Crawl space Covering dirt floor of crawl space

Published on August 30th, 2017 | by Henri

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Ventilating a crawl space

Q.  Should we ventilate our camp crawl space or not? Our camp measures 24 x 40 feet, has a 22 x 8 foot enclosed porch and seems to have been built sideways along a slight rise of fill adjacent to, or partially over, a ledge. The space has a dirt floor with clearance varying from about 3 to 4.5 feet; the upside foundation is mostly below ground level while the opposite foundation is at ground level. The camp is centrally sited on about .6 acre with lake and marsh along two sides.

About 10 years ago we replaced floor joists and a central beam, which seem to have rotted out (since construction 30 years ago); they run the length of both the camp and porch. At the same time our contractor installed a continuous, construction-grade, plastic mat over the dirt floor. He also mounted an exhaust fan in one of the two openable windows located on opposite walls in the space. When we have lived in the cabin, we’ve ventilated the space with both window and floor fans, and the crawl space and air seemed dry.

Is powered ventilation of the crawl space necessary? Is passive or powered ventilation necessary when the camp is occupied during late spring, summer and early fall? Even when the camp is unoccupied? I’ve encountered conflicting opinions about ventilating crawl spaces, usually without explanations, so I’m equally interested in learning reasons why ventilation is or is not important. Thank you.

A. The rot you experienced was likely due to a previously bare dirt floor. The environment near a lake and a marsh provided ample moisture to contribute to the rotting problem.

Since your contractor laid a heavy plastic over the dirt floor, there should be no need to ventilate the crawlspace.

In fact, research conducted many years ago, in the ‘70s, by the National Association of Home Builders (NAHB) Research Foundation, research shared with me by my good friend, the senior analyst, resulted in the recommendation that a dirt floor crawlspace thoroughly covered with plastic does not need ventilation and that ventilation actually encourages problems.

The reason ventilation was found to cause serious problems, especially in the southeast, is that it brings in a considerable amount of moisture during the warm and hot weather. The framing absorbs this moisture and no ventilation is able to dry the framing to a point below which deterioration occurs.

Consider leaving the windows closed year around, but check the crawlspace when you move in and when you move out: The best test is the “nose knows best.” If the crawlspace smells OK, you have nothing to worry about.

When I built our house in the late ‘70s, following the research mentioned above, we didn’t install any vents in the crawlspace and covered the bare soil with two layers of 6 mil plastic. I checked the crawlspace regularly and it always smelled sweet.

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