Table 16.1 Range of variation in indoor climate

Solar radiation absorbed in

the walls Solar radiation penetrating

through windows Indoor air temperature

amplitude Indoor maximum air

temperature Indoor minimum air

temperature Indoor surface temperature

Average internal air speed,

windows open Actual air speeds at any

point in room Indoor vapour pressure

The climatic variable Range of variation

15-90% of incident radiation 10-90% of incident radiation 10-150% of outdoor amplitude

— 10 to +10°Cfrom outdoor maximum

0 to -Ь7^СС from outdoor minimum

— 8 to + 30°C from outdoor maximum and minimum

15-60% of outdoor wind speed

10-120% of outdoor wind speed

0-7 mm Hg above outdoor level

16.2. Methods to Determine Human Requirements and Satisfactory Design Principles in Relation to Climate

16.2.1. the method of olgyay.—Olgyay [16.9] was the first to propose a systematic procedure for adapting the design of a building to the human requirements and climatic conditions. His method is based on a "Bioclimatic Chart" showing the zone of human comfort in relation to ambient air temperature and humidity, mean radiant temperature, wind speed, solar radiation and evaporative cooling. On the chart, dry bulb temperature is the ordinate and relative humidity the abscissa. The comfort zone is in the centre, with winter and summer ranges indicated separately (taking seasonal adaptations into account). The lower boundary of the zone is also the limit above which shading is necessary. At temperatures above the com­fort level the wind speed required to restore comfort is shown in relation to humidity. Where the ambient conditions are hot and dry, the evaporative cooling necessary for comfort is indicated. Variation in the position of the comfort zone with mean radiant temperature is also marked.

The analysis of climatic data and evaluation of appropriate human requirements and the design principles to satisfy them, pro­ceeds according to the following steps:

a. Compilation of local climatic data, including temperature, wind, radiation and humidity.

b. Tabulation of climatic data on an annual basis, and con- struction of a series of charts showing annual distribution of the climatic elements.

c. Plotting of the tabulated data on air temperature and humidity on to the bioclimatic chart.

The lower limit of the comfort zone on the chart (70°F) divides the climatic conditions into two categories; the area above this limit is known as the "overheated" period, in which shading from solar radiation is necessary, and below it is the "underheated" period where irradiation is required. Thus the climatic type is determined and, from the other variables shown on the chart, the comfort requirements of ventilation, evaporative cooling, shading or solar radiation can be evaluated.

d. Planning of design factors, such as the building forms and orientation, the location, size and shading of openings and glazed areas, etc., to compensate for disadvantages in the ambient climatic conditions by maximising heating in the underheated period and minimising it in the overheated period. Application of this pro- cedure, for evaluating the performance of fixed shading devices, was demonstrated in Section 11.5.

As mentioned, Olgyay's method was the first attempt to system-ize the incorporation of climatic conditions into building design. However, the system is limited in its applicability as the analysis of physiological requirements is based on the outdoor climate and not on that expected within the building in question. It was shown in the previous section that the relation of indoor to outdoor conditions varies widely with different characteristics of the building construc­tion and design. The method is thus suitable for application in humid regions, where ventilation is essential during the day and there is little difference between the indoor conditions and those out of doors. But application in hot dry areas, particularly in the sub-tropics, could lead to erroneous conclusions.

This important point may be clarified by the following example. Let us consider a sub-tropical inland region where the daytime temperatures are around 32-35°C (90-95°F), the minimum about 17°C (63°F), and the relative humidity about 40%. According to the bioclimatic chart, comfort is unattainable during the day, unless the indoor wind velocity is very high, or evaporative cooling pro­vided. Tn fact, comfort could easily be achieved, keeping the indoor temperatures below 28°C (82°F), by employing suitable structural materials of white exterior and by closing and efficiently shading the windows.

A similar limitation applies to conditions below the comfort zone, where heating is recommended for periods in which the outdoor temperature is lower than 20°C (70°F); here the actual indoor ten.peratures may be considerably higher, again depending on the properties and external colour of the structural materials.

