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Understand the underside of crawl space moisture

House Doctor | Gary Dymski

August 26, 2004

Q. I live in a split-level home on the South Shore. The front entrance door is on a slab, and the first floor has a living room, dining room and kitchen over a crawl space. My problem is the foyer inside the front entrance. The floor is ceramic tile, and the den to the right has a carpeted floor. To the left of the ceramic floor is an office with a vinyl floor. The tile floor collects large amounts of condensation during humid weather. It has actually puddled in areas, even when there is cross ventilation from the open front and rear sliding door in the kitchen. There is a dehumidifier in the crawl space and ceiling fans in the office and living room.

I am thinking about putting a floating floor, like those manufactured by Pergo, over the tile. Would this reduce the level of condensation? The den and office floors do not seem to have the same condensation problem as the tile. ---V. Lehane, Oakdale

A. The real problem seems to be the moisture buildup in your crawl space. Why not tackle that situation instead of putting in new flooring? Colder floors, like tile, will often develop more visible condensation. Your other flooring is more porous and is probably absorbing more of the condensation. But moisture still is entering your home via the crawl space. To control moisture, a vapor retarder made from polyethylene or heavy plastic sheeting should be spread over the soil in the crawl space. Ventilation also should be present. The number of vents and the amount of exterior air they allow into the crawl space is important. Too much hot, humid air in the crawl space can actually add to the problem. Follow these guidelines:

Without a vapor retarder present, 1 square foot of free vent area is required for every 150 square feet of crawl space ground area.
If a vapor barrier is down, 1 square foot of free vent area is required for every 1,500 square feet of crawl space ground area.
In some vapor barrier installations, if the soil is too moist or too high, a thin layer can be removed. It also can be graded level. The plastic is then spread across the floor. Make sure all debris is removed and you have a flat surface. The plastic sheeting is overlapped by 6 inches and not always laid to the end of the foundation. In homes with wood flooring, some moisture is needed to keep the wood from drying out. Generally speaking, the vapor barrier should cover about 75 percent of the crawl space. To read a fact sheet on crawl spaces, visit the Forest Products Laboratory Web site, http://www.fpl.fs.fed.us/documnts/techline/Crawl_Space_Ventilation.pdf

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Understand the underside of crawl space moisture

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