Water Vapor Transmission Rate (WVTR) - ASTM F1249
The rate at which water vapor will pass through a material under specified conditions and specimen geometry. The volume of water vapor that will pass through a unit thickness of material per unit area per unit time per unit barometric pressure.
The test specimen is held such that it separates two sides of a test chamber. The "wet side" of the specimen is exposed to a high relative humidity atmosphere, while the "dry side" is subjected to a zero relative humidity atmosphere. Infrared sensors on the "dry side" detect the amount of water vapor present. Testing is complete when the concentration of water vapor in the dry side atmosphere is constant.