layer property that describes the ease with which vapor molecules diffuse through it. It is defined as the quantity of vapor flow across a unit area that will flow through a unit thickness under a unit vapor pressure difference. It is to vapor diffusion what conductance is to heat transfer. The unit of measurement is typically the “perm” (gr/h•ft2•in. Hg); in metric/SI units, it is stated in terms of ng/(s•m2•Pa).

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