A layer property that describes the ease with which vapor molecules diffuse through it. More specifically, vapor permeance 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).