The ISO 9932:2021 standard specifies a method for measuring the water vapor transmittance of paper and Cardboard sheet materials, using the dynamic gas method or the static gas method. The dynamic gas method is used for the measurement of water vapor transmittance through continuous air flow, which is suitable for high transmittance materials; the static gas law is used for closed environment measurement, which is suitable for low transmittance materials. The standard applies to paper and Cardboard up to 38 mm thick, and the test results are expressed in g/(m ² · d). This method is widely used to evaluate the moisture resistance of paper and Cardboard, especially in the quality control and product development of Encasement materials.