A. Climates

1. Hot-arid climates

In such climates where the daily (diurnal) temperature range often exceeds 20 degC the most important principles are to have a large thermal mass (masonry or concrete walls and concrete floors), a well insulated external skin and effective summer shading of windows and walls where possible. As winter daytime temperatures are relatively low. good solar access to the northerly-facing windows is most important. East and west windows should be avoided unless full shading and thermal insulation can be provided. The insulation is necessary for both the heat of the summer day and the cold of the winter night. The design should include breezeway-style living areas for summer evenings. Evaporative cooling is very effective and appropriate in this climate.

2. Cool-temperate and temperate climates

The majority of the country's population lives in either the temperate or the cool-temperate zones. In design terms the winter requirements vary little because of the relatively sunny winters experienced in Australia compared with many other countries. The major differences are related to insulation levels and the like.

In summer there is a greater need for thermal mass in the warmer temperate areas to suppress daytime temperature peaks. In the cool-temperate areas such as Tasmania and the New South Wales tablelands the outdoor temperature in winter rarely rises above the comfort range and so thermal mass is less important for summer as natural ventilation and good shading are quite adequate. In winter the thermal mass is important where solar energy is desired for heating. Where intermittent heating is used in non-sunlit rooms, thermal storage materials will tend to increase the energy needed for heating.

Where east and west windows are required for views etc., effective shading must be provided for summer. With the exception of very hot periods there should be no special requirements for ventilation other than those of fresh air and air movement to avoid stuffiness.

3. Hot-humid climates

The problem of high humidity levels is not easily solved with building design. Although temperatures may not be as high as in the arid zone climates the combination of high humidity and moderately high air temperatures causes discomfort. This is especially so at night when lower temperatures are needed for a sound sleep. The aim is, first, to ensure that indoor temperatures do not exceed outdoor-temperatures. This is achieved by extensive shading (especially on the eastern and western sides of the building), insulation of roofs (reflective foil is appropriate) and most of all, unimpeded natural ventilation. Shading of east and west faes by heavy planting can be most effective in providing both the sun protection and a cool place to sit during the day. The use of ceiling fans to induce air movement is strongly recommended. Elevating the building above the ground has been found effective in low-density areas, but it makes shading from planting more difficult. The design of openings to facilitate airflow is important. The building structure should be of lightweight construction to aid cooling whenever the temperature drops. A concrete slab on the ground is effective in providing a limited heat sink but this must be weighed against the greater need for relief by air movement.

B. The sun's movement

To obtain the best use of the sun's energy, the designer must be aware of the pattern of the sun's movement as well as the specific considerations for house and site design.

The pattern of the sun's movement over Sydney (latitude 34 South) may be taken as an example. In mid-summer, approximately 22 December (the summer solstice), the sun rises at about 5 a.m. slightly to the south of east. From there it climbs sharply to be nearly overhead at noon, and sets at about 7 p.m. to the south of west.

In mid-winter, the sun rises to the north of east and travels low through the northern sky to set north of west.

Figure 22. Elevations of the sun at different periods of the year at Sydney.

Figure 23. The azimuth of the sun.

At the equinox (21 March and 23 September) the sun's path is between the winter and summer paths. Therefore the sun rises and sets due east and due west. At noon the altitude of the sun (its angular height above the horizon) is 90 degrees minus the latitude of the observer.

C. Orientation for solar access

Because the sun's path across the northern sky (in the southern hemisphere) is low in winter, and high in summer, a house can be designed to allow the sun to enter and warm the house and warm outdoor living areas in winter and prevent the sun from striking walls and roof or from penetrating the house in summer.

Windows facing in a northerly direction receive useful sunshine for most of the day in winter and the desired period for access to the sun's rays in most of Australia is 9 a.m. to 3 p.m. In summer, unwanted sunshine can be very easily blocked by an overhang, pergola or other horizontal shading device.

Although the winter sun in the mornings and the evenings coming through windows facing east and west is pleasant visually, it provides very little useful heating. In summer those east-and west- facing windows receive a high proportion of the sun's energy and because the sun is low in the sky it is difficult to screen it out with conventional shading devices.

South-facing windows receive no direct radiation in winter and very little In summer. The low evening sun in summer may cause problems on an open site.

For the best passive solar design, the windows to living rooms and bedrooms should face in a northerly direction. Some flexibility of orientation is acceptable, however it has been found that the optimum orientation is within 20 degrees either side of north. A building oriented outside this range loses the benefits of winter sun. This is clearly demonstrated in figure 26 which illustrates graphs of mean daily solar radiation on vertical surfaces of various orientations in Sydney. Notice how the solar radiation received on a vertical surface facing north- east or northwest is almost the same year round. The importance of orientation is self-evident.

Figure 24. Optimum window orientation.

If there is a preference, then about 12 degrees east of north (approximately magnetic north in New South Wales) is best to let some sun in for an early warm-up in winter. Because north orientation is essential to passive solar design, it is important to choose a house block that allows north orientation to at least the major daytime living spaces. Trees or buildings could block access to sunlight, and this needs to be checked when siting dwellings.

The orientation of a building is determined usually by the position of the windows and the proportion of the plan. Excluding the internal spaces at this point the objective is to locate most glass on the north fae and to design the building so that its north and south fae are larger than the east and west faes.

Figure 26 also illustrates the relationship between Sydney's heating load profile and solar radiation incident on surfaces of different orientation. A diagram such as this is admittedly simplistic but it shows the effect of window orientation very clearly. The north orientation is the only one that is able to combat the heating load without creating enormous penalties in the summer.

It is often suggested that the optimum building plan proportions for a temperate climate is about 1: 1.5 with the longer fae to the north. This may be correct on the basis of simply the heat gains and losses but it will not necessarily be applicable to the requirements of a particular design and its site. The area of north-facing windows is perhaps more important and in Australia's temperate and cooler areas there is still a need for good cross-ventilation in the summer months. This requirement may well dictate a longer, thinner plan than the aforesaid optimum.

The way in which living units are planned effects the overall thermal efficiency. The greater the exposed external surface the greater the potential heat loss. Figure 25 shows that medium- density building is subject to smaller heat losses per living unit than detached cottages and likewise high-density building is subject to even smaller heat losses. In residential buildings this can be advantageous, provided the individual units have a reasonable access to sun from the north.

In high-density commercial buildings this compactness is often a disadvantage because it results in a year-round cooling load due to internally generated heat, and so a need for air conditioning. Figure 25 illustrates how density of planning relates to conductive heat loss. The heat loss rate will be proportional to the external surface area and its resistance (A_{ex ×} U). Solar access usually becomes more difficult as density increases.

Likewise, the solar gains in winter can be enhanced by the orientation and grouping of the various units. In developing these arrangements, it needs to be remembered that the sun is low in winter and high in summer. A roof's exposure is therefore high in summer. Light-coloured roofs will reflect much of that radiation.

D. What is solar access?

Solar access can be described as allowing the sun to penetrate a building or be utilized by a solar collector on the surface of that building between 9 a.m. and 3 p.m. in midwinter. There are varying degrees of solar access. There is whole-site access where the area of yard to the north of the building, as well as the north wall and rooftop are protected from shading by other buildings and vegetation in midwinter. North-wall access refers to the protection from shadows in midwinter of only the north facade, which includes the north roof and north wall.

Although whole-site access is desirable for outdoor garden use, it can be very costly in terms of the use of land and may not affect household energy use. Energy-efficiency encompasses more than Just energy savings in houses and so the decreased density that results from whole-site access cannot be justified.

There is a third level of solar access, rooftop access, which aims to protect rooftop solar collector systems from shading at certain times. Although this level of solar access allows maximum density to be achieved, it forecloses too many options for future development. The definition of solar access depends on the definition of the solar collector (whether passive or active).

Figure 25. Heat losses from different residential building types.

Figure 26. The effect of window orientation and winter heating load.

Figure 27. Solar access in Sydney.

North-wall access is the level of access designed for in this handbook.

The protection of solar access to dwellings is crucial to the performance of passive solar architecture. The period between 9 a. m. and 3 p.m. in mid-winter, as shown in figure 27, has been generally accepted as a measure of solar access.

For mid-winter in Sydney, the azimuth of the sun is approximately 45 degrees at 9 a.m. and at 3 p.m. At noon, the altitude of the sun is approximately 30 degrees. The protection of solar access is discussed in a later chapter under detailed design guidelines.

E. Solar energy collection

1. Sun and solar radiation

The energy (or power) from the sun is received by radiation. The sun is 1.4 million km in diameter and the temperature of its core is 14,000,000°K. The outer layer is called the photosphere and its temperature is only 6000°K. It can be assumed from the point of view of a person on Earth that the suns rays are parallel due to the vast distance between Earth and the sun which varies a little throughout the year from 150 million km to 155 million km. Even though it is 53 times larger than the Earth it appears as a spot. due to this enormous distance. The electromagnetic rays of energy emitted by the sun are Earth's sole source of energy except for a small amount of energy emanating from the radioactive decay of the Earth's minerals. This constant emission is called insolation (not to be confused with insulation, which means isolate). The value of the sun's energy at the outer surface of the Earth's atmosphere is known as the solar constant and is 1.353 kW/m².