16.2.2. method proposed by the author [16.6].—The method described here uses the Index of Thermal Stress (see Section 5.6) to evaluate the human requirements for comfort, from which the necessary features of building design to achieve this comfort are determined; it also involves an estimation of the indoor climate expected under the given ambient conditions. The analysis proceeds as follows.

a. Analysis of the climate

This is carried out on a diurnal basis, for the periods of most extreme physiological stress, enabling the most important aspects of a climate, for considerations of stress, to be specified. These include problems of overheating in the summer, of underheating or excessive cooling in winter, of wetness during rain seasons, etc. In some cases a single aspect is of overriding importance, whereas in others several requirements are of particular significance during different periods of the year. Typical diurnal patterns of outdoor temperature, vapour pressure and wind velocity are compiled and summarized for the hottest and coldest months, and if necessary for other periods of stress conditions, noting features requiring specific attention.

b. Choice of approach in hot climates

The initial examination is a comparison of the indoor con­ditions of comfort obtained by two methods: by effective ventilation, and by a reduction of the internal temperatures below the outdoor level. The criterion for comparison is the thermal stress of the occupants of the building considered, indicated by their evaporative weight loss: thermal comfort corresponds to a weight loss rate of 40-60 g/h, but provided that the skin remains dry thermal stress is very slight below 100 g/h.

The possibility of achieving thermal comfort during the day using ventilation is examined through the Index of Thermal Stress (s<?e Section 5.6), the values of which are computed for the critical hours in the building interior. The assumption is made that, with efficient ventilation, the indoor air temperature and vapour pressure during the day are identical with those outside, and hence only the outdoor values are taken into account. The mean radiant and air temperatures are assumed to be close enough for the additional effect of radiative heat exchange to be neglected, and therefore structural materials must be chosen to justify this assumption. The indoor air speed is taken to be 30% of the free wind speed, with 1-5 m/sec a fixed limit above which the air motion causes annoyance.

At night the required indoor temperatures for comfort are lower than the level necessary during the day, on account of the drop in wind speed and also the higher internal mean radiant temperature which may be maximal at this time. The latter factor requires a compensatory 1-2 deg С reduction in the indoor air temperature.

If it is concluded from the above examination that comfort is attainable using ventilation, under conditions in which the assump­tions made are valid, then the primary requirements, to be met through the design of the building and the choice of structural materials, are that these conditions be satisfied (i.e. internal surface temperatures below the level causing thermal stress) and that good cross-ventilation be provided.

Application to warm-humid and to Mediterranean marine climates is described in detail in Subsections 17.3.2 and 17.4.2.

If the results of this examination imply that ventilation would not provide the required comfort level, or in cases where ventilation is for some reason undesirable during the day, then a second approach is taken. This involves reduction in indoor temperatures below the outdoor level, effected by the specific selection of building materials.

To obtain this reduction the windows must remain closed and the indoor air is assumed to be still, with the vapour pressure 2 mm Hg higher than that outside. The upper limiting air temperature for comfort is then that temperature producing a weight loss of 100 g/h under the conditions specified, computed from the Index of Thermal Stress. Thus the required reduction in temperature below the outdoor level is evaluated.

The possibility of achieving this temperature with the most favourable design conditions is then examined, i.e. with an externally white building ventilated during the evening and night. The potential reduction varies with the outdoor temperature range, and with suitable materials the indoor maximum may be 50-60% of this range—below the maximum outside. Thus for the ambient internal conditions anticipated, of air temperature, elevated vapour pressure, still air, and elevation of mean radiant temperature above that of the air, the expected sweat rate is computed. If this is below 80 g/h (taking into account the elevated radiant temperature), it is possible to effect the required reduction in internal air temperature by the correct choice of building materials and external colour.

Further details of this procedure are given in Subsection 17.4.1 for a Mediterranean continental climate.

If, however, neither of the above examinations reveals a system able to ensure comfort, or even to come close to it, then the necessity for mechanical thermal control is indicated.

In arid regions this is best provided by either air-conditioning or water evaporation (desert coolers). Where the air is more humid air-conditioning is the only suitable method.

Heating requirements for the winter are determined on the basis of expected indoor temperatures. Those below which heating is necessary are 18°C during the day and 15°C at sunrise, although higher temperatures would be desirable (see Section 17.5).

c. The Building Bioclimatic Chart

As mentioned earlier, the potential reduction of indoor below outdoor temperature increases with the external temperature range. The range, however, is inversely related to the ambient vapour pressure. Thus the upper limit of outdoor temperature at which indoor comfort can be achieved is raised by a drop in the vapour pressure.

