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This standard specifies a method based on cup tests for determining the water vapour permeance of building products and the water vapour permeability of building materials under isothermal conditions. Different sets of test conditions are specified. The general principles are applicable to all hygroscopic and non hygroscopic building materials and products, including those with facings and integral skins. Annexes give details of test methods suitable for different material types. This standard is not applicable in the case of test specimens with water vapour diffusion-equivalent air layer thickness values less than 0,1 m, as a result of increasing uncertainty in the measurement results. If the measured water vapour diffusion-equivalent air layer thickness is greater than 1500 m the material can be considered impermeable. The results obtained by this method are suitable for design purposes, production control and for inclusion in product specifications.