Crawl space Crawl space vapor barrier

Published on February 15th, 2017 | by Henri

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Keeping a crawl space dry

Q.  I have a 4-foot crawl space with a rock base. A few years ago I had insulation installed in the floor joists. There is never any water but the crawl space often smells musty and is noticed in the carpeting under beds and couch.

There are four vents that I open in summer and close in winter. I am having new carpeting installed this week and am wondering if there is anything I can do to prevent this smell from happening again. A friend suggested a dehumidifier in the crawl space but it would be difficult for me to empty out the water. Any suggestions?

A.  A dehumidifier in the crawl space is never a solution in the long run. It can be helpful for a very short period of time to reduce a serious problem.

Please close the four vents year-around. In the summer, they admit a considerable amount of moisture from the outside hot air into a cool space where the relative humidity soars and condensation can occur, causing the mildew smell.

This exterior moisture is absorbed by the framing and, if the insulation was installed by stapling the vapor retarder to the bottom of the floor joists, as is unfortunately commonly done, it is trapping moisture in the joists’ cavities, exacerbating the situation and causing the carpet smell in the poorly-ventilated areas under the beds and the couch.

As you close the vents for the winter, there is no way for the elevated moisture level to dissipate, so it permeates to the floor above as well as in the rest of the house.

If the crawl space floor is thoroughly covered with a plastic sheet, there should be no need to have the vents open at any time.
This is the result of research and recommendations from the NAHB (National Association of Home Builders) Research Foundation. Decades ago, the framing in many crawl spaces, especially in the more southern climates, was found rotting from the practice of ventilating them. Building codes requiring venting of crawl spaces are behind the times.

You mention that your crawl space has a rock base. Do you mean that it is ledge or a layer of stones spread on top of the soil? If ledge, can it be covered with the recommended plastic sheet, as there may be subtle moisture from outside following the ledge inside and evaporating?

If the soil is covered with a stone bed, it may be more difficult to install a plastic vapor retarder, but it should be attempted.
A single layer of six mil plastic may not be effective, as it may be subject to punctures from walking on it. Try two or more layers, or get much heavier plastic from agricultural stores.

During my construction career, I used to get that type of heavier plastic, when needed, from farmers and horticultural centers who had to replace regularly the plastic covering their green houses. I simply asked them to save it for me and would to get it promptly.

An alternative we have used is to rake the stones to one side, install the plastic over the bared earth and cover it with the stones, repeating the process for the other half of the crawl space.

Consider pulling down the insulation to allow the joist spaces to dry. Once the moisture is properly controlled, either reinstall the insulation with the vapor retarder up against the subfloor or follow the better practice of insulating the crawl space walls instead of the joists.

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