Critical Moisture Conditions for Mould Growth on Building Materials

Abstract: Materials that are stored or used in damp climates may be subject to mould growth. However, all materials are not equally susceptible to mould growth. For each specific material, there is a critical moisture level. If this is exceeded, there is a risk that mould fungi will develop on the material. In a building, different constructions are exposed to different climatic conditions. To minimise the risk of microbial growth, building materials should be chosen that are tolerant to the expected climatic conditions. In this study, the critical moisture levels for ten building materials were evaluated in constant climates, favourable for mould growth, in the laboratory. Samples of the building materials were inoculated with mould spores and incubated in climate chambers at different relative humidities and temperatures; growth of mould was analysed weekly for at least 12 weeks. In order to verify that the laboratory test is relevant also for real life conditions a field study was performed where pieces of the same materials as in the laboratory test were placed in three outdoor ventilated crawl spaces and three outdoor ventilated attics, where the climate was varying, and mould growth on the test pieces was studied over 2.5 years. Material specific mould growth curves were produced based on critical moisture levels. Overall, there was agreement between the laboratory test and the field study. When the climate in a test site exceeded the mould growth limit curves, there was also mould growth on the test pieces if the time was sufficiently long. 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 that the same test method is used. It is also important to state the temperature at which the critical moisture level applies and how long the material is exposed to the climatic conditions during the test. Antoher conclusion from the study is that although conditions in laboratory studies are simplified and accelerated, the results serves well to indicate mould growth in within a building construction. The methodology, as well as experiences and conclusions, from the laboratory test were used when formulating a new test method for determining the critical moisture level of a building material.