The susceptibility for mould growth varies among different building materials. One way to describe the susceptibility is the lowest RH at which mould can grow on a specific material, the critical moisture level (RHcrit). Determining RHcrit for materials provide the basis for material choice in designs where moisture and temperature conditions are known. In this study, RHcrit of 21different products were determined according to SIS-TS 41:2014/SPMet 4927. This test method is developed based on the results of a variety of laboratory studies and validated by field studies. Test specimens were inoculated with a suspension containing spores from six different mould fungi and were then incubated in moisture chambers at four levels of RH at 22 °C. After 12 weeks specimens were analysed for mould growth. RHcrit was determined based on the lowest RH at which mould grew on the specimens. RHcrit varied among different products, even between product belonging to a similar group of material, for example, calcium silicate boards or gypsum boards. The results show, and confirm, previous findings that it is not possible to estimate RHcrit for a specific product based on material group. Instead, each product must be tested. © The Authors