The presence of mould can cause problems in the indoor environment, including bad odours, and affect people's well-being and health. Significant amounts are spent annually on repairing moisture and mould damage, so it is important to prevent damage. Knowing the critical moisture condition of the materials in a building envelope makes it possible to assess the risk of mould growth when exposed to moisture and is an important part of carrying out a moisture safety design.
The Swedish Building Regulations (BBR 18 6:52) state that well-researched and documented critical moisture conditions should be used during the design and construction period. If this does not exist, 75% RH should be used as the critical moisture condition for mould growth. Moisture levels in building components vary in different parts of a building, as different activities and premises are more humid than others. Choosing a product that has a higher critical moisture condition than the expected moisture level in the building element reduces the risk of mould growth.
Mould growth in a building is a consequence of moisture either as humidity or the introduction of free water through, for example, water damage or condensation. If the relative humidity RF in a building section exceeds 75% at room temperature, there is a risk of mould growth on the building materials. This level is a general minimum level but not all products will grow mould at this level. This is because the threshold for mould growth on a material varies between different products. Some products can be exposed to high moisture levels without mould growing on them, while others grow at lower levels. This property is described by the critical moisture state of the material with respect to mould growth. The difference between different products' critical moisture conditions for mould growth is that they contain different amounts of organic nutrients for the moulds to grow on.
There are no standardised values for the critical moisture condition for groups of building materials. The critical moisture condition can be affected by various factors, including any additives in the material and layers on the material surface. Research shows that different products within the same material group, such as plasterboard, wood fibre board or wood fibre insulation, can have significantly different properties and limit values for mould growth. Therefore, it is impossible to estimate or guess the critical moisture condition for a product group; it needs to be determined for each individual product within the group.
The critical moisture content of a material is determined by testing the product in the laboratory in a standardised way. According to the test method, the products are tested at four different moisture levels at +22°C The critical moisture state is affected by temperature and if it is colder, more moisture is needed for mould to grow. The method includes an equation to calculate the critical moisture condition at temperatures other than the constant temperature of +22 °C used in the test. It is important to point out that the critical moisture condition of a product is not a measure of how good or bad it is. Instead, it is an aid to making informed decisions on the choice of materials for building elements where the moisture level is known. To reduce the risk of mould-related damage in buildings, more actors in the industry would need to recognise the importance of using critical moisture conditions in the design of buildings. To facilitate this work, more material manufacturers need to choose to test and report the critical moisture condition on their products.
This is an article from our magazine Trävärden, view it here! (Link)