When the "mass. is located on the floor (i.e., an insulated timber building. with a concrete slab) then the ground under the building contributes a stabilizing effect. Figure 44 shows how the temperature isotherms move as the mean temperature outside gets colder. The influence of temperature outside, via the ground and the floor slab, is considerably delayed a time lag in the order of one month. The ground under the house is being heated by the energy entering the house, either from solar energy or other energy sources such as space heaters. The ground acts as a thermal store or buffer where the house is thermally well coupled to the ground with, for example, a concrete slab floor. This is very useful in winter when there is plenty of solar radiation entering the house during the day. The heat is stored and then some is given back as the night-time temperatures drop and the building begins to cool down. In summer the floor acts in the same way by soaking up the heat energy that gets into the house. so helping to keep the inside temperatures down.

Figure 44 shows the influence of perimeter slab insulation. A considerable amount of the heat lost through a concrete slab floor flows out through the edges of the slab because it is in much closer contact with the cold outside air. Such measures are not thought to be economic in Sydney. In areas such as the Australian Capital Territory, Bathurst, Tam-worth and other such cold areas, perimeter floor insulation will help to reduce the loss of heat stored in the floor slab. For improved comfort conditions on tiled floors, however, it has been found effective in Sydney.

2. Thermal storage capacity

When developing a thermal storage system or simply comparing materials it is useful to look at the storage capacity of the proposed building materials which is referred to as the volumetric heat capacity (J/m².degC) or more commonly the specific heat v and the rate at which the material can take up and store heat (the Y-value). Some examples of common storage materials are given in table 8.

From these values can be determined the heat-storage capacity for a given temperature rise or if the heat gain is known, how much the material will rise in temperature. In an ideal situation. for any room into which sunlight enters. the surface area of the thermal mass should be as large as possible. even where not directly exposed to solar radiation. It must, however, be well Insulated from the outside so that it is mainly interacting with the interior.

In the case of a room that does not receive sunlight directly and is used and heated intermittently, it is advisable to insulate any heavy material surfaces from the room. In such places large amounts of heat would Just be soaked up by the thermal mass each time the heating is turned on and so the room might feel rather cool.

In the case of a building with a concrete slab floor on the ground, the ground underneath is being heated by the energy entering the slab from the room, either from solar energy or other energy sources such as space heaters. The ground adds to the thermal store of the floor slab where it is in contact with that slab. This is very useful in winter when there is plenty of solar radiation entering the house during the day. The heat Is stored and then some is given back as the night-time temperatures drop and the building begins to cool. In summer the floor acts in the same way by soaking up the heat energy that gets into the house, so helping to keep the inside temperatures down. The need for insulation under the slab and around the perimeter has been discussed under insulation, earlier in this chapter.

The thermal behaviour and energy consumption of 15 cottages was studied in a major research and demonstration project. Each of the houses were typical of a particular construction type in Australia and the plan layout of some was also similar to permit cross-comparisons of the results. Hourly data were collected from each dwelling over a three-year period with the houses unoccupied for the first eight months. Graphs of the daily minimum and maximum temperatures recorded in four of the houses are illustrated in figures 45 to 48. Daily minimum and maximum temperatures in the living areas are plotted against similar outdoor temperature data over approximately 18 months during the occupied period. The impact of the various quantities of thermal mass In each house is shown by the marked reduction in the temperature swing and the general suppression of the temperature extremes. In each of the figures the data above the X=Y line illustrate heat being returned to the interior of the space from the thermal mass at a time when the outdoor temperatures are low. The graphs show how each dwelling behaves under the same climatic influences. They are plots of the internal temperature against the outside temperature for each day using both the maximum and the minimum values. The data used include both summer and winter time measurements.

Cottage No. 359. for which the data in figure 48 are illustrated, is of standard timber frame construction with a timber floor over a crawl space. The only insulation is 4-inch thick glass fibre on the ceiling. The external cladding of this house is fibro-cement sheet, commonly used on many houses built In Eastern Australia in the 1940s and 1950s, whilst the interior linings are 3/8-inch plasterboard sheet. The living room windows are oriented to the west, south and north (north windows have a large fixed overhang). The results show that this house is unable to modify the

Table 8. Examples of commonly used thermal storage materials external climate to any extent and in summer it is often hotter inside than out.

The data illustrated in figure 46 are from cottage No. 357 of standard brick-veneer construction, with a timber floor and ceiling insulation. This construction system is essentially a timber-frame construction lined internally with plasterboard and with a single layer of brick built around the outside of the timber frame. The living room in this cottage has only equator- facing windows (north in Australia)

The performance of this house is not significantly better because it still lacks an adequate amount of thermal-storage material associated with the interior.

Figure 47 illustrates the results of data collected from a timber-framed cottage. No. 351, with a concrete floor slab in direct contact with the ground. As with the others the internal linings are plasterboard and the ceiling is insulated with 4-inch thick glass fibre blanket. The exterior wall lining is 10mm thick compressed wood-fibre planking laid over double-sided reflective foil insulation. All living area windows face towards the equator and the floor finish is hard vinyl tile with scatter rugs. The impact of the concrete floor slab is clear. The temperature swing is reduced and the interior conditions are buffered against the extremes of the outside. The "narrowness of the cluster of dots shows that the concrete floor slab is acting in conjunction with the ground as a substantial heat sink.