Figure 28. The NASA (1971) standard spectral irradiance at a point approximately midway between the sun and Earth.

Solar radiation or insolation is made up of various wavelengths mostly in the range of 0.22.5 microns with a peak at 0.5 microns. Figure 28 is based on data collected by NASA outside Earth's atmosphere. As the sun's rays pass through the atmosphere it is degraded and so at Earth's surface the shape of the graph is a little different. Some energy is absorbed by the water and ozone in the atmosphere. The ozone layer in Earth's atmosphere is the major element that absorbs the ultraviolet wavelengths and the use of CFCs is being blamed for its degradation.

Much of the sun's energy that reaches Earth is either absorbed by the atmosphere or is reflected back out into space. Figure 29 shows how that energy is dissipated. About 50 per cent of the sun's energy at the outer surface of the atmosphere reaches Earth's surface and is absorbed into the ground which must be dissipated otherwise the Earth would overheat. Earth's temperature is held in a very delicate balance as recent discussions about the effects of carbon dioxide build-up and the ozone layer depletion have shown. The energy absorbed by the Earth is dissipated through three mechanisms and approximately 20 per cent by long-wave length re-radiation, 20 per cent by evaporation and 10 per cent by convection - total 50 per cent.

Figure 29. The passage of radiation through the atmosphere.

Figure 30. Approximate division of solar radiation at Earth's surface.

Table 6. Diffuse radiation compared with direct radiation based on CSIRO data for "clear sky" conditions in Sydney

When the sun's rays reach Earth's surface if is typically 4.6 per cent in the ultraviolet wavelengths, 46 per cent in the visible wavelengths and 49 per cent in the infrared wavelengths, depending on weather conditions.

The radiation reaching Earth is either of direct or diffuse in form, the latter reaching Earth's surface after being reflected off the particles of the atmosphere. The sum of the radiation reaching Earth's surface is referred to as the total or global radiation.

There are only a few places in Australia where solar radiation data are recorded and so CSIRO has developed a computer model to calculate the values for a large number of places based on a more commonly measured value - the precipitable water in the atmosphere. This is one of the more common Hems of weather information that is collected across the country. Tables of solar radiation give values for what is called a "clear sky day. and so do not take into account any dust or pollution that may be present in the atmosphere. In Sydney, the measured values tend to be about 70 per cent of the "clear sky day" values due to pollution.

Figure 31. The 'cosine law' states that the intensity of energy coming In on a fitted surface (area 2 which represents Earth's surface) can be calculated by multiplying the intensity of the incoming energy (the sun's parallel rays) by the cosine of the angle of Incidence, I.e., intensity on area 2 = cos× Intensity on area 1.

On cloudy days (especially when the clouds are at a high level), the diffuse component can be almost the same or greater than the solar constant due to refractive focusing. Extremes of 110 per cent of the solar constant have been measured.

Figure 32. How the sun's intensity is reduced when it has to travel through more of Earth's atmosphere.

Figure 33 shows the relationship between the altitude angle and the solar radiation levels. The larger the altitude angle, the greater the radiation. Also the higher the reference point above sea level, the higher the radiation intensity. This is because the sun has less atmosphere to travel through. As the angle of incidence on a surface gets smaller (i.e., the solar altitude angle) the effective atmosphere thickness gets greater and so more energy is absorbed and reflected away from Earth's surface.

Figure 33. Solar intensity as a function of altitude and altitude angle.

Figure 34. Typical graph of solar radiation over a full day.

The graph shown in figure 34 is the result of combining both the effect of the atmosphere and angle of incidence throughout a single day (remembering that the sun's output is unchanging throughout time).

So far the sun has been considered as a source of energy with a temperature of about 6000K, as has the path of that energy to and from Earth's surface.

Radiant energy is absorbed into the surface of objects it strikes and it then must be rejected later. The second law of thermodynamics states that energy must flow from a hotter body to a cooler body not vice versa. For the purpose of explaining how heat radiates from one object to another and the rate at which it does that, use is made of the term black body. A black body is a theoretical object with properties such that:

(a) It absorbs all radiation incident on its surface. i.e., it does not reflect or transmit any energy;

(b) It is able to re-radiate all that energy away again and the rate at which it does so is a function of its absolute temperature (°K).

Stefan's law

F = emittance rate
s = Stefan Boltzman constant
T = absolute temperature

An example:

The sun radiates energy from its surface (photosphere) which is at approximately 6000°K and Earth has a mean temperature of 283°K (10C) and its atmosphere has a mean temperature of about 250°K (-23°C). As a result the sun's energy warms the Earth.

Another characteristic of this so called black body is that the wavelength of the radiation being emitted varies inversely with its absolute temperature.

Wien's law

d(max) = wavelength
T = absolute temperature

In figure 35 the spectral distribution of energy from three bodies of different temperature is shown. The sun at 6000K and two others at 1000K and 400K (400K = 127C which is the temperature of a solar collector). The sun's radiation peaks at about 0.5 microns and cuts out at about 2.5 microns. Terrestrial radiation, i.e., from objects at earthly temperatures, peaks at about 10 microns with a range of about 4 to 100 microns.

Radiant energy with wavelengths of less than 2.5 microns is known as shortwave radiation while radiant energy with greater than 2.5 microns is known as longwave radiation. The implications of this are discussed further in the section on transparent elements.

Figure 35. Wavelength for different bodies at different temperatures.

All materials have certain qualities that define their thermal characteristics. as shown below but in this chapter consideration need only be given to the first three on the list.

Absorptivity
Emissivity
Reflecffvity
Mass/density
Conductance
Transmittance
Acceptance
Specific heat

Absorptivity is designated by the Greek letter a and is defined as the ratio of thermal radiation absorbed per unit area of a surface to the thermal radiation absorbed per unit area by a black body or perfect absorber.

Emissivity is designated by the Greek letter S and is defined as the ratio of thermal radiation emitted per unit area of a surface to the thermal radiation emitted per unit area by a black body at the same temperature. As a black body can emit all energy that it has absorbed, which is all the energy incident on its surface, then the emissivity is in effect also the ratio of the energy incident to the energy emitted. In other words it is a comparative value of its ability to give off or let go any heat energy stored in H.

An example of an application of a low emissivity surfaced material is the use of aluminium foil to keep food hot. It has an emissivity of 0.05, i.e., it emits only 5 per cent of what would be emitted by a perfect emitter (a black body). Black paint on the other hand has an emissivity of 0.9 and so it is used on radiators to help release heat. Motor car radiators are painted black for this reason. If they were chrome plated, they would not be as effective as a heat dissipator and there is a good chance that the engine would overheat.

Reflectivity is easier to define by deduction as follows. The first law of thermodynamics states that energy must always be accountable or energy cannot be destroyed, it can only be changed in state.

Figure 36. Radiant energy striking an object.

That being the case, then reflectance is the energy that is neither absorbed or transmitted through the surface in question. Aluminium foil is a poor emitter (S=0.05) and a good reflector whilst black paint is a good emitter (S=0.9) and a good absorber. White paint is a good reflector and a good emitter. The thermos flask is an excellent example of how use is made of all these factors. The escape of energy stored inside a thermos flask filled with hot water is restricted by the low emissivity of the mirrored glass linings and the vacuum between them that will only permit the flow of energy by radiation; not conduction or convection. The low emissivity surfaces limits the amount of energy that can be radiated to the outside, thus dramatically slowing down the rate of cooling of the contents.

Figure 37. Graph of the absorptivity versus emissivity of various building materials.

The properties of absorptivity, emissivity and reflection relate to the surface qualities of a material. This is demonstrated by the fact that the emissivity of painted surfaces is unaffected by colour whilst the absorptivity is colour-dependent. Most organic surfaces have a relatively high emissivity. Note how some materials such as galvanized iron are rather deceptive given their shiny appearance.

2. Transparent elements

When the sun's rays strike a transparent element such as a window, some are transmitted. whilst some are absorbed into the glass and the remainder are reflected away. The ratio of transmitted, absorbed and reflected rays will vary according to the characteristics of the particular glazing being used. In passive solar design the transparent elements (windows, sunspaces etc.) are usually the main source of heating for the building and so their design is most important. The graphs in figure 38 illustrate the relative intensity of the energy emitted by the sun and a body at terrestrial temperatures at various wavelengths in comparison with the transmission characteristics of standard window glass.

Figure 38. Graphs of sun and terrestrial bodies compared to transmission through glass.

3. The glasshouse principle

The wavelength of solar radiation received at the surface of the Earth is in the range of 0.2-2.5 microns, whilst terrestrial radiation (from various earthly temperature ranges) peaks at about 10 microns with a range of about 4-100 microns depending on the particular temperature.

Standard window glass will generally transmit radiant energy in the 0.3-3 micron wavelengths. At greater wavelengths it is more or less opaque. As a result, the energy reradiated from the interior and objects inside the building will not be transmitted back through the glass. This is known as the glasshouse principle. Energy can, however, still be lost by conduction as discussed later in the section on opaque elements.

4. Thermal behaviour of transparent materials (glass and plastics)

In the case of a transparent element in the external fabric of a building there can be two independent and separate heat exchange processes operating at the same time as is shown in figure 39. The temperature difference between the inside and the outside will cause a flow of heat energy from the hotter side to the colder side by conduction (this Is the same process of heat exchange that occurs in an opaque element and therefore is discussed fully in the section on opaque elements). At the same time, if the sun's rays are striking the glass, there will be a flow of solar energy (radiant energy) in through the window.

