Crawl space Covering dirt floor of crawl space

Published on March 18th, 2017 | by Henri


No need to encapsulate crawl space

Q. Eighteen months ago, I bought a house that is built on a crawl space. During my purchase, the inspector was disappointed to see that it was not “encapsulated” and highly recommended that I do so as soon as I can.  In the summer, I can smell an earthy smell and worry about potential mold. Do you recommend encapsulation as the preferred method for preventing odor and/or mold seepage into the first floor? Thanks so much.

A. If by encapsulation, the inspector means lining the floor and walls of the crawl space with a special fabric, I am not in favor of it for two main reasons.

  1. Encapsulation may trap moisture between the fabric and the walls, encouraging the development of mold. Moisture may be coming through the walls, unseen if it is minor, and evaporates into the crawl space now. Soil moisture may also migrate around the fabric, as the fabric is not sealed tightly at the joints of floor and walls. This moisture, and mold, may then travel upward and affect the framing, and migrate into the living space.
  2.  It is very expensive.

The mustiness can be handled by removing any sharp object on the crawl space floor, making sure that it is reasonably smooth, and covering it thoroughly with 6-mil or heavier gauge plastic, which you can buy in building-supply, agricultural-supply and hardware stores. If the soil is not smooth, you may need to spread out two layers of 6-mil plastic or use heavier gauge plastic. The most important thing is to seal the plastic around the perimeter of the crawl space. This is easily done by digging a shallow trench with a garden trowel, temporarily placing the soil on top of the plastic, lining the trench with the plastic and placing the dirt over the plastic.

During my active construction days, I got heavier plastic free from garden centers, which had to replace their greenhouse coverings every once in a while and needed to dispose of it. This was a win/win for both of us.

Once this is done, if there are any vents, close them and leave them closed year-around once the mustiness has dissipated. If it does not do so over a dry fall, you may need to use a dehumidifier until the mustiness is gone, but it should not be necessary.There is no need for ventilation once the floor is fully covered and the soil’s moisture is controlled by the plastic. The soil takes care of itself.

Years ago, building scientists realized that leaving vents open was the reason for rot on crawl-space framing, especially in the southeast where ambient moisture is high in the summer and the hot, humid air is absorbed by the wood framing with no chance to dry out since there is no let up in the humidity. Decay begins over time. In cooler regions, the crawl space vents have often been opened in the moist summer months, then closed in the winter, sealing the moisture in the air.

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