The rate at which water vapor will diffuse through a unit thickness and area of material, induced by a unit differential in water vapor pressure. Permeability values are related to a common thickness and can be used to compare various materials. Units are usually grains•in/ft2•hr•in Hg or perm•in. SI unit for permeability is ng/s•m•Pa (1 perm•in = 1.46 ng/s•m•Pa).