Determination of the Critical Moisture Level for Mould Growth on Building Materials

Sammanfattning: The susceptibility to mould growth varies between building materials. The factors that most affect mould growth, the relative humidity (RH) and temperature also vary in different parts of buildings. One way of preventing the growth of mould in buildings is therefore to choose building materials that can withstand the expected conditions. It is thus crucial that data are available to allow the correct choices of materials to be made, especially information on the critical moisture level for mould growth is needed. This is the lowest RH at which mould can grow on the building material. In this work, a variety of laboratory studies on mould growth on building materials at different combinations of RH and temperature were performed. Based on the results, the critical moisture level for the tested materials were determined. This made it possible to predict the propensity for mould growth on these materials in parts of building subject to known RH and temperature. To validate these predictions, the same materials tested in the laboratory were exposed to the conditions in three crawlspaces and three attics, with varying RH and temperature, for 2½ years. Good agreement was found between the predicted and observed mould growth. A new test method for determining the critical moisture level of a material was therefore developed based on the results of these studies. It was also shown that this method will make better prediction than traditional mould resistance tests, which evaluate the resistance to mould growth in a “worst case scenario”, i.e. at relatively high RH and temperature. The RH and temperature in buildings fluctuate, as does the length of time that the critical mositure level is exceeded. A simplified approach, considering the cumulative time that conditions had exceeded this level gave sufficient information to validate the laboratory tests. Using this approach will not underestimate the risk of mould growth, but will include a margin of safety. However, it was also shown in this work that to make more precise predictions of the mould growth, the length of the favourable conditions of RH and temperature must also be taken into account. Test specimens of wood were exposed to alternating conditions of favourable and non-favourable RH, either on a short-term basis (12 hours) or a longer term basis (1 week), while maintaining the temperature at a favourable level for mould growth. The results were compared to those obtained following exposure to constant, favourable RH. It was shown that both the cyclic conditions slowed down the process of mould growth on wood; the long-term cycling more than the short term. Fluctuating temperature, while keeping the RH constant at a favourable level, also had an effect on mould growth, as it was slowed down. In order to determine the critical moisture level of a material, it is assumed that this property is the same for all samples from that particular material. Wood is a commonly used building material in Sweden. It is an inhomogeneous material and it was shown in a meta-study in this work that several characteristics of wood affect its susceptibility to mould growth; surface structure, wood species, sawing pattern and if the surface was recently planed or sawn. It is therefore not possible to predict the general susceptibility of wood to mould based only on a few data as it is affected by several parameters and it is therefore probably not possible to determine a general value of the critical mositure value for wood. Also, the susceptibility cannot be described by one single parameter, as it depends also on other parameters.