T he inverse relationship between temperature range and vapour

Abidjan and Conakry in West Africa, (b) in several regions in India, and (c) in the relatively small area of Israel, using data derived from various sources. The importance of this interdependence between vapour pressure and temperature range is clear from the correlation between the vastly different areas studied, and quantitatively the relation approximates to:

At (°C) = 26 - 0-83 v.p. (mm Hg).

A high humidity necessitates ventilation in a building and, as the corresponding temperature amplitude is low, little reduction in internal temperatures can be obtained. On the other hand, when vapour pressures are low and the temperature range correspondingly high, considerable temperature reductions are possible, and the necessary absence of ventilation is not detrimental at the low humidity. The choice of control method to avoid thermal stress within a building is therefore facilitated to a great extent by con­siderations of the local temperature range/vapour pressure relation. For the ambient combination of these factors, the indoor climate to be expected using each of the approaches to control can be esti­mated, and that yielding the most satisfactory thermal conditions selected.

For practical use, the suitability of ventilation, air temperature reduction, evaporative cooling or air-conditioning, for ambient conditions combining different temperature amplitudes and vapour pressures, are plotted on a psychrometric chart to form what has been named a Building Bioclimatic Chart (see Fig. 16.2). On each part of the chart is shown the summer comfort or neutral zone, for acclimatized people at rest or engaged in sedentary activity (area bounded by N), and the margin of permissible conditions (area bounded by N'). The temperatures at the upper limit of the accept­able zone are respectively 26° and 28°C for vapour pressures 20 and 5 mm Hg.

In Fig. 16.2 (a) the range of conditions is shown under which comfort is attainable by control of the indoor temperatures alone in the absence of ventilation. The area on the chart in which comfort­able conditions can be achieved by this method is marked off by the line M, and the range of attainable acceptable conditions by line M'. Both are found at a vapour pressure below 17 mm Hg. At higher humidities, the still air conditions within a building necessary for the temperature reduction would cause the discomfort of moist skin. The temperature limits of these zones are inversely related to the vapour pressure, varying up to 31° and 33°C at 17 mm Hg and up to 37° and 39° С at 5 mm Hg.

The conditions under which comfort may be achieved using ventilation are shown in Fig. 16.2(b). The zone bounded by the line V applies to buildings not specifically designed to minimize heating (standard buildings), and that enclosed by V is for structures of medium to high thermal resistance with white external surfaces. The two zones extend respectively to 28° and 30°C at 25 mm Hg and to 30° and 32°C at 5 mm Hg.

Below 17 mm Hg and 32°C there is an overlap area in which the use of ventilation and of temperature control are both applicable, although lower night temperatures might be expected with the former, as less heat is stored within the structure during the day.

The conditions under which neither method is adequate and some system of heating or cooling must be provided are given in Fig. 16.2(c). Such systems can be used of course also at the climatic conditions where comfort is attainable by either ventilation or pre­vention of undue elevation of the indoor temperatures. EC and EC mark the regions where evaporative cooling is suitable, for standard and for well-insulated white buildings.

Beyond all these regions in which ventilation, building materials or evaporation can be utilized to ensure thermal comfort within a building, mechanical cooling or air-conditioning must be used. On the chart this zone is marked off by AC. Above 17 mm Hg vapour pressure dehumidification is required and below 8 mm Hg some humidification is advisable. Evaporative coolers provide this in­crease in humidity but their cooling effect is insufficient outside the range shown on the chart.

At temperatures between 20° and 27°C and v.p. below 5 mm Hg, extra humidification is required to avoid irritation (zone W). Below the neutral zone (20°C) there is a region in which the indoor minimum temperatures are sufficiently higher than the outdoor minima to make heating unnecessary. The limits of this zone (H and H') depend on the structural properties of the building, and extend to include lower temperatures as the humidity falls. This is because of the higher physiological sensitivity to wet-cold than to dry-cold. Below this region some form of artificial heating is required.

The component parts of the chart described above and shown in Fig. 16.2(a), (b) and (c) are recombined in (d) to give the complete chart, from which the alternative methods suitable for comfort provision, under any given combination of temperature and humidity conditions, can be seen simultaneously.

However, the limits of the chart zone should only be con­sidered indicative of the suitability of these thermal control methods, as inaccuracies arise from local deviations in climatic conditions from those assumed for the construction of the chart, particularly in the temperature range and wind velocity. In addition, the effectiveness of the methods suggested also depends on the design and structure of the building.