GB/T 17146 - 2015 specifies the test method for the water vapor transmission performance of building materials and their products. It provides detailed guidance on the selection and treatment of samples to ensure that the samples are representative. Clarify the specifications and accuracy requirements of various test equipment, such as moisture permeable cups. It covers various test methods such as drying agent method and water method, and standardizes the operation process to strictly control the temperature and Humidity of the test environment. According to this standard, key indicators such as water vapor transmission rate of materials can be accurately determined, providing a scientific basis for building waterproof and thermal insulation design, and helping to improve the moisture resistance and energy saving performance of buildings.