This International Standard specifies a method of determining the water vapour transmission rate, water vapour permeance, water vapour permeability and water vapour diffusion resistance index for rigid cellular plastics. The scope of this method provides for the testing of rigid cellular materials that have thicknesses from 10 mm upwards and which may, as an integral part of the material, contain natural skins or adhered facings of some different material. Three different sets of temperature and humidity conditions are provided, as follows: a) 38 °C and a relative-humidity gradient across the test specimen of 0 % to 88 %; b) 23 °C and a relative-humidity gradient across the test specimen of 0 % to 85 %; c) 23 °C and a relative-humidity gradient across the test specimen of 0 % to of 50 %. The results obtained by this method are suitable for design purposes and production control, and for inclusion in product specifications. The method is suitable for materials which have water vapour transmission rates in the range 3 ng/(m2·s) to 200 ng/(m2·s).