many of these materials are only vapor retarders as they have varying degrees of permeability.
Materials have a moisture vapor transmission rate that is established by standard test methods. One common set of units is g/m²·day or g/100in²·day. Permeability can be reported in perms, a measure of the rate of transfer of water vapor through a material (1.0 US perm = 1.0 grain/square-foot·hour·inch of mercury ≈ 57 SI perm = 57 ng/s·m2·Pa). air barrier fil