The rate at which water vapor will diffuse through a unit area of material induced by a unit differential in water vapor pressure. Permeance values are reported for specific thicknesses (usually recommended application thickness). Units are usually grains/ft2•hr•in Hg or perm. SI unit for permeance is ng/s•m2•Pa (1 perm = 57.4 ng/s•m2•Pa).

SPFA believes that the path to a healthful, safe, high-quality and performing SPF installation travels through the hands of a knowledgeable, trained, experienced, and ideally PCP-certified professional contractor.