This Technical Specification describes a laboratory test method for assessing the critical moisture levels at one specific temperature after 12 weeks of humid exposure regarding mould growth on building materials. The results can be used to theoretically estimate critical moisture level at any other temperatures than the tested. The method is applicable for clean materials that have not previously been exposed to conditions that will enable mould growth. The method — is not applicable for prediction of mould growth beyond the time tested in the laboratory; — do not consider health effects of mould exposure in the indoor environment; — cannot be applied to outdoor building structures, e.g. facades, fences etc., where rain, wind, radiation from the sun etc. may affect mould growth; — do not include growth of e.g. bacteria, although this also contribute to the microbial fouling of materials in damp buildings.