The permeance of the materials is important when examining the vapor control strategy of the wall assembly. Materials can be separated into four general classes based on their permeance:

Vapor impermeable 0.1 perms or less (Class I vapor retarder - considered a vapor barrier)
Vapor semi-impermeable 1.0 perms or less and greater than 0.1 perm (Class II vapor retarder)
Vapor semi-permeable 10 perms or less and greater than 1.0 perm (Class III vapor retarder)
Vapor permeable greater than 10 perms (Not considered a vapor retarder)

For unfaced insulation, the permeability is a function of the material thickness. In general most productmanufacturers list the permeance of the material based on a thickness of 1 inch. Increasing or decreasing the thickness of the material will affect the permeance. This can become an issue when using XPS foam insulation. 1 inch of XPS has a permeance of 1.1 perms (borderline Class II and Class III vapor retarder), increasing the thickness to 2 inches decreases the permeance to 0.55 perms (middle of the Class II vapor retarder). Therefore, 1 inch of XPS is considered to be vapor semi-permeable, while 2 inches is considered to be vapor semi-impermeable.

For faced rigid insulation boards (such as foil faced or glass fiber faced polyisocyanurate), the permeance of the facing is often much lower than the permeance of the polyisocyanurate and will govern the overall permeability of the sheathing board. For these products, the permeance will not change with increasing thickness.