Q&A: Is a Vapor Barrier Necessary?

A.Corresponding editor Paul Fisette responds: In a heating climate, you should include a vapor barrier on the warm, wide side of the insulation, even if the home has plastic housewrap.

Housewraps and poly vapor barriers are different materials that perform different functions in a wall. Housewraps are installed on the outside surface of the exterior wall sheathing. Housewrap manufacturers often tout them as a barrier to air leakage. While there have been some reports that housewraps do help minimize air leakage, the manufacturers’ claims may be overstated. Many building scientists believe that the holes punched through housewraps by staples, siding nails, and other fasteners compromise the effectiveness of wraps as air barriers. Envelope testing has shown that sealing the interior drywall (airtight drywall approach), or carefully installing a polyethylene barrier under the drywall, and/or sealing the exterior sheathing is a more effective way to minimize air leakage. I think that the greatest value of housewrap is as a redundant weather barrier (see "Housewrap vs. Felt," 11/98); it should be required in all homes.

A layer of polyethylene under the drywall, if very carefully installed (with taped joints and sealed penetrations), is an effective air barrier. However, in most homes the polyethylene is casually installed, and its main function is as a vapor diffusion barrier. Water vapor is transported by two different mechanisms: air movement and vapor diffusion. Air moves in response to air pressure differences. As it moves, it carries with it an assortment of gases, one of which is water vapor. So when air moves from one location to another, it brings water vapor along for the ride.

Water vapor moving via diffusion moves independently of air. Water vapor moves from areas of higher temperatures and moisture concentrations (higher vapor pressure) to areas of lower temperatures and moisture concentrations (lower vapor pressure). That is why we put vapor barriers on the warm side of the wall. Vapor pressure can drive water vapor directly through materials. Materials with low permeability (lower than 1) are classified as vapor barriers because they are considered to have a permeability that is low enough to retard the diffusion of water vapor to negligible levels. While the amount of moisture that is moved by diffusion is relatively small compared to the amount moved by air pressure, it is significant and should be controlled. Usually, the easiest way to control diffusion is by using a low-perm material. Kraft-facings on insulation, polyethylene, and vapor barrier paints are some common materials that function as effective vapor barriers.