It is possible, therefore, to have a situation where heat is flowing out through a window by conduction (outside colder than inside) whilst at the same time energy is flowing in through the glass by radiation. In most cases of north-facing windows, this inflow from the sun will be greater than the outflow by conduction. However in some cases the sun's energy that does penetrate over a 24-hour period will not be greater than the heat lost over the same period (in winter, windows to the south and most skylights fall into this category). If the radiant energy that passes through the glass is not absorbed then it may well be reflected back out through the glass. Energy that is absorbed and then re-radiated will however, not pass back out by radiation. Glass is one of the few materials that transmits solar radiation but not terrestrial radiation of longer wavelengths. Most of the clear plastics such as acrylic, mylar and the like, tend to be less selective.

Figure 39. Diagram of heat flow through 3mm glass.

The standard conditions of 3mm plain glass are given as:

Reflectance (r) = 0.08
Absorptance (a) = 0.05
Transmittance (t) = 0.87

For standard 3mm clear sheet glass

Shading coefficient = 88/88 = 1.00

From figure 39 above it can be seen that the total heat gain by radiant energy Is 88 per cent.

Figure 40. Graph showing temperature rise of different coloured materials exposed to direct sunlight.

The quantity of energy transmitted through glass depends on its composition. Australian window glass has a transmission of approximately 85 per cent. If it was low-iron (also known as "water white") the transmission could be as high as 98 per cent, but such glass is extremely expensive. For calculation purposes it is assumed that the transmission of standard 3mm glass is 88 per cent.

The shading coefficient is the ratio of the solar heat gain through the selected window to the solar heat gain for standard 3mm glass under exactly the same conditions.

5. Opaque elements

The process of heat exchange through an opaque element is by conduction due to the temperature difference between one side and the other, i.e., if the outdoor temperature is lower than the indoor temperature then there will be a flow of heat from inside to outside proportional to the difference in temperature as explained in the earlier section on fundamentals. The solar energy striking the surface of a roof or external wall element is absorbed causing the outer surface to be warmed above the temperature of the outside air, resulting in a change to the effective temperature difference. The resultant effect of the air temperature and the incident solar radiation is known as the sol- air temperature.

Figure 37 shows the relative absorptivity, emissivity and reflectance of different surfaces. If a material has a high alpha (a) and a low sigma (S), i.e., galvanized iron, then it will heat up much more than a material with low alpha and high sigma such as white paint. As a rough guide the darker materials will absorb more than light materials. However, some are deceptive such as aged galvanized iron.

Figure 40 shows the relative daily mean temperature rise of various materials exposed to the sun under the same conditions of wind and ambient temperature. The rate of temperature rise is dependent on the quantity or intensity of solar radiation falling on the surface and the absorptivity (a). Likewise, the rate at which the surface cools off or loses heat will also be dependent to some extent on the air movement over the surface (convective losses) and the surface emissivity (S), i.e., radiative losses (long wavelengths).

So, there is an outer surface which is heated by the sun's energy and "cooled" by air movement etc.. and heated or cooled by the ambient air. All of this is important when considering heat flow through a building element such as walls or roof exposed to the sun.

The simple heat flow Q through wall, say, of area A m due to conduction is:

Q = A × U × Dt

When a surface is exposed to both ambient temperature and the warming effect of the sun's rays, then there is a situation where the effective value of At is going to be different from the simple value of To - Ti

There are a number of variables that will influence the situation; solar irradiance (1), absorptivity (a), emissivity (S) and ambient temperature (To), the combined effect of which is called the sol-air temperature.

Sol-air temperature is an imaginary temperature of a layer of air adjacent to the surface being considered. It is the equivalent of the effect of solar radiation and air temperature combined. It will vary throughout the day and year just as the solar radiation on a surface does.

Figure 41. Sol-air temperature.

The formula for sol-air temperature is given by the American Society of Heating Refrigeration and Air-conditioning Engineers (ASHRAE).

T_s = sol-air temperature C
T_a = ambient temperature (outside) °C
I = Radiation incident on the surface W/m².°C
a = absorptivity
f_o = surface conductance reciprocal of 1/f_o
S = emissivity
R = difference between longwave radiation incident and radiation emitted by black body at temperature T_a

Part of this equation can be standardized. It has been found that for horizontal surfaces (roofs) an optimistic view can be taken as follows:

S = 1; DR = 63 W/m² and f_o = 20

The answer then becomes -3

For vertical surfaces it has been found that AR = 0 and so that part is cancelled out.

Therefore for horizontal surfaces assume -3

(related sky temperature) to and for vertical surfaces assume 0

For surfaces in between interpolation can be made between 0 and -3.

The term given to this value is T_sky.

This generally makes the calculations complex and very long. If the building Is reasonably insulated, it is reasonable to assume a pessimistic view for winter and ignore the solar temperature effect. Especially if the walls and roof are well insulated and the absorptivity () is low, because the extra heat flow into the building due to solar gains will be quite small.

Consider an example of a white roof a = 0.2 and f_a = 20

assume I = 540 W/m² at noon on June 22

T_a might be about 1 5C. Therefore T_s = 17.4C.

Note: This is a maximum effect - at other times the effect is much less. It is possible to estimate the daily overall effect. It may be found that the effect overall will be very marginal.

At best the sol-air temperature must work to help in gaining comfort in winter. However, the effect in summer is not to be neglected. In winter its effect improves the situation slightly and so helps. In summer it is the other way round. This excess only aggravates the already higher than desirable ambient temperatures, especially on the roof. The graphs in figure 42 show that the radiation is higher on a horizontal surface in summer compared with winter.

Instantaneous or hourly integrated values of solar radiation on the particular surface are needed to calculate instantaneous sol- air temperatures. At present the only data available for this purpose are the CSIRO "clear sky" solar tables. They do not take into account the local conditions of pollution and cloud cover and so need to be modified. A value of 0.7 - 0.8 is a suitable modifying factor for much of New South Wales. A more useful value in heating load calculations is the "daily mean sol-air temperature', that can be used when calculating the total daily or monthly structural heat loss or heat gain. In solar architecture design evaluation it is easier to handle.

In many calculations it will be necessary to derive a value for the mean sol-air temperature. The values given at the back of this publication for radiation incident on various surfaces are daily totals. These will have to be converted to mean hourly values. The equation below allows calculation of the mean daily sol-air temperature of a surface using those daily totals.

Figure 42. Graphs of sol-air temperatures on various surfaces

T_s = T_o+ ((G × 1 /f_o × 10³)/(24 × 3.6)) - T_sky

Note: The solar radiation value (G) is expressed in MJ/m² .day

Surface absorptance (a)

The absorptance of a surface may be apparent from visual inspection. The following are some values that may be appropriate for use in sol-air temperature calculations:

White paint	0.2	White washed roof	0.2
Polished aluminium	0.05 - 0.15	New aluminium paint	0.2
Cream paint	0.4	Old aluminium paint	0.5
Aluminium sheet	0.45	Red brick	0.55
Light concrete	0.6	New galvanized sheet	0.65
Aged galvanized sheet	0.8	Earth or sand	0.8
Dark concrete	0.9	Black paint	0.95

Table 7. Outside surface resistances (1/fo) (m2.degC/W for "sheltered" "normal" and "severe" exposures.

Building	Emissivity	Surface resistance for stated exposure
		Sheltered	Normal	Severe
Walls	High	0.08	0.06	0.03
	Low	0.11	0.67	003
Roof	High	0.07	0.05	0.02
	Low	0.09	0.053	0.02

F. Energy storage (heat)

Thermal mass incorporated in the construction of a building interior can improve thermal comfort and reduce energy consumption. Thermal comfort is improved by a reduction in the daily temperature swings and the maintenance of temperatures closer to the comfort zone. Energy consumption for heating and cooling can be reduced or even eliminated if correctly designed. In spaces that are intermittently heated such as is often the case in many temperate climates. thermal-storage materials may have no significant effect on energy consumption. Thermal mass is especially important in hot-arid climates with high diurnal temperature swings. where the heat of the day can be stored for release at night to cool breezes on summer nights that will drain it away. or to the interior space in winter to maintain comfort.

1. Thermal mass and slabs on ground

The use of a suitable thermal mass in all buildings is vital. This is especially so in hot-arid climates where, with high diurnal temperature swings. excess heat can be stored when not needed for later use (winter) or disposal (summer). In temperate areas. the mass of the structure works in the same way to produce greatly reduced temperature fluctuations.

The ideal arrangement for the thermal mass of a structure is inside a protective envelope of insulation. This infers a "reverse brick-veneer" wall, where a light insulated shell is on the outside instead of the inside. In such a configuration the mass interacts more with the internal environment than the external environment.

Figure 43. A schematic diagram of soil temperature profiles under a lightweight building with concrete slab on ground floor

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. Schematic profiles of a concrete floor slab with and without perimeter insulation

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.

