Laboratory study to determine the critical moisture level for mould growth on building materials

The susceptibility of building materials to mould growth varies. Some are tolerant to high relative humidity in the ambient air without mould growth occurring, while others are less tolerant, and mould can grow in relative humidity as low as 75%. Within a building, constructions are exposed to different temperatures and relative humidities. To minimise the risk of microbial growth, building materials should be chosen that are tolerant to the expected conditions. In this study, the critical moisture levels for ten building materials with a range of expected critical moisture levels (wood-based materials, gypsum boards and inorganic boards) were evaluated. Samples of the building materials were inoculated with spores from six species of mould fungi (Eurotium herbariorum, Aspergillus versicolor, Penicillium chrysogenum, Aureobasidium pullulans, Cladosporium sphaerospermum, Stachybotrys chartarum) and incubated in test cabinets at specified temperature (10 °C and 22 °C) and relative humidity conditions (75–95%); growth of mould was analysed weekly for at least 12 weeks. One of the conclusions is that two similar building materials or products may have considerably different resistance to mould growth, and so the results from one type of building material cannot be applied to the other. Also, in order to compare results from different tests, it is important to use the same test method. It is also important to state the temperature at which the critical moisture level applies and how long the material is exposed to the temperature and relative humidity conditions during the test.

► Mould growth on ten building materials was studied.
► The critical moisture level (RHcrit) for each material was determined.
► It is important to state the temperature and incubation time when reporting RHcrit.
► Results from one material cannot be applied to another, similar material.