The ASTM E398-20 standard provides a Water Vapor Transmission Rate (WVTR) test method using dynamic relative humidity measurement for flexible barrier materials. This method allows the vapor barrier performance of sheets to be evaluated under known and controlled test conditions and enables conversion to results obtained by Miscellaneous test methods such as weight analysis. Testing is suitable for materials that are completely hydrophobic or have at least one hydrophobic surface. Although this method is not suitable for referee testing, it is suitable for material comparison and control testing. Test results are expressed in SI units and it is required that the sealing of the test instrument be maintained during testing to ensure accuracy.