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Q.Our drywall contractor argues that installing a poly vapor barrier behind the drywall will trap moisture, where it will condense, causing the drywall to degrade. Is there any truth to this claim?

A.Clayton DeKorne responds: The poly isn’t the problem. Moisture in wall cavities is only a problem if it condenses into liquid water. To condense on the poly, the humidity would have to be very high (over 50% relative humidity), and the poly very cold (due to poor insulation or lots of cold outside air leaking into the wall). In this case, you’ll have condensation problems, with or without poly.

To understand this, let’s look at a few principles. Vapor retarders, such as poly, are installed on the warm side of walls to prevent moisture diffusion into the wall cavity where it can condense-and cause moisture damage. Diffusion is the movement of moisture through the tiny pores in a material and is only a problem in homes with high indoor moisture levels. For example, if a homeowner dries laundry or stores firewood in the basement, boils lots of pasta water, has many pets and houseplants, or a large family that takes frequent showers, the indoor humidity will be high, especially in a small house. The best way to prevent moisture problems under these conditions is to remove the source — install a vented clothes dryer, build a wood shed, and install good bath fans and a range hood. A vapor retarder, such as poly, is a second line of defense to keep moisture from seeping into the wall cavity and condensing.

In the vast majority of cases, however, moisture problems in homes are caused by air leakage, rather than diffusion. Warm, moist indoor air that leaks into a wall or ceiling cavity can condense in cold weather when it reaches a cold surface — typically the backside of the exterior sheathing. Or, cold air leaking into a house can cool interior surfaces, causing moist interior air to condense on the inside surfaces — often leading to mildew growth. Wet areas, such as bathrooms, corners in unheated closets, and wall or ceiling areas near band joists, soffits, windows, and doors are among the most vulnerable areas. To prevent these problems, concentrate your efforts on installing adequate wall insulation, good ventilation in wet areas, and air sealing at the gaps around windows, doors, outlets, and the band joist. One way to do this, of course, is to install poly under the drywall, carefully caulking or taping the edges around penetrations. In this case the poly serves as both an air barrier and a vapor barrier.

Clayton DeKorne is senior editor of the Journal of Light Construction.