Air Humidity

Indoor air always contains humidity in the form of water vapour. For a comfortable indoor climate the relative air humidity level is decisive. During the heating season 30% to 45 % of relative humidity are ideal. When the building structure is poor, levels above 45 % may already lead to the formation of mould. Levels below 20 % should be avoided for health reasons.

The warmer the air is the more water vapour can it absorb. Absolute air humidity indicates how many grams of water vapour are dissolved in one cubic metre of air. Relative air humidity indicates at what percentage indoor air is saturated with humidity. Air completely saturated with humidity has a relative air humidity of 100 %. Beyond that level air can no longer absorb humidity. If more humidity is added water vapour will condense by forming small droplets. In nature this phenomenon leads to the formation of fog, in heated rooms water vapour from the air condenses on cold surfaces. This can give rise to mould.


The ideal air humidity level is 30 % to 45 %
Relative air humidity is an essential condition for a comfortable indoor climate. The ideal air humidity level during the heating season is a relative humidity of 30 % to 45 %. Values clearly exceeding 60 % of relative humidity are regarded as unpleasant. For reasons of building physics, air humidity should not exceed 55 %. When the building structure is poor, air humidity levels above 45 % may already lead to the formation of mould. While levels below 30 % of relative humidity are outside of the ideal human comfort zone, short-time levels down to around 20 % are no reason for concern. Fundamentally, lower levels of relative humidity are advantageous as they do not allow mite to grow rapidly.



Illustration: Humidity comfort values according to Leusden and Freymark (source: Heinz Gabernig, Energie- und Klimatechnik, 1995 edition; including supplements of the upper and lower limits (winter))