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Q.Where does the vapor barrier go when finishing a basement? Since we use a vapor barrier on the inside of exterior above-grade walls to keep moisture in, it’s my contention that a plastic vapor barrier should be used against the wall to keep out the moisture in the earth. Many disagree and say the basement is no different than the upstairs. My feeling is that the earth’s moisture will penetrate the concrete and will condense on, or at least be trapped by, the outside of the interior vapor barrier, thereby lowering the insulation’s R-value and possibly producing mildew in the wall cavity.

A.Stephen Smulski responds: I agree with those who feel that a basement is no different than an above-grade living space: The vapor retarder should be installed on the warm side of the wood frame wall. Sandwiching the vapor retarder between the insulation and the interior finish ensures that the vapor retarder is always warmer than the dew point. As a consequence, moisture can’t condense on it regardless of whether the water comes from inside the basement or from the soil outside the foundation.

The vapor retarder inside the wood frame wall works in conjunction with dampproofing (a bituminous liquid, for example) applied to the exterior of the foundation to keep the wood frame wall dry. By clogging micropores in the surface of concrete and masonry, dampproofing hinders both vapor diffusion and capillary transport of soil moisture through the foundation.

Placing a vapor retarder under the floor slab does the same thing. As an added precaution, you can apply a low permeability coating on the inside of the foundation before building the wood frame wall. This is no different than laying polyethylene sheeting over a concrete floor slab that lacks a vapor retarder before putting down a finish floor.

Keep in mind that basement moisture problems can be largely avoided by installing perimeter drains, by applying dampproofing, by sealing cracks, by backfilling with free-draining soil, by grading soils so that they slope away from the foundation, and by installing gutters and downspouts.

Stephen Smulski is president of Wood Science Specialists of Shutesbury, Mass., a consulting firm specializing in wood performance problems in light-frame structures.