Air and vapour barriers
The Complete Guide to Smart Vapor Barriers and Vapor Open Membranes to Help Keep Walls Dry
...permeance, often shifting from less than 0.7 perm when dry (Class II vapor retarder) to over 13 perms when wet (vapor open). This large range provides the maximum safety and drying capacity for the wall assembly.
The Perm rating: The perm rating tells you how much water vapor can pass through the material. Seek a memb...
