(1) The rate of water vapor transmission per unit area at a steady state through a material, membrane or assembly; (2) the time rate of water vapor transmission through unit area of flat material or construction induced by unit vapor pressure difference between two specific surfaces (values in reference sources are quoted for specific material thicknesses) under specified temperature and humidity conditions. The English (inch x pound) unit of measurement for permeance is (grains)/(h x ft2 x in. Hg), which is commonly referred to as perm units.

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