Material	Density	Volumetric heat capacity
	kg/m³	(Ib/ft³)	J/m³. degC	(Btu/ft³.°F)
Water	1000	(62)	4186	(62)
Concrete	2100	(131)	1754	(26.2)
Brick	1700	(106)	1360	(20.1)
Store: marble	2500	(156)	2250	(32.8)
Materials not suitable for thermal storage:
Plasterboard	950	(59)	798	(11.8)
Timber	610	(38)	866	(12.9)
Glass fibre matt	25	(1-5)	25	(0.37)

Figure 45. Cottage No. 359 of standard timber construction with timber floor and insulation only on the ceiling. The living room windows are oriented to the west. south and north (north windows having large fixed overhand).

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.

Figure 46. Cottage 357 of standard brick-veneer construction with a timber floor and ceiling Insulation only. but with north-facing living-room windows

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).

Figure 47. Cottage No. 351 of timber-frame construction with a concrete floor slab with Insulated walls and roof and all living area windows facing north.

Figure 48. Cottage No. 341 fully brick construction with a concrete floor slab. Both the roof and the wall cavities are Insulated and all living area windows face north.

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.

Figure 49. Periodic temperature swing showing time lag and decrement

Figure. 50. Time-lag versus diffusivity and U-value.

Figure 51. Decrement factor versus diffusivity and U-value

G. Heat retention

1. External fabric resistance

The U-value of single-sheet window glass has been found to be 5.98 W/m². degC. Whilst windows are usually uncovered during the day to let in daylight (and direct sunlight in winter) overall heat losses can be reduced at night by covering with curtains or blinds. Thermal comfort can also be improved by covering windows at night, because rates as high as 5.98 W/m.degC will cause a lowering of the overall mean radiant temperature (MRT). It is important that curtains or blinds be well fitted to windows to minimize heat losses at night. Ideally, curtains should wrap around to the wall at the sides, be fitted with pelmets and finish close to the floor or on a projecting ledge as shown in figure 52.

Figure 52. Restrict air circulation across windows to reduce conduction of heat

If sealed. Insulated shutters were fitted to a window then the reduced U-value can be easily calculated. However, the effect of less substantial elements Is more difficult assess. For that reason some basic values are given. First, it is assumed that one of two conditions prevail; either the space between the curtains or blinds and the glass is closed off at the perimeter as described above or it is open for free circulation to the room. The difference is quite marked. When the edges of the curtains are closed and the material is suitably lined so as to restrict air circulation, the "trapped" pocket of air is quite effective as insulation. These values, known as U-value modifiers (M) are as follows:

0.33	Heavy drapes with restricted air circulation
0.60	Light drapes with restricted air circulation
0.75	Heavy drapes with free air circulation
0.85	Light drapes with free air circulation

To determine the modified U-value for thermal evaluation calculations over at least a 24hour cycle the following equation can be used.

U_m= U/24 × ((M × H_d) + 24 - H_d)

where

U_m = overall modified U-value
M = modifier from above table
H_d = hours per day curtains are covering windows (i.e., 1800 hrs to 0700 hrs = 13 hours)

2. Thermal insulation materials and their application

Thermal insulation materials generally available for building purposes can be classified into two generic groups - bulk materials and reflective foil laminates (RFL). The first of these relies on the resistance of air trapped in pockets between the fibres of the blanket type materials (mineral fibre materials) or the cells formed in the foamed structure of board or slab type materials (usually made from plastics such as polystyrene and polyurethane foams). The second reflects radiant energy away from the object or surface being protected. The basic principles of heat transfer by radiation and conduction have been covered earlier, along with the principles of operation of such materials.

Thermal insulation in the outer fabric of a building is a vital component of an energy-efficient design strategy. The key to successful energy-efficient design is the control of heat flow through the external fabric. All the solar energy gained could be easily lost from an inadequately insulated building before it is able to be of benefit.

(a) Roof insulation

The major heat path in both cold and hot weather is through the roof. Generally the roof is the largest single exposed surface and is usually built of relatively light-weight materials. The basic insulation of roofs should be resistive material to minimize heat loss in cold weather with the addition of a layer of reflective insulation under the roof cladding where summers are warm enough to cause overheating inside the building (which is the case in most localities except those with cool summers). In predominantly warm-hot climates where no winter heating Is required. the use of reflective insulation only may be appropriate. Reflective insulation has a greater resistance to heat flow down (summer) than to heat flow up (winter) because it resists radiant energy flow better than conductive flow. The use of resistive insulation will reduce the conductive losses available from any nightsky cooling effect or air cooling of the roof surface. which is undesirable in warmer climates.

The air space below the reflective insulation in the attic space of a pitched roof need not be ventilated for summer where resistive insulation is included on top of the ceiling. The temperature of a ventilated roof space may be maintained at close to the outside air temperature and although this may be beneficial in summer to reduce heat build-up in the roof space. in cold weather it tends to negate any insulating contribution provided by the roof cladding and the associated reflective insulation. The difference in heat flow through a well-insulated vented roof and a well insulated non-vented roof into the occupied space below is very small. The U-value of a pitched roof with only reflective insulation under the roof cladding is in the order of 1.06 W/m².degC for heat flow in an upward direction and 0.64 W/m².degC for heat flow in a downward direction.

(b) Wall insulation

Insulating framed external walls is generally not difficult because the outer cladding material is usually designed to be a barrier to moisture. In such construction it is important, however, to ensure that a vapour barrier is installed on the warm side of the insulation layer (in cold climates this will be near the inside lining and in the hot humid climates near the outer lining).

Heat bridges in metal frame construction could be a problem in cool temperate climates where condensation will occur, and in hot-arid climates where the walls are exposed to the sun. In such circumstances it is advisable to use an outer layer insulation that covers and thermally isolates the framework from the external cladding material.

(c) Insulation d framed floors over ventilated crawl spaces

In cold climates it is advisable to insulate the underside of framed light-weight floors.

Generally the air under such floors is ventilated to minimize problems caused by dampness. In winter months this results in such spaces being at temperatures close to ambient, hence the need to Insulate to reduce heat losses down through the floor.

(d) Insulation of floor slates on ground

In passive solar building design it should not be necessary to insulate fully between a concrete slab and the ground except in extremely cold climates where in-floor central heating is being installed. The disadvantage of insulating the whole area under the floor slab Is that the house is isolated from the ground, which in winter is warmer and in summer is cooler than the external air conditions. The free heat storage benefits of the ground under the building is lost if full Insulation is used.

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. An alternative is to insulate the edge and the perimeter strip of the floor for approximately 600mm. Such measures may only be necessary in cool temperate climates. Perimeter floor slab insulation is recommended in areas of 2000 degree days to base 18.3°C, or greater (a description of heating degree days is covered elsewhere). Such insulation will help to reduce the loss of heat stored in the floor slab. Details of the Installation of edge-of- slab insulation is illustrated in figure 53.

Figure 53. Edge Insulation of a concrete slab on ground

{e) Insulation materials

The minimum insulation levels desirable in roofs, walls and floors will be determined by building codes and regulations In most countries. Optimum levels will be higher and will depend on the installed cost of the products being considered, the local cost of energy for space heating or cooling and the accepted discount rate for finance in the particular country.

Thermal insulation to restrict heat flow into and out of buildings has been well demonstrated to be economically worthwhile. In most situations the optimum levels of insulation will repay their capital outlay in energy savings over a short time. The improvement in thermal comfort of an insulated building compared with an uninsulated dwelling is quite significant, although it can be difficult to evaluate In economic terms when the users are accustomed to lower than average comfort standards. This is often the case in the more temperate climates where it is possible to manage with lower comfort levels. The value of energy savings over time can be determined using conventional discounting techniques as adequately described by Markus and Morris.

Typically uninsulated walls have a U-value of approximately 2.0 W/m² .degC whereas correctly insulated walls have a U-value 0.6 W/m² .degC in temperate climates and lower in more severe climates. Roofs are typically 4.5 W/m² .degC when uninsulated and 0.5 W/m² .degC when insulated in temperate climates.

Thermal insulation generally available for building purposes can be classified into three groups:

(a) Bulk materials;
(b) Reflective foil laminates;
(c) Rigid lightweight boards.

(i) Bulk materials

Bulk materials are available in either flat batt form, blankets or loose fill. The materials most commonly used are rockwool or glass fibre (yellow batts and pink batts). Materials such as eel grass (fine sea weed), acrylic fibre and cellulose fibre (from waste paper) are sometimes used; the latter has been quite popular in recent years due to its lower installed cost.

Rockwool is usually irritating to the skin if handled (during installation) without protection. It withstands high temperatures and is used in boilers etc. It used to be used in buildings in past years but then it went out of favour. It now seems to be coming back. The conductivity of rockwool is 0.035 W/m.degC at the usual density of 48 kg/m .

Glass fibre is most commonly used for bulk insulation of buildings and is known to most people in Australia as either pink batts or yellow batts. It does not withstand such high temperatures as rockwool because glass fuses at about 600C. As a product, it tends to be most resilient and not fall to pieces on the building site if maltreated. It does tend to be irritating if not handled carefully. A popular concern is that it is carcinogenic although evidence seems to show that problems relate to manufacturing conditions only (large quantities of loose fibres) and not to site conditions. where the product is bound together with acrylic or epoxy binders. It is not used in hospitals because of these dangers (especially with regard to operating theatres). The conductivity of glass fibre is 0.042 W/m.degC at a density of 12 kg/m which is the usual value for building grade material. Material with a resistance of R1.2 is approximately 50mm thick whilst material with a resistance of R2.0 is about 90-mm thick, depending on the manufacturer.

