Condensed Moisture-Another Source of Wood Decay

Build Green: Wood Can Last for Decades warns the homeowner of yet another danger: moisture that can condense, accumulate in wood, and lead to decay in walls, attics, and crawl spaces.The causes of this condensation are a source of moisture, a way for moisture to be transported into wood, and a lack of drying capability.

Many houses are built over a crawl space foundation. Crawl spaces can be built in a number of ways: 1) open pier-and-beam construction; 2) enclosed crawl space with vents; 3) enclosed crawl space without vents. Moisture management differs for each type of crawl space. The primary source of moisture is usually evaporation from damp soil under the house. In general, the soil around the foundation should be graded so that water drains away from the building.

Moisture from soil beneath the house can condense on the subfloor and floor joists in enclosed crawl spaces. Eventually, the subfloor and floor joists can become wet enough to decay in as little as 5 to 15 years.

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Decayed subfloor and floor joists. (Photo provided by Advanced Energy, Raleigh, N.C.)

This problem can be prevented by placing a vapor-resistant covering, such as polyethylene sheeting, over the soil in the crawl-space area.

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Polyethylene ground cover professionally installed. (Photo provided by Advanced
Energy, Raleigh, N.C.)

The vapor-resistant covering should extend at least 8 inches up the foundation walls and any protrusion such as a chimney to minimize moisture as much as possible. Ground covers are recommended for enclosed crawl spaces everywhere except in the driest climates.

Obviously, a ground cover will not be effective against other sources of water intrusion from improper drainage. A ground cover does little good under a house if water collects on top of the cover.

Another potential source of moisture in crawl spaces is humid outdoor air during summer months, particularly for air conditioned houses in humid climates. At times, venting will introduce moisture to the crawl space rather than get rid of it. This can lead to moisture issues when the crawl space is cool relative to the warmer, humid outdoors.


Polyethylene sheeting installed by the homeowner can be quite effective at reducing moisture in a crawl space if care is taken to extend the sheeting up foundation walls and all protrusions, such as a chimney.

Enclosed crawl spaces without vents are designed to keep the humidity out. They rely on supplying conditioned, dehumidified air to maintain a dry crawl space.

Open pier-and-beam construction generally does not require a ground cover because the amount of air flowing under the house is sufficient to carry away excess moisture.

crawlspace and piping

Open pier-and-beam crawl space generally does not need a ground cover. (Photo by Samuel Glass, FPL.)

Like so many of the issues that Carol Clausen and Samuel Glass discuss in this paper, condensed moisture is another potential problem that the homeowner must guard against. Armed with this knowledge, homeowners can keep their houses dry and their wood intact.