"JG/T 309-2011 Determination and Classification of Water Vapor Transmittance of Exterior Coatings" specifies the determination method and classification standard of water vapor transmittance of exterior coatings, including terms and definitions, measurement principles, required instruments and materials, specimen processing steps, Accuracy of result presentation, grading standards and test report requirements. This standard applies to exterior coatings on porous substrates such as brick, stone and concrete used in industrial and civil buildings. Coating products that do not form a film layer or affect the porosity of porous substrates are excluded from the standard. During the implementation process, a number of necessary normative documents are cited to ensure the accuracy and comparability of the measurement results.