The weight of water vapor allowed through a tape within a specified time period.
The rate of water vapor transmission through containerboard indicates the ability of the finished container to protect its contents from undesirable effects of high humidity or moisture loss of the product.
is the rate at which moisture will pass through a homogeneous material. The rate may vary on exposure conditions. Water Vapor Transmission is usually expressed as "per-inches" which is the grains of water transmitted per square foot per hour per inch differential of mercury for a one inch specimen thickness. (ASTM C355)