What is the difference between permeance and permeability?
Permeability is the water vapor transmission of a homogeneous material under unit vapor pressure difference between two specific surfaces, per unit thickness. An acceptable unit of permeability is perms-inch: grain-in/h*ft2*in*Hg. Permeability is important for specifying insulation materials. Permeance is the water vapor transmission of a specific sample under unit vapor pressure difference between two specific surfaces. An acceptable unit of permeance is perm: grain/h*ft2*in*Hg. Permeance is important for specifying vapor retarders. It is important to ONLY use permeance when specifying vapor retarders. The industry standard requirements for vapor retarder permeance in mechanical systems are typically 0.02 perms for below 32°F service temperature and 0.03 to 0.05 perms for above 32°F service temperatures. When selecting a vapor retarder, make sure that permeance meets these requirements. Saran* 540 and 560 vapor retarder films easily meet the more stringent of these two requirements (0