Eelgrass, the botanical name for which is Zostera marina, is marketed as alpinete. It is not commonly used in New South Wales, but is more common in Victoria where the material is readily collected from the beaches. It used to be used in South Australia also before the Second World War. It is a fine grade long-strand seaweed and was used extensively in older buildings in Australia and overseas. Eelgrass is dried and treated with a chemical such as borax to make it fire-retardant and resistant to vermin. Its conductivity is 0.048 W/m.degC at a density of 20 kg/m³. It may well be possible to collect it from some of beaches in New South Wales but it needs checking and treating for fire and vermin. It would seem to be an appropriate material for the appropriate technologist.

Cellulose fibre is marketed in Sydney by a number of companies. It has been in use for many years in both Melbourne and Sydney. It is made from waste paper and chemically treated with ground borax powder to make it fire-resistant and rot-resistant. The main difficulty in using it has been quality control when the material is made on site. The industry is working on this problem, which, if not already solved. will be solved soon. Cellulose fibre is non-allergic. Its advantage over batts and blankets is that it will fill crevices and can be blown into confined spaces. The problem is that it is very hard to ensure that there is sufficient material in the right place. Correctly manufactured and installed the conductivity of cellulose fibre is 0.035 W/m.degC. Its cost is competitive with glass fibre at about half to two thirds the price of the latter. It is made from recycled material which is a considerable attraction to some.

Vermiculite was formerly used as an insulator for bolters, hot- water tanks etc. It is too expensive to use as building insulation. It is a naturally-occurring material that expands into a loose flaky material in a kiln. It is generally used today for sprayed ceilings and fire insulation. Its conductivity is 0.067 W/m.degC at a density of 80 kg/m³.

Acrylic fibre is marketed in Australia as Wonder Wool, among other names. It is made from 3 denier × 54-mm long acrylic fibres which have been fused into a matt. It Is treated with a frame retardant (ignitability 14, spread of flame 0, heat evolved 1, smoke developed 5). It resembles fluffy wool and is used industrially behind lining materials such as in railway carriages. It looks like orion pillow filling. It is supplied in various widths and thicknesses. Like cellulose fibre it is non- allergic. Its cost per value of resistance is competitive with other materials. Its conductivity is 0.023 W/m.degC which is better than rockwool (so that a thinner layer is required).

(ii) Reflective foil laminates (RFL) and composites

Aluminium foil is sold under various names such as Sizalation and Renfoil among others. It is a material made up in a sandwich construction as follows:

Aluminium foil
Polyethylene film
Kraft paper
Adhesive and fibre reinforcement grid
Kraft paper
Polyethylene film
Aluminium foil

(The old-style material was Jute and bitumen but today the core is a flame-retarded adhesive and glass-fibre reinforcement).

The aluminium foil sheet is bonded to the kraft paper and the polyethylene film before it is in turn bonded to a second set of the same materials with the reinforcement grid in between.

RFL sheet is available in double- or single-sided laminate, and supported or unsupported foil. It can be fire-resistant or non-fire-resistant, and anti-glare treated or untreated. Single-sided material should only be used in buildings where it is being laminated to another material. Single-sided material is not weather-resistant and so is not suitable for use in roofs where it may also have to be a sarking. Unsupported foils are only used when they are being laminated to another rigid material. Most building codes now require that all RFL laminate be of the fire- retarded quality.

Anti-glare coatings increase the emissivity and reduce the reflectivity of aluminium foil.

Material with an anti-glare coating should be used when the material is being applied in sunny conditions to protect the applicators' eyes from the sun's reflection during Installation. In roofs it should be placed with the anti-glare coating upwards as this side will soon be covered by a dust layer and so will be less effective anyway.

Some manufacturers also make a "vapour-stop" material with a heavier plastic film to ensure a high level of moisture stop. This material can be used in cool rooms and the like where a high level of moisture resistance is required.

The SAA standard for the installation of reflective foil is AS 1904 and it is manufactured to AS 1903.

Aluminium foil is only effective as an insulator when coupled with an air space (minimum 25mm air space). Sheets should overlap 150-mm or be sealed with tape.

RFL used in a ceiling with dust on the top surface has the same effect as 50-mm mineral fibre in summer and 12-mm mineral fibre in winter.

Aluminium foil is often bonded on to a number of products to form a sandwich. The core is usually glass fibre batts or urea formal-dehyde foam. This system provides the benefits of both materials in terms of summer reflectance and winter insulation against heat loss. When used in roofs over ceilings it is important to ensure that such products are placed over the top of joists so that there is an air space under the foil.

(iii) Rigid lightweight boards

Polystyrene (Isolite) is a white, (usually) rigid sheet which can be ordered to any thickness. It is the same material that many architectural students use for model making and from which cheap ESKYs are made. it can be obtained in either standard grade or fire-retarded grade (which is needed for building use). It is reasonably effective in moist situations but it will absorb some moisture (which increases its conductivity). It is commonly used as an edge insulation for concrete slabs and in slab form for wall insulation. When used in that way it can be ordered with a spring edge to assist with securing it between studs. Its conductivityis 0.036 W/m.degC at a density of 24 kg/m³. It is difficult to use in oddshaped spaces because it has to be cut and fitted which is time-consuming. It is often used in bead form (bean-bag-chair filling) for hot-water cylinders and other cavity filling.

Polyurethane (Isothane) has a closed-cell structure. i.e., each bubble of gas is enclosed, unlike polystyrene where the many air pockets are only separated by a thin film that is not impervious. When new, polyurethane Is filled with nitrogen and so has a lower conductivity than if it was filled with air. As it ages this gas leaches out and the conductivity increases gradually. After some years however it is still better than, say. glass fibre but it is more expensive. As a result of the closed-cell structure it tends to be more impervious to moisture. It is available in both standard grade and fire-retarded grade. Its conductivity when new is 0.016 W/m.degC, and 0.025 W/m.degC when aged. The flexible form of this material Is not generally used in building but rather in furniture and the like. Its conductivity is 0.035-0.039 W/m.degC. The rigid form is used In building but more often In cool room construction as it is generally more expensive than the other materials available on the market. It is also available as an in situ foam for use where access to a cavity space is difficult.

Woodwool is marketed in Australia by Stramit Industries as Woodtex, made from regular sized wood shavings or "wool" matted together with a Portland cement binder. The natural colour is grey but it can be supplied painted or coloured on the surface only to give various effects. Generally it is available as 25-mm and 50-mm thick sheets which are usually mounted in a patented steel suspension grid system. These slabs are quite heavy and are used mainly as acoustical absorber panels. Its conductivity is about 0.08 W/m.degC which is about half as good as the same thickness of glass-fibre thermal insulation.

Strawboards come in two principal forms. Solomit is manufactured in Adelaide and is often used as a ceiling material where a straw finish is desired. It is simply straw bound together with wire in such a way as to form a 50-mm thick bats. it is often seen as the ceiling in primary school buildings of the early 1970s period and in some child-care centres. Some architects have used it as a ceiling in domestic work where all the other materials are natural-finished.

Stramit is another of these products but has a paper covering to which various coatings are applied, including chopped straw. This material is also sold in 50-mm thick sheets. As a nonstructural ceiling it will span the width of the sheet which is 1.2m.

The conductivity of these materials is generally higher than glass fibre and so additional material is required to achieve an added resistance of R2.0 as required for Sydney. The conductivity of Solomit is 0.041 W/m.degC at a density of 213 kg/m³ and of Stramit board it is 0.081 W/m.degC at a density of 320kg/m³.

Fibreboard is marketed as Canite and is generally 12-mm or 20-mm thick. In some places such as Queensland It is available in thicker sheets. Sheets are usually 1200-mm wide and 1.8. 2.4 or 3.0-metres long. Fibre-board is often used as a ceiling but it must be well supported because it sags or settles with time. It is used in schools as pinboard material. Its conductivity varies with temperature and moisture, i.e., k = 0.062 at 23°C and 0.048 at 21°C. The density is usually about 215 kg/m and an average conductivity of 0.06 w/m.degC can be assumed.

Urea formaldehyde can be obtained in slab form formed between two sheets of foil but it is generally formed in situ It looks rather like pressure pack shaving cream or mock dessert cream when first made. It is marketed in New South Wales by two main suppliers through a number of outlets - HEIMAX and ICI. The foam is made on site by adding a foaming agent (liquid) to the ureaformaldehyde resin in very carefully measured quantities. The mixing occurs in the dispensing gun and must be tested on site for the correct mix (springy lump on the ground, not limp). As the chemical reaction takes place some water is liberated which is normally absorbed by the surrounding materials. There is some shrinkage on setting which should not exceed about 3 per cent by volume; although the installation standard states 5 per cent. After it is placed from a hose it takes 4 to 5 minutes to set, and 24 hours for a complete set and cure. It has been in use In industry for about 25 years although its use in the building industry is quite recent (since about 1975). Its cost is competitive with glass fibre but it should be used in places where it is not subject to mechanical damage. as it crushes to a powder after curing. It is ideal for use where the cavity is too inaccessible to place other batt type materials as it can be pumped through a long 12-mm hose over quite a distance. There has been some concern about the safety of the product as it does release a small amount of formaldahyde gas during the curing. The amount released is very small when compared with particle-board flooring and furniture. There is no evidence that it acts as a bridge for water to cross a brick cavity. Experience with seven houses has revealed no problem. Due to the bad press publicity it has received in the past it tends to be used more as a pre-cured slab material.

