Plastic Soil Covers Reduce the Moisture Content in Basementless Homes

Moisture contents of the sub-floor wooden members in two basementless, air-conditioned, brick veneer houses were studies to determine the extent to which a 6 mil polyethylene soil cover could aid in reducing them. one house (A) had a 2 to 2-1/2 foot crawl space with vents which were blocked by dense plantings of shrubbery; insulation had been installed between the floor joists. The other house (B) had a 3 to 4 foot crawl space, the vents were not blocked, and insulation was not present between the joists. The average moisture content of 22 sampling points on the joists and sill plates in house (A) varied from 13.7 to 18.7 percent during the 12 months prior to the application of a soil cover over about 70 percent of the crawl space area; individual sampling points had moisture contents ranging between 11 and 22 percent. After application of the soil cover, the average moisture content of the joists and sill plates dropped to a low of 10.0 percent. The presence of a soil cover over 70 percent of the crawl space area, under house (A) was associated with an average drop of 3 to 4 percent in wood moisture content, even without good ventilation; the moisture contents of individual sampling points decrease by as much as 8 percent. The average moisture content of the subfloor, joists, and sill plates in house (B) varied from 13.9 to 17.1 percent before the application of a soil cover. After the soil cover was applied, the moisture content of the wood decreased steadily and reached a low of 8.5 percent. It therefore appears that soil covers can aid in decreasing the moisture content of the sub-floor wooden portions of houses even where there is an adequate crawl space and good ventilation.

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