Critical moisture level for mould growth on building materials. Mould can grow on building materials in 75-100% relative humidity (RH) at room temperature. How sensitive materials are to mould growth varies. One way to describe this sensitivity is the critical moisture level, RHcrit, the lowest RH at which mould can grow on a material. The critical moisture level for different material groups was proposed based on the current research situation in 2005, in the report ” Microbiological growth on building materials – critical moisture levels. State of the art” (SP Rapport 2005: 11). Based on new research results, these values are no longer valid. This report provides a general overview of the conditions for mould growth on building materials, focusing on the critical moisture level and the new research results. The main conclusion is that RHcrit is a product-specific property. It is impossible to estimate RHcrit for a product based on that it belongs to a group of materials, such as plaster or wood-based boards. Instead, RHcrit must be determined by laboratory tests for each product. The report also discusses how the results of a laboratory test can be used to prevent mould growth in buildings with known RF and temperature and the benefits of using RHcrit instead of traditional mould resistance tests.