How To - Address Moisture in a Crawl Space

Q: I live in a split-level house with an unfinished crawl space below the living room. I want to replace the carpet in this room with engineered hardwood. When I consulted a contractor, he measured the moisture level at one of the vents in my floor and told me that he cannot install hardwood because the moisture level is too high. Can I use a vapor barrier? Are there any other remedies for this problem?

Herndon

A: If the contractor merely checked for moisture at vents, you might want to have another contractor come and do a more thorough assessment. Frank Kroupa, a technical adviser for the National Wood Flooring Association, a trade group, says contractors are supposed to check the moisture readings in the plywood subfloor. Perhaps that is what the contractor was trying to get to at the vent. If the reading is high, or from 8 to 12 percent depending on the area of the country, the contractor should look under the house and assess what's going on.

If the crawl space does have a moisture problem, you can address it by diverting gutter water farther away and converting the crawl space to something that functions more like a basement. That means spreading thick (at least 6mm) plastic over the floor and up the foundation walls six inches above the level of the surrounding soil. You'll also need to close off vents, which allow humid air to rush in during the summer. Dominion power company in Virginia provides good instructions, with diagrams, in a document titled "Crawl Space Moisture Control." Go to its Web site, http://www.dom.com, and search for "crawl space." You'll also find information about insulating foundation walls.

Once you're done, have a flooring contractor recheck moisture levels. To avoid buckling or gaps, the wood flooring association recommends that the moisture level of the flooring be within 2 to 4 percentage points of the moisture level in the subfloor at the time of installation. The flooring is typically more dry. A vapor retarder, such as a 15-pound building felt, goes between the subfloor and the flooring. Don't use a vapor barrier, such as plastic, however.

Jeanne Huber

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