The natural conditions of relative humidity (RH) and temperature (T) in different parts of a building is rarely constant over time. Instead, RH and T often vary cyclically and may pose a risk of mould growth as these conditions are the two key environmental parameters that controls mould growth. Consideration to both humidity and temperature conditions and the susceptibility to mould in a material enables to minimize the risk of future mould damage. The critical moisture level is to be regarded as a temperature dependant material property. As building mate¬rials vary widely in their resistance for mould growth it is not possible to state a critical moisture level of a material without performing tests. The overall aim of this project was to find a way to determine at which level of RH different building materials tend to develop mould growth in order to provide a basic knowledge of the process of designing buildings with low risk of mould growth in accordance with the Swedish building regulations. The results of laboratory tests, together with results from a long field test formed the basis for a new method. This method is designed to evaluate the critical moisture level for mould growth on clean building materials at one specific temperature. In the method, four sets of 7 specimens for each building material are infested with mould spores and then incubated in four levels of relative humidity at 22°C during 12 weeks. During these weeks mould growth analyses are per¬formed regularly. The critical moisture level at the specific temperature is defined by the tested levels of relative humidity. Critical moisture levels at other temperatures may be calculated by using a formula given in the method.

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