This standard describes test methods for the measurement of water vapor transmittance of organic films, which is essential for evaluating the performance of coatings in practical applications. Determination of water vapor transmittance helps to understand the permeability of coatings under different environment conditions, which is particularly important for applications such as exterior wood and masonry. The standard includes two test methods: the cheering method and the wet cup method, which are respectively suitable for measurement under different Humidity conditions. The test results can be used to design and optimize Coatings formulations to ensure the effectiveness of coatings in actual use.