3. Draught control

The infiltration of cold air in winter can result in considerable discomfort for the occupants and a large additional energy consumption if they try to combat the problem with heating. Research has shown that on many occasions the most cost- effective strategy to reduce heating loads is to reduce unwanted infiltration. During hot summer days when outdoor temperatures exceed indoor comfort temperatures substantially, unwanted infiltration will also cause discomfort and drain the interior of stored coolness.

The calculation of heat flow due to infiltration has been explained fully in chapter III under "Air infiltration".

The flow of air into and out of a building should be at the discretion of the occupants who can choose the conditions they desire. Fixed ventilation does not allow that choice and can too often be a source of discomfort for the occupants. Often a lack of ventilation is blamed for condensation problems caused by chilled surfaces. Such problems are considerably reduced in correctly insulated buildings. In such rooms as bathrooms and kitchens, where condensation is a problem, it is better to provide positive ventilation (exhaust fans) at the time the moisture is generated than to build in fixed ventilation which cannot be controlled.

Unintentional infiltration can be the result of choice of construction details and building design. The following points are provided as a design and detailing checklist.

(a) Major entrances and commonly used doorways from outside should be isolated by lobbies or vestibules. Such lobbies should have doors to isolate them from living areas and other habitable rooms;

(b) Fireplaces should be fitted with dampers to close off the flue when not in use;

(c) Exhaust fans should be fitted with positive action shutters to close when off;

(d) Windows should be selected or detailed to allow locking in the partially open position in preference to fixed ventilation;

(e) Care should be taken to ensure the junction of different materials is sealed. Common areas of difficulty are windows installed into face brickwork inside and out, junctions of walls and exposed-beam roof structures and Junction details where open joint shadow line detailing is used. Exposed timber floors should be sealed at the perimeter with a flexible sealant.

The amount of fresh air required in a space depends on the concentration of pollutants and the number of occupants. In houses, about 20-30 m³ /hour of fresh air per person is sufficient for most activities. In terms of the volume of an average dwelling of 100 m of habitable space with an average of four occupants, one air change every two hours is quite adequate in cold weather. Research has found that in older dwellings with fixed ventilators in each room and exposed timber floors, the air change rate can be as high as 10-15 air changes per hour. In modern dwellings with concrete slab floors the figure is more commonly 1-2 air changes per hour.

H. Heat distribution

7. Thermo-circulation

As air is heated it becomes less dense and floats upward to be replaced by cooler air. Research by Balcomb and others has demonstrated that this effect can generate considerable energy flows from a double height sunspace into a two-storey building behind. This heat distribution effect is due to the relatively high temperatures achieved at the lower levels of a well-designed sunspace. In taller buildings the flow will be even greater. The flow due to the vertical distance between openings can be calculated and is described later in chapter VII.

2. Mechanical circulation

Whilst natural air movement Is desirable it is often more effective to use mechanical means to circulate warm air (air + energy) from one place of collection to the place needed. When used to move air at low flow rates, fans can be very effective and economical to operate.

Simple exhaust fans (with or without ducting depending on the application) can be used to move warm air that collects at the top of high volumes to occupied spaces at lower levels. They can be used to move warmed air from a sunspace to a cooler non-sunlit space behind.

The rate at which air moves with mechanical devices depends primarily on the fan design and the power of the motor. Such information can be found in design guides for air handling equipment or sometimes In the manufacturers literature.

Worked example No.2

To calculate the daily heat gains or losses through 1 m of north-facing window In Sydney during July, select the following information from the climate section:

Mean daily ambient temperature

T_o = 11.7°C

Assume mean internal temperature = 21°C

Solar heat gain (SHGF) = 1 2.2MJ/m² (from Sydney insolation tables)

To calculate or select the U-value for glass = 6 W/m².degC: assume glass 3mm clear shading coefficient A = 1.00

To calculate daily heat loss by conduction:

H	= A.U (Ti-Ta) × 24 × 3.6 × 10-3
	= 1 × 6 × (2.72) × 24 × 3.6 × 10-3
	= 4.8 MJ/m2.day

To calculate daily heat gain by radiation:

H	= SHGFx SC
	= 12.2x 1.00
	= 12.2 MJ/m2.day

The total daily heat gain as a result of conduction and radiation exchanges is 12.2MJ radiant gain less 4.8 MJ conductive loss/m².day

= 7.4 MJ gain per m of window per day.

If curtains are drawn at night then it is possible to use a modified U-value (Um). Assume curtains to be lightweight with restricted air circulation. Refer to section H.1 above. The modifier for light drapes with restricted air circulation is M = 0.6. Assume also that curtains are closed for 13 hours per day (say 6 p.m. to 7 a.m.).

Therefore:

Um	= U/24 × ((M × Hd) + 24 - Hd)
	= 6/24 × ((Q.6 × 13) + 24 - 13)
	= 4.7 W/m2.degC.

The revised daily heat loss by conduction is therefore:

H;	= A.Um (Ti - Ta) × 24 × 3.6 × 10-3
	= 1 × 4.7 × 9.3 × 24 × 3.6 × 10-3
	= 3.8 MJ/m2.day

The revised total daily heat gain is now (12.2 - 3.8)

= 8.4 MJ per m² of window per day.

Now consider windows on other orientations and see why the north windows are so important for winter heating.

I. Passive solar heating strategies

The three systems for passive solar heating generally accepted today are direct gain, thermal storage walls (masonry or water), and the attached sunspace. These are described below. Further Information and design details can be found in the literature on passive solar design listed in the bibliography. The reader will also find that opinions about precise details will vary from author to author in much the same way that opinions vary about construction details. Passive solar design does not have just one correct solution. Correct solutions are as varied as the designer's imagination.

1. Direct gain

Direct-gain systems are those where the solar radiation enters the habitable spaces via equator-facing windows and is absorbed by the materials inside the space for later dissipation when the ambient temperature falls. This is by far the most commonly used system and in spatial design terms the most flexible because it can accommodate daylighting and views to the exterior as well as providing a source of heat. Whilst it is the easiest for the designer to work with, it does have far more limitations than some of the other systems.

Figure 54. An example of direct gain

To operate effectively, the area of the equator-facing direct-gain window is closely related to the area of thermal-storage materials within the space and the climate characteristics of the particular location. The thermal-storage area must be matched to the incoming solar radiation to maintain the diurnal temperature swing in the range of 5-6C for acceptable thermal comfort. Insufficient thermal storage could result in daytime overheating even in quite cold climates. In temperate climates, where heating loads are modest. this system should provide high levels of thermal comfort without any significant auxiliary heating. In very cold climates there is likely to be insufficient solar contribution due to the limitations of glass area and it may be more appropriate to use a combination of systems. Manual techniques for the design of appropriate window size and area of thermal-storage material are given in many publications such as the SLR-method. A computer-based calculation method, CHEETAH, is also available.

The equator-facing windows must be designed with appropriate shading and means of minimizing conductive heat loss as described earlier. Direct sunlight entering the space can be a problem in terms of fading of interior finishes and also glare for the occupants. The latter can be countered in most domestic situations by appropriate selection of surface finishes. Direct sunlight inside a room has many psychologically beneficial qualities which have been recognized down through the ages and so this should not be overlooked.

Roof-level glazing can provide a useful contribution to the equator-facing glass area provided its thermal resistance is adequate to minimize heat losses. In temperate climates and those locations where the summers are quite hot, it is important to avoid sloping glass. In any event, all glass must be well shaded to stop the entry of summer sun as discussed in more detail later.

The floor is most commonly the main termal 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 (see chapter 111) 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 too distribute the sun's energy to the other heat storage surfaces.

2. Thermal storage walls (Trombe wolfs)

Thermal storage wall systems usually comprise a dark-coloured heavy wall (masonry, concrete, mud brick etc.) erected in the solar aperture with a double-glass system mounted approximately 1 " (20-30mm) in front of the wall surface. The sun's radiation passes through the glass and is mostly absorbed by the dark surface of the wall (preferably of high absorbency and low emissivity). The amount of energy that is lost back through the glazing depends on the insulating properties of the glazing and the surface properties of the wall. Special surface coatings for thermal storage walls with low emissivity and high absorbency are manufactured In North America specifically for this application.

The solar energy absorbed into the wall raises its temperature slowly throughout the day. By night-time the heat has penetrated to the inside surface where it can radiate Into the room raising the environmental temperature so providing warmth to the occupants. The time taken for the heat to reach the inner surface depends on the thermal properties and the thickness of the wall material, typically about 10 hours per foot of thickness for masonry. The same design guidelines and computer-based design tools listed for direct-gain systems also provide for thermal storage walls.

In early examples a range of movable night insulation systems were employed but Balcomb now reports that experience has shown double-glazing and selective surface coatings on the outer face of the thermal storage wall to be more reliable and cost-effective. Current recommendations are that the air space between the glass and the wall surface should be well sealed. The earlier examples had various systems of venting either to the room being heated for winter or the outside to reject heat in summer.

