This International Standard describes general test methods for determining the water vapour transmission rate of sheet materials by means of a dynamic gas method or a static gas method. Depending on the method and specific apparatus employed, materials up to 38 mm thick and with water vapour transmission rates in the range 0,05 g/(m2 - d) to 65 g/(m2 - d) can be tested. The basis of the function of the instrumental techniques is briefly described. Advice on calibration is given in annex B.