16.3. Required Thermal Properties of Structure in Relation to Climate

The physical properties of principal interest are the thermal resistance and the heat capacity, and their required characteristics depend of course on the external conditions, giving different criteria for hot and cold climates. Where the annual climate includes only one season of thermal stress, the others being within the comfort range, requirements may be determined according to this season alone. Where two or more such seasons occur during the year, for instance a cold winter and a hot summer, the required resistance and capacity should be evaluated for each period and the most extreme values adopted to comply with both requirements.

The functional requirements for hot and cold climates will be analyzed here, some existing regulations summarized and, in the following section, new formulae presented for evaluating the thermal resistance and heat capacity required according to the climatic conditions.

In a hot climate the function of the building envelope is to moderate the daytime heating effects of the external air and solar radiation on the structure and its interior. At the same time, the rate of cooling during the night should not be over-reduced. When some form of mechanical cooling is employed the "cooling load" should be as low as possible, and when the interior is ventilated during the day provision should be made to minimize any elevation of internal surface temperature above the outdoor level caused by solar radia­tion.

In the winter of a cold region, heating is always used and the construction of a building must ensure healthy and comfortable indoor thermal conditions, and a reasonable fuel economy with the heating methods locally employed. Structural specifications should be given in terms of two criteria, based on the severity of the climate:

a. The required thermal resistance of the external walls, and possibly of the windows

b. The permissible overall coefficient of heat loss of the building unit (e.g. apartment).

The required thermal resistance also depends on the heat capacity of the structure, a relationship discussed in greater detail later.

Regulations specifying minimum thermal requirements for buildings exist in many European countries, but the wide differences between specifications in countries of quite similar climate suggest that local tradition played a considerable part in their designation. However, some of these requirements are summarized below.

Austria

In Austria [16.8] the requirements with respect to thermal insulation are based on the average annual minimum outdoor temperature (or the degree-days) of the locality. The standard used in their formulation was the maintenance of an interior air tem­perature of 20"C, and the prevention of surface condensation, which is ensured with the internal surfaces of the external walls at a tem­perature above 10-8°C [16.2]. This last corresponds to the dew point of air at a relative humidity of 55% and temperature 20°C.

The general formula on which the Austrian requirements are based is:

_R ²⁰ ~ '<«) ™in ₌ ²⁰ ~ '(»> -i- _(m2 j, _{d C/kcal)}

5 (20-10-8) 46 ^{1 5} ' '

where R is the required thermal resistance, 20 and 10-8 the tem­peratures specified above, and f₍₀₎ min the outdoor minimum design temperature. The thermal resistance required for roofs is about 1-7 times that for walls.