Indoor air design conditions

Indoor air design conditions are required for building physical analyses, which have been determined by the Building Physics research group during several research projects since 2002. The key design conditions are indoor temperature, moisture regain and pressure difference.

Temperature

In the buildings where the indoor temperature is the standard ambient temperature, the temperature recommended to be used in analyses as the indoor temperature is constant 21 °C during all seasons. In reality, during the summer the indoor temperature is higher, but this design temperature also takes into account the effect of indoor cooling. This indoor temperature design value is also presented in the current RIL 107-2012 Water and moisture insulation instructions of buildings (in Finnish). In other buildings the indoor temperature used in the analyses must be estimated case-by-case.

Moisture regain

The indoor air moisture regain design values recommended in different building types are presented in the picture and table below. These same tables are also included in the current RIL 107-2012 Water and moisture insulation instructions of buildings (in Finnish). In the buildings of the indoor air moisture class 1 the moisture regain value is estimated case-by-case. In this case the summer and winter conditions should be considered separately.

Building types belonging in indoor air moisture classes determined based on indoor air moisture regain. 1) In cases belonging in indoor air moisture class 1 the moisture regain and temperature should always be estimated case-by-case during the design process. The moisture regain can alternate between 6-20 g/m3 depending on the use of the building. 2) In cases belonging in indoor air moisture class 3 the hygrothermal design process is done using the moisture regain value 3 g/m3 in the winter, unless it can be reliably proven that a smaller moisture regain is adequate in the building analysed. 3) When designing a building it must be taken into account that the use of the building might change later, in which case the moisture class might also change. 4) When choosing the moisture class of cooled spaces it must be taken into account that the indoor air moisture regain might rise high as a result of the possible changes in the indoor temperature. Ice arenas and other cooled exercising spaces, whose temperature is occasionally raised high and which are occasionally used in moisture class 1 conditions, belong in moisture class 1. 5) When designing the envelope structures of cooled spaces the water vapor coming in from the outside should also be taken into account, since it might cause condensation of moisture and conditions favorable to mould growth near the surface of the structure.

When designing a building not presented in the above table, one must assess which building type in the table corresponds to the analysed building the best when it comes to indoor air conditions, and to choose the moisture class accordingly. In uncertain cases it is recommended to choose the moisture class with the larger moisture regain.

ATTENTION! the moisture regain design values mentioned earlier deviate from the values presented in the international SFS-EN ISO 13788 standard, but according to research performed these moisture regain design values better correspond to moisture regains in Finnish buildings.

Pressure difference

When studying the moisture moving into the structures through air leakages, the design value that can be used on the inside of the building during all seasons is +10 Pa of overpressure. When examining the effects of an air flow moving from the outside in on the performance of the structures, the design value to be used indoors is -15 Pa of underpressure during all seasons.