the rate of water vapor transmission per unit area at a steady state through a material, membrane or assembly, expressed in Grain/Square Foot.Hour~lnch Mercury (grain/ft2.h~in Hg [ng/Pa~a~m2]).

The physical property that defines the ease at which water molecules diffuse through a material. It is to vapor diffusion what conductance is to heat transfer. The unit of measurement is typically the "perm."

A unit of measurement for the ability of a material to retard the diffusion of water vapor at 73.4 F (23 C). A perm, short for permeance, is the number of grains of water vapor that pass through a square foot of material per hour at a differential vapor pressure equal to one inch of mercury.

The measurement, in grains per square foot per hour per inch of mercury, of the rate of water vapor being transmitted through a membrane.

an index of a material's resistance to water vapor transmission. (See PERM. )

Permeance, in general, is the degree to which a material admits a flow of matter or energy.