Since all materials allow some water vapor to diffuse through them to some degree, the term "vapor diffusion retarder" is a more accurate term. The ability of a material to retard the diffusion of water vapor is measured in units known as "perms" or permeability. Any material with a perm rating of less than 1.0 is considered a vapor retarder.
Therefore, Kraft paper coated with asphalt attached to insulation does qualify as a vapor retarder, having a perm rating of 0.40. Insulation faced with aluminum foil has a perm rating of 0.05. When it comes to controlling moisture in insulation, it is also important to understand that effective moisture control includes air sealing around the insulation, not just the use of a vapor retarder.
