This International Standard specifies a method for the determination of the water vapour transmission rate (often erroneously called "permeability") of sheet materials. This method is not generally recommended for use if the transmission rate is expected to be less than 1 g/(m2•d) or for materials thicker than 3 mm. In such cases the method specified in ISO 9932 is preferred. The method cannot be applied to film materials that are damaged by hot wax or that shrink to an appreciable extent under the test conditions used. For some purposes it may be necessary to determine the transmission rate of creased material; a procedure for this is given in annex A.