The ISO 7783-2018 standard specifies a method for the determination of water vapor transmittance of Paints and varnishes using the wet cup method. This standard applies to both self-supporting and non-self-supporting coatings and describes the process for the measurement of water vapor transmittance of coatings under specific Humidity conditions. Water vapor transmittance is a key indicator for evaluating the moisture resistance performance of coatings. The standard states that water vapor propagation under high Humidity conditions is particularly important, so the wet cup method is used as a reference method. This method helps to evaluate the performance of coatings in the actual use environment.