The behaviour of a traditional passive solar house is illustrated in figure 48. Cottage No. 341 is constructed of double-brick walls with the cavity filled with urea formaldehyde foam. The floor is a concrete slab on the ground similar to cottage No. 351 in figure 46. The roof is insulated similarly to the other houses and all living area windows face toward the equator (north). This house is the best in terms of overall thermal performance as can be seen from the small number of dots above and below the accepted comfort lines. The dots in this example are clustered in a "fatter" pattern than those In figure 45 which displays the impact of the internal walls which are not ground connected. The energy stored in them must come from and go back into the interior space, thus causing the interior temperature to rise more slowly as the day warms up and to fall more slowly as the night comes on. It should be noted that some of the energy that enters the floor slab in both this example and that in figure 46 will flow away to the ground and be lost.

The following is a review of some important characteristics of materials that can store energy. First, when energy is absorbed into a material that material will rise in temperature. The effect of letting the sun into a room that has no significant storage capacity will be to cause the room temperature to rise - if the room is well insulated then the rise will be quicker because little heat is lost.

If the room surfaces comprise a significant area of high storage capacity material in the path of the sun. then some of the sun's energy entering the room will be absorbed without having any immediate effect on the temperature of the room. The temperature of the thermal mass will rise and the heat energy will be held until the temperature of the room begins to fall later after the sun is no longer entering the room.

If some surfaces are warmed by the sun's energy then the mean radiant temperature (MRT) will be higher and so people will sense that the room is warmer even though the air temperature may not have changed. Since the surface temperatures of a room have a significant bearing on thermal comfort, this factor can be utilized by careful placement of heat storage materials (refer also to chapter IV on thermal comfort).

If the thermal mass in the room has been cooled during the night, then during the day it will act as a sink and soak up the excess heat that flows into the room. In summer this can be very beneficial as a means of keeping the room cool (i.e., the environmental temperature is kept lower).

Where it is desirable to level out large swings in temperature from day to night, this can be achieved by the storage of heat from the warmer part of the day until a later time when it can offset the colder temperature of the evening. The admittance values (Y-value) give an indication of the performance of elements in this respect. They can also be used in calculations to predict internal temperatures using a technique known as the admittance method.

From all this some recommendations can be made about the desirability of thermal mass and its preferred qualities according to the climate of the location. In temperate climates and desert climates which are usually arid the desirable wall surfaces will have a low U-value, a low T-value and a high Y- value. In areas where it Is necessary to heat with auxiliary energy then it may be desirable to minimize all three values to maximize the heating effect when its needed.

3. Surface colour and texture

The floor is most commonly the main thermal storage medium in direct-gain systems because so often it is the most economical place to locate heavy masonry materials. Unless there is an adequate amount of thermal storage in the walls of the space (perhaps at least half the walls). the floor should be of a material with a high thermal admittance and have the least amount of thermally-resistive covering (i.e.. carpets or cork tile). The surface should have a low reflectance to absorb as much of the incoming energy as possible. Where there is a significant thermal storage component in the walls. the floor may be more reflective the better to distribute the sun's energy to the other heat storage surfaces.

4. Coverings and sheltering (interior finishes and furniture)

The type of floor covering or finish used on a ground-connected floor (i.e.. concrete slab. masonry or stone paving) will have an important bearing on the way such a floor interacts thermally with the room. Materials such as carpet. cork or foam-backed vinyl materials act as an insulator (see chapter IV) and effectively reduce the internal admittance of the floor surface. During winter an undesirable temperature rise can occur in spaces without alternative thermal mass that are heated by the sun entering directly into the interior (known as a direct-gain system and discussed elsewhere) because such coverings reduce the admittance of heat into the floor slab. Likewise, in summer a floor with an insulated covering is also less effective in acting as a heat sink to soak up the daily heat gains for disposal at night.

5, Time lag and decrement

As the outdoor temperature cycles up and down through each 24-hour period so the energy flow through the exterior elements of a building will also cycle (see figure 49). The time taken for heat to flow through each building element will vary according to the storage properties and the resistance and so some of the heat energy will still be passing Into the material when the external temperature has dropped below the inside temperature. This repetitive cycling produces what is called a periodic heat flow defined by the time lag and decrement factor.

Time lag is the time delay between when a temperature rise occurs on one side of a material and the onset of the temperature rise on the other side. The ratio of the two temperatures achieved over a cycle is known as the decrement factor. The two terms are illustrated in figure 49. Heavy materials such as masonry and concrete with a high volumetric specific heat capacity will take time to conduct the heat energy from one particle to another because each in turn can absorb considerable energy for a given rise in temperature. The term that describes this function is thermal diffusivity or temperature conductivity and is a function of the conductivity divided by the volumetric specific heat capacity. These are interrelated in figures 50 and 51.

In practical terms each element of the external building fabric such as the roof. walls and doors will have a different time lag and decrement. Heavy masonry walls of say 230 mm brickwork will have a time lag of about 8 hours whilst the lighter roof will have a time lag of only 1 or 2 hours. Each of these elements work together to generate a building time-lag (usually about two hours or so for a typical brick-veneer construction). Very heavy constructions such as earth-sheltered buildings well below the surface may have a time-lag of months. The benefit of heavy materials with a long time-lag, is that they can be used to soak up solar energy during the day and release it back at night when it is cool.