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principles: general considerations

Chapter 16

Principles of Design and Choice of Materials to Adapt Building to Climate

general considerations

The final two chapters present a synthesis of the numerous systems of interaction between the human comfort and the functional require- ments, the architectural features and the characteristics of the struc- tural materials, to form a guide to principles and specifications of building design. '

The general principles and methodology involved in determining the interacting factors as functions of the ambient climatic con­ditions are the considerations of the present chapter. In the following chapter these principles are applied to specific climatic types.

16.1. Range of Climatic Control through Building Design

Specific features of design and of structural materials affect the response of a building to exposure to climatic elements: the quantity of solar radiation absorbed in and penetrating the building, the air and surface temperatures, the air velocity and the vapour pressure. Approximate ranges of variation are summarized below for the relation between indoor and outdoor climatic characteristics.

Solar radiation absorbed in walls

This varies with the absorptivity (colour) of the external surface. Whitewashed surfaces absorb only about 15% of incident radiation; standard light colours, such as cream or light grey, absorb 40-50%, medium dark shades (dark grey, green, red, etc.) 60-70%, and black surfaces 80-90%.

Solar radiation penetrating through windows

This depends on the types of shading employed and to a lesser extent on the composition of the glass. With dark, external shading

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as little as 10% of impinging radiation may enter the building. Using internal shades, 40-70% enters, and without shading the proportion is about 90%.

Internal temperatures

The relation of internal to external air and surface temperatures depends on external colour, materials, size and shading of windows and ventilation; it may be expressed by the amplitude ratio, by the deviation of indoor from outdoor maxima and minima, etc. (see Chapter 6).

Using a white exterior with thick walls and small, closed shaded windows it is possible to reduce the internal range to below 10%, and the internal maximum by 50% of the outdoor temperature range. The minimum is then elevated by about 40%. With the addition of ventilation at night the indoor amplitude is slightly increased, as the minimum is subject to further reduction than the maximum. Possible decrease in maximum is then 60%, and elevation in minimum 20%, of the outdoor range. In arid regions this reduction in maxima may reach 10 deg С (18 deg F).

To the other extreme, with thin, low resistance and capacity walls, externally dark, and/or with large unshaded windows, indoor temperatures are often more than 10 deg С above the outdoor level; if ventilation is also absent, the elevation of indoor above outdoor maximum may rise above 20 deg C. The amplitude of the interior air temperature is then 1-5-3 that out of doors. Temperature fluc­tuations of the internal surfaces are even greater than those of the air, elevations in the maxima reaching 30 deg С and over (e.g. for a single layer sheet roof). But the minimum on these surfaces may fall to 8 deg С below the outdoor minimum (where the climate is dry and nights still).

Indoor air velocity

With effective cross-ventilation, average internal air speeds can reach 60%, and the maximum speed 120%, of the outdoor free wind speed. If there is no cross-ventilation, even when windows are open, the average may only be 15% and the minimum 10%.

Indoor vapour pressure

The vapour pressure in a ventilated building is equal to that outside: In crowded buildings, and in winter when the windows are closed for days on end, the indoor vapour pressure may rise to 7 mm Hg or more above the outdoor level.

The ranges of variation of the indoor climatic elements, which may be expected for different types of buildings, are summarized in Table 16.1.