Protection from solar radiation in the summer is best provided by some form of cover over the glazing. The protection that is appropriate for direct-gain windows may not be sufficient to keep out the diffuse- and ground-reflected forms of radiation. Unwanted solar heat in direct-gain heated spaces can be dissipated by natural ventilation whereas in thermal storage walls it will be absorbed into the walls to heat the room when not wanted.

In some cases water has been used as the thermal storage material. This technique of passive solar heating is similar to the thermal storage wall except for a number of key points. The thermal storage capacity of water Is approximately three times that of brickwork by volume (see table 8). Whilst it is inexpensive as a material it is both difficult and relatively expensive to store securely. The time-lag effect of water in containers is shorter than in masonry because it is a fluid: the transfer of energy is part by conduction and part by convection. Many examples built so far use water in vertical cylinders painted a dark colour or finished with a selective coating as described for the thermal storage walls. The cylinders can be placed with spaces between them to allow some solar radiation and daylight to penetrate directly into the space. The water-storage walls generally perform better than thermal storage walls because the convection mixing of the water results in lower surface temperatures and a more even distribution of energy. Most of the design tools that have been developed for use with thermal storage walls are also suitable for the design of water-based heat-storage systems.

Figure 55. A thermal-storage wall system

Figure 56. Sunspace enclosures

3. Sunspace enclosures

An attached sunspace system comprises an additional enclosure, usually with all surfaces glazed, attached to the equator-facing side of the building to be solar heated. The additional space so created can be used for various purposes that are suited to exposure to high levels of sunlight and a wider temperature range than aimed for inside the rest of the building. The built form of the sunspace may range from simple attachment to the face of the building to full integration into the building envelope. A number of sunspace configurations have been defined in the literature. for use in design tool analysis. They are essentially variations on a theme that have been defined for computer-modelling purposes. In reality, an attached sunspace is a room on the equator-facing side of a building with a significantly high glass area compared to the floor area (glass/floor ratio > 1).

The critical components of a sunspace solar-heating system are the floor (usually ground connected), the wall separating the sunspace from the remainder of the building to be heated and the area and nature of the glazing. The wall dividing the sunspace from the rest of the building is similar to the thermal storage wall described before. In this case the glass has been moved out to form an accessible space for other purposes. thus making the overall system more economically viable. The dividing wall can be either a solid mass, uninsulated, insulated on the inside, insulated on the outside or simply light-weight heavily insulated. If the wall is of uninsulated heavy materials then the same rules apply as with the thermal storage wall. If it is insulated then a venting system is required to transport the collected heat to the interior of the building. The use of thermal mass materials inside the sunspace will help to modify the diurnal temperature swing.

In all locations except where summers are very cool, it is important to provide adequate through ventilation to remove summer heat build-up (minimum vent area should be 10 per cent of the total glass area for mild summer climates and more for warm to hot summer climates). In most areas where summers are warm (mean temperatures within the comfort range) it will be necessary to shade the glass area fully to avoid overheating and degradation of the interior surfaces and fitments. The preferred design for such areas is with a fixed opaque roof and only the vertical surfaces glazed. Even in colder slimates there is likely to be problems with overheating in autumn as the sun is passing low in the sky and yet the temperatures are not commensurately lower. This can be overcome to some extent with suitable shading of the roof glass and the east and west glass, if any. The current trend is to build such enclosures as a visual element in an otherwise conventional building. Unfortunately, too often little thought is given to the protection of the glass areas from solar gain in summer and, as a result, many unsatisfactory buildings are being built.

In many applications an attached sunspace is used as a multi- level connecting space (lower floor with mezzanines above to the upper floors). In such cases the designer must be aware of the chimney effect of the must be aware of the chimney effect of the tall space as the air in the sunspace will be much warmer than the remainder of the house. The design should allow for the cooler air inside to flow down to the lower area and out to the sunspace. With large glass areas, overheating can be quite a serious problem without correct shading.

J. Natural cooling strategies

This section considers strategies that can be adopted to regulate overheating. Whilst some strategies can be quantified using reasonably accurate design methods such as the admittance method to predict peak Indoor temperatures or computer-simulation techniques, others are more elusive although well known in use. One of the main difficulties is that most cooling strategies rely on user action and accurate knowledge of their behaviour and such climatic information as detailed wind patterns. If these were really known to the designer then the quantifying process would be simpler.

In many cases of poor design, the internal thermal climate Is more severe than the external climate due to uncontrolled external environmental gains (windows not adequately shaded and external walls and roof not adequately Insulated) or unnecessarily large internal energy gains. A good design solution will provide an indoor thermal climate that starts from the position of being better than the outdoor environment.

In much of the year overheating the inside of buildings is the result of excess solar heating and internally-generated heat reaching the interior spaces. This is certainly the case where the ambient air temperatures are no greater than about 27 °C. In such cases, it is usually practicable to maintain comfort conditions with appropriate control of those heat gains (such as shading of windows and exhausting internal heat) and stood ventilation and air movement patterns.

Where air temperatures are above reasonable comfort levels it is necessary to apply other strategies which will collect or soak up the excess heat for disposal into the cooler earth or to the cooler night air. In these cases the design approach should be as follows:

(a) Reduce solar gains to the interior by correctly designed shading;

(b) Minimize conductive gains by shading walls and other surfaces as appropriate and Insulating the external fabric of the building:

(c) Minimize the effects of internal gains (lights and other appliances) by exhausting the heat;

(d) Design night ventilation openings to optimize the cooling of thermal sinks (thermal mass):

(e) Allow for appropriate air movement (ceiling fans and the like) to raise the occupants' comfort threshold;

(f) Design for minimum air infiltration during the day when external air is 3C greater than the upper comfort limit.

1. Control of heat gain through external fabric

Discomfort from overheating is the result of the body not being able to dispose of the heat generated at an appropriate rate. This state in turn may be the result of external heating influences or a person's activity rate. In the design of buildings the aim Is to cater for people whose activity rate is appropriate to the situation, i.e., when designing a gymnasium allowance is made for the increased activity rate of the occupants by providing more ventilation than might be the case in another situation. In this section the primary concern is with excess heat from the sun or from appliances being used within the building enclosure.

The unwanted heat generated by appliances (cooking stoves in houses or computers and the like in an office environment) can usually be dealt with by ventilation in temperate or cool climates. This technique will at least bring the surrounding conditions to that of the incoming ventilating air. In excessive cases other techniques are applied using heat pumps and the like to remove the excess generated heat.

In most cases for the building designer, the major source of excess heat is the sun in two forms; incident solar radiation and ambient air temperature above an acceptable comfort level. In both cases, the first strategy should be to provide some form of barrier or filter to reduce this overheating effect. The second is to provide positive means of effective natural cooling. The strategies chosen for a project should suit the specific characteristics of the climate and also enhance the architectural design solution.

The reduction of overheating is best achieved by exclusion of the unwanted heat rather than its later removal (often by air conditioning). Therefore the external fabric should be insulated as discussed earlier (including the appropriate use of RFL insulation in summer driven climates). Appropriate shading of opaque external elements is desirable especially where the sol- air temperature effect is significant. The solutions available to the designer range from suitable landscaping to shelter the walls and roof from solar gains (unfortunately these solutions are slow to establish unless already existing) to more sophisticated screens and vented wall linings built as an integral part of the building structure. Transparent elements of the external fabric such as windows and skylights are usually the main source of concern and so will usually require shading from direct sun in the non-heating seasons. The design of shading is discussed separately in this book as it is both a key factor in the reduction of overheating and in visual design of the building. Unwanted solar energy can be excluded by various techniques that can all be grouped into the following general categories.

(a) External devices or systems that deflect the sun's rays (either fixed or adjustable, natural or man-made, to suit the particular application). This is the most positive and effective approach with the least chance of failure;

(b) Specific treatment of the transparent element to change its transmission properties. Generally such solutions are fixed and so will also minimize sun penetration at times when it may be desirable, i.e., winter. A number of variable transmission glasses are being developed such as electrochromic glass, where an electrical force is used to alter the transmission properties of the glass. These should be available in the next few years;

(c) Internal devices that reflect some energy back out and convert other energy from radiant to sensible heat by heating up the air around the shading device. These devices, such as reflective blinds and curtains. are less effective than external barriers but are easier to operate and maintain;

(d) Each of these has its place in the designers "palette" and the successful designer will choose the best one for the job.

2. Passive cooling

In many locations it is necessary to apply some positive cooling strategies to overcome overheating resulting from climatic factors, even after reducing direct solar heat gain to a minimum. This situation usually occurs where the ambient air temperature is either close to the upper limit of the comfort zone or above it and is, sometimes, exacerbated by high humidity levels.

Where the ambient air temperature is within the comfort zone and humidity levels are low it may be possible to satisfy comfort needs with the heat control strategies discussed above. If these limits are exceeded in one way or another the following approaches may be appropriate. either individually or in combination. Some are more climate-dependent than others, such as evaporative cooling which is only applicable where the wet bulb temperature is in the range of 12°C to 24.5°C and relative humidity is less than 80 per cent.

