The measure of a material or assembly’s ability to limit the amount of water that passes through the material or assembly 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