WVTR – Water Vapour Barrier

The abbreviation WVTR stands for “Water Vapour Transmission Rate” and describes how much moisture diffuses through a material per unit of time. The lower the measured value, the better the water vapour barrier. WVTR is typically expressed in g/m²/day under defined test conditions. In practice, WVTR is particularly important whenever moisture uptake can affect product quality, for example through clumping, loss of crispness, or reduced suitability for consumption.

Common test methods include the IR sensor method according to ASTM F1249 (for barrier films) and gravimetric “cup” methods (measuring weight change over time). WVTR measurement depends strongly on temperature and relative humidity, which is why values can only be compared when the test conditions are identical. Therefore, the WVTR value of a material is never static, but always variable and dependent on the test climate (temperature, relative humidity, and test method).

The following values are normalized WVTR ranges for a 25 µm film at 38°C / 90% RH: