This International Standard specifies a method for determining the water-vapour transmission properties of coatings of paints, varnishes and related products. It supplements ISO 12572. As far as possible, the procedure, the definitions and the calculations have been taken over from ISO 12572. It is recommended that ISO 12572 be consulted, if necessary, to obtain a better understanding of the procedure specified in this International Standard. Water-vapour transmission rates of more than 680 g/(m2·d) (i.e. water-vapour diffusion-equivalent air layer thicknesses, sd, of less than 0,03 m) will not be accurately quantified by the test method described in this International Standard.