The measure of a material or assembly’s ability to limit the amount of water that passes through it by vapor diffusion. The test procedure for determining vapor control layer class is ASTM E-96 Test Method A (the desiccant or dry cup method).

Class I:       Materials that have a permeance of 0.1 perm or less

Class II:      Materials that have a permeance of 1.0 perm or less and greater than 0.1 perm

Class III:     Materials that have a permeance of 10 perms or less and greater than 1.0 perm