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Q.To dry out damp crawlspaces in Virginia, I have had success closing the crawlspace vents and installing a dehumidifier. Are there any other measures I could be taking? What is an appropriate relative humidity level for a crawlspace?

A.Bruce Davis, senior building science specialist at Advanced Energy in Raleigh, N.C., responds: Before beginning work to dry out a crawlspace, there should be a discussion and agreement (ideally, in writing and signed) between the contractor and the homeowner about the potential for house shrinkage as a result of solving the moisture problem. The next step is to confirm that there is not a liquid moisture problem. Is there a live spring, a plumbing leak, a surface drainage problem, or a hydrostatic source for liquid moisture?

Once those potential moisture sources are removed, you will probably want to seal the crawlspace vents. In the Southeast, outside summer air generally holds more moisture in vapor form than the air in the crawlspace does. When this outside air enters a ventilated crawlspace, it can contact a surface cool enough for condensation to form. Any combustion appliances in a sealed crawlspace should be sealed-combustion units, unless ducted exterior combustion air is provided.

Closing the vents and installing a dehumidifier can control the situation if the moisture flow into the crawlspace is less than the capacity of the dehumidifier to remove it. Dehumidifiers installed in a sealed crawlspace do not use a noticeable amount of energy in the summer and generally do not run during the winter. I advise setting a dehumidifier in the 45% to 50% humidity range. In some sealed crawlspace retrofit jobs, dehumidifiers are used only temporarily, until the excess moisture has been removed.

In addition to sealing the vents, you should consider the following steps:

  • installing a ground vapor retarder (6-mil polyethylene), sealed at seams and piers with fiberglass scrim and duct-sealing mastic, and held in place with spikes;
  • installing a polyethylene vapor retarder on the masonry walls, sealed under the ground poly, leaving a 3-inch termite view strip at the top of the wall; and
  • installing weatherstripping to make the access door reasonably airtight.