Irrespective of their air leakage rate, permeable air barrier membranes are categorized according to their water-vapour permeance. The degree to which water vapour passes through a wall is specific to each type of air barrier. The diagrams below show the effects of water-vapour diffusion, per type of air barrier, for hot summer weather and cold autumn temperatures. Note that the more permeable an air barrier, the more it allows water-vapour diffusion in the assembly.