The ASTM D6701-21 standard defines a test method for determining the water vapor transmission rate (WVTR) of nonwoven fabrics and plastic barriers. This method is suitable for the measurement of water vapor through various barrier materials with a thickness of not more than 3 mm, including single and multi-layer films, nonwoven fabrics, etc. Water vapor transmission rate is a key indicator for evaluating material shelf life, Stability, vapor transmission rate and wearing comfort. If the buyer and seller agree on the sampling, test conditions and acceptance criteria, the data of this method can be used as the basis for judgment. Standard values are expressed in metric units, usually g/m ² · day.