(Water Vapor Transmission Rate)
WVTR (also referred to as MVTR, Moisture Vapor Transmission Rate), is a measure of the rate at which water vapor in the surrounding atmosphere will pass through a paper product. A specified relative humidity, % RH, is on one side of the sample and a desiccant is on the other side. The change in weight of the desiccant over time is a measure of the transmission of water vapor through the substrate. The test is usually run for 3 days, with a measurement taken each day. The water vapor transmission rate is calculated and is reported in g/m²/day. The test is run according to TAPPI Test Method T448 or T464, depending on the specified relative humidity.