Varying degrees of cooling in a building can be achieved using natural sources such as:

(a) The night air: night-time convective cooling of a thermal mass or heat sink within the building fabric;

(b) The upper atmosphere: radiant nocturnal cooling of the building fabric to the night sky, causing a chilling of the roof structure which can then take up heat the next day;

(c) Water evaporation: evaporative cooling of the air and/or the building structure;

(d) Earth cooling: storage and recovery of energy from the sub- surface soil.

Cooling systems using these sources have been in existence for a long time but in some cases little is known about how to quantify their effectiveness. Water evaporation for cooling is well understood when used in a defined environment of cool air within a mechanical systems. Little is known about how to define the evaporative cooling effect of water sprayed into the surrounding landscape or the free use of water in semi-enclosed spaces such as an atrium or centrally-located courtyard. Some work has been done on this by authors such as Givoni and Lesuek but there is still a long way to go. Many designers have their own knowledge of specific cases from personal experience: they have tried ideas built on the available knowledge and their own developed sense of logic. The young designer can learn from studying the natural and the built environment, noting the cases where a space is comfortable in an otherwise unpleasant environment and observing what makes it better; the use of sprayed water. a large shaded mass that soaks up the heat of the day etc.

Night-time convective cooling used to cool the structural mass of a building can be most effective where the vapour pressure is generally below 2.27 kPa and the diurnal temperature range is large (above about 10 deg.C.) and night time temperatures fall below about 20°C. In areas where the vapour pressure is between 2.27. kPa and 2.67 kPa, nighttime convective cooling is still workable but it may be necessary to use air circulation techniques whilst the building is closed to maintain comfortable conditions. These limits are discussed in detail in chapter VI - Bioclimatic analysis. Almost all places in Australia that require cooling. except for the warm-humid areas, can achieve a useful cooling contribution from this technique. Usually natural ventilation during the day is undesirable under such conditions and so indoor air movement must be created using fans until the day cools sufficiently to open the building to the outside air. The night-time cooling of the structural mass of a building by convection will lower the daytime temperatures by about 2-4C compared with the same building not ventilated at night. The external fabric of such a building should be well insulated to minimize daytime conductive and solar heat gains to the interior and the structural mass in particular.

Radiant nocturnal cooling can be achieved in areas where the night sky tends to be clear during hot weather, generally in all Australian climates except the warm-humid areas. The power of such systems is limited to about 70 W/m², which is minimal for most purposes. In most areas of Australia where there are clear night skies sufficient for such a system there is also a large diurnal temperature swing which will provide cool night air as discussed above, which can be used more simply and at less capital expense.

Evaporative cooling has been used extensively throughout the hot-arid areas of Australia for many years. Mechanical systems that force evaporatively cooled air through the building in much the same way as an air-conditioning system are most effective and economical to run. The main energy requirement is to drive the ventilating fan. In many of the places in Australia where evaporative cooling is most needed there tends to be a shortage of clean water suitable for such equipment. As a result the dirty or brackish water that is often used tends to clog the evaporator pads and increase the maintenance costs. In the more temperate areas of Australia, when on certain occasions the wet bulb temperature is below about 17°C, it is possible to use evaporative cooling but more often the humidity levels are too high resulting in unpleasant warm humid conditions inside.

Passive uses of evaporative cooling such as open water in roof ponds and courtyards have been effectively used throughout history, but as stated before it is difficult to quantify their benefits. Even today most of the design details are based on anecdotal experiences. The water in a pond will follow about 1C above the wet bulb temperature with diurnal swings dependent on the depth of the pond. If water is used in a concrete roof pond then the ceiling below can be expected to reduce the indoor temperature by up to 3C below the average outdoor dry bulb air temperature.

Earth cooling can be utilized in many climatic areas and its effectiveness has been demonstrated by those who have constructed their whole building underground. It is possible to draw air through tubes or passages beneath ground that has been cooled by water evaporation and then into a building for cooling. This technique was used in Alice Springs in a dwelling built for the Flying Doctor Service earlier this century. There have been other examples tried but generally the capital cost is quite high and the passages difficult to maintain free of harmful bacteria.

3. Ventilation and air movement

Natural ventilation of a building will initiate passive cooling during the summer. The exhaustion of excess warm air and the intake of cool air help to lower interior temperatures. To facilitate the exit of warm air, dampered vents should be located at high points in a building.

Natural thermal pressure differentials provide airflow in domestic dwellings. In still conditions ventilation occurs due to this effect. In windy conditions thermal pressure differentials are insignificant compared with the effects of the wind. Ventilation due to temperature differences can be increased by extra storey heights or by providing heat to the upper end of a chimney which is the principle of the solar chimney. If natural thermal pressure differentials do not produce sufficient flow velocities, fans, turbines, or plenums can be used to accelerate them. Attic fans will, if needed, more quickly remove hot air that accumulates during the day. Operable vents, connecting the building space with the attic, provide a pathway for the upward flow of hot air. The motion of outside air can often be used to induce interior air movement without the aid of fans. Wind can be used to power a turbine or can be directed in such a way that pressure changes result which move inside air.

Buildings themselves act as large-scale ducts and should be designed with options for unobstructed movement of air.

Figure 57. Air flow over a building to induce air flow

Ventilation will generally occur if a pressure differential exists across a building. This differential may be enhanced by the shape and orientation of the building with regard to the prevailing wind direction and openings placed to utilize those pressure differences.

Openings in opposite walls produce a different interior air movement than openings on adjacent walls. Similarly the position of the opening in each wall will influence the path of the interior air stream. The area of openings may be varied for different effects. However, maximum ventilation occurs by having equal size of inlet and outlet openings.

Internal air flow may be further affected by roof overhangs, awnings and fenestration. The main air stream can be directed towards the ceiling for winter ventilation and towards the occupied zone for summer comfort.

Internal walls, partitions and floor-to-ceiling furniture will affect the air flow pattern. Whether the air flow is perpendicular to or parallel to the direction of the main stream flow will modify the air flow. The number of internal openings that the air must flow through to reach the outlet opening will further modify the ventilation produced within the building by reducing the volume of flow.

Figure 58. Effects of the position of openings in a room on ventilation and cross air-flow

4. Circulating fans for air movement

Where natural wind patterns are not available or buildings have to be closed to keep out an overheated environment. it may be desirable to use mechanical aids to induce air movement throughout the space or building. The advantage of using a fan for air movement compared with natural ventilation is that its effect is more controllable and so can be directed as needed away from work areas where papers or other materials can be disturbed.

Personal low level fans can be used close to the occupant and directed in such a manner that others are not affected by H. Ceiling fans are economical to operate and provide a broad area of air movement. Generally the blades move slowly creating a gentle movement which need not be disturbing. This type of fan is ideal in cases where air movement is needed for comfort in an otherwise closed space such as a massive building in a hot-arid climate. This fan type can effectively mix the air in the space bringing it in contact with the structural mass cooled the night before. The values in figure 59 illustrate how air movement at comfortable rates can be provided over a reasonable area from one ceilingmounted fan. The distribution covers a much wider area than does the vertical type fan. The only disadvantage being that for safety the units should be mounted at about 2.5-m necessitating ceiling heights of 3-m (regulations in Australia require that the blades be mounted at not less than 2.2 m above floor level.

Figure 59. The impact of mechanical fans for Induced air movement within a space

5. Daylighting

Daylight is important for more than just vision. Studies have shown that daylight is important to satisfy many human physiological reactions. It is most important that designers consider both daylight quality as well as daylight quantity. In this section it is intended only to highlight the need to incorporate good daylighting design with passive solar and natural cooling technologies. A brief summary of sources of daylight are given for general guidance. Designers should refer to other more detailed texts for design guidelines.

Table 9. Summary of ventilation functions

Sources of daylight include direct sunlight, (high intensity), diffuse skylight and diffuse reflected light from buildings. ground. objects

Direct daylight is the result of allowing direct solar radiation into a space, and so in hot times of the year it will be more often excluded or at least filtered to reduce the heating effect of the sun. Direct sunlight often be a serious source of glare unless internal surface colours and textures are chosen to avoid the problem. Direct daylight/sunilght is a valuable design modelling tool as it is always on the move and changes in intensity as the day passes. The designer should not lose sight of this magnificent component of a designers vocabulary.

Diffuse light comes from the sky hemisphere (that component reflected and refracted through the Earth's atmosphere) or is reflected from the surfaces of other objects and if too bright it can cause glare. Diffuse

Figure 60. Use of light shelves or other reflective surfaces to direct sunlight Into a space without glare

Good daylighting design is to do with the quality of light rather than just quantity. Glare from daylight can be a serious source of discomfort especially when direct sunlight is reflected off another object either inside or outside the building. In a residential situation, occupants are more able to adjust their position to avoid glare, whereas in other spaces where people are working in a fixed pattern it is more difficult to move and so potential glare sources must be considered in the basic design.

Direct sunlight may be necessary for heating and as a source of light and so it may be better to direct it away from work surfaces and other potential glare-producing surfaces. The use of reflective surfaces outside the viewing range such as light shelves and louvres are an excellent way to overcome this problem. Many designers have utilized these techniques, as Is shown below, to produce glare-free interiors with good natural daylighting levels and a dynamic visual quality that exploits the changing nature of natural light.