on the insulated lining. In some situations such as factory roofs and some timber flat roofs, ventilation is provided above the insulation to remove any water vapour that has penetrated that far.

Fire hazard When fire breaks out in a compartment the contents of the whole room are heated up, leading to accumu- lation of flammable gas. Eventually a point is reached when these gases, together with the materials evolving them, suddenly ignite and thus involve the whole room in fire.

Tests done many years ago at the Fire Research Station (Raynham 1973) investigated the factors

Physics in Technology July 1975

Uninsulated Max. Min. Range Upper concrete surface 37.8 15.5 22.3 Centre of slab 34 18 16 Insulated Upper concrete surface 29 21 8 Centre of slab 27.5 20 7.5

Table 3 Temperature ("C) variation in concrete roofs

leading to a short, and therefore dangerous, flashover time. Tests involving fires in domestic sized rooms with insulating board and hardboard wall linings, showed that flashover occurred at between 8.5 and 12min. Two further tests made with a noncombustible sprayed insulating lining gave flashover times of 8 and 4 . 5 min, thus suggesting that the thermal insulating characteristics of a lining are probably more significant from a fire development point of view than its combustibility. If this is true to any extent, significantly improved standards of thermal insulation, such as we are about to adopt in this country, are almost certain to accentuate the fire risk, especially in dwellings.

Conclusions It can be seen that clear benefits accrue from insula- ting existing buildings to a higher standard, and that when doing so the first priority should be reduction of adventitious ventilation. The insulation techniques and materials required are readily available and easy to apply.

In new construction the prospect of improved standards is welcome. The scale of the improvement is such as to call for new techniques and combin- ations of materials. The result may indeed be such as to have a profound effect on building design and construction generally.

The matter will need constant review as fuel costs change. Moreover if the maximum benefit of all these efforts is to be achieved a more sophisti- cated approach to heat loss calculation and control is required.

Finally the problems of interstitial condensation and increased fire hazard which may result from improvements in thermal insulation must not be ignored.

References Dick J B 1950 IHVEJournallS (17.9) 123-34 IHVE Journal 1946 14 (135) 103-12 IHVE Journal 1974 42 63 IHVE Guide Book 1970 Raynham E A 1971 Proc. First Nat. Symp. on Structural

Insulation (London : Structural Insulation Association) Raynham E A 1973 Fibre Building Board Development

Organization Technical Bulletin TB 10-73 Thomas D A and Dick J B 1953 IHVE Journal 21 (214)

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