The Building Foundation Design Handbook (Labs et al. 1988) notes that ground cover membranes such as 6-mil (0.006 in.) polyethylene that restrict evaporation of soil moisture are the single most important way to prevent condensation and wood decay problems in crawl spaces. The ground cover material should have a perm rating of no more than 1.0 and must be rugged enough to withstand foot and knee traffic.

All debris must be removed and the soil should be leveled before laying the membrane. Overlap edges 4 to 6 in., and continue the membrane up the face of the wall to the grade level if the interior grade is below the outside grade.

Traditional building practices and building codes have included provisions for crawl space ventilation. In recent years, some researchers are finding that crawl space vents may not be necessary, especially in southern climates where vents can introduce hot, humid air into the crawl space. As more information becomes available, building codes may change their ventilation requirements. The following vent area recommendations reflect the current prevalent building code requirements.

A rectangular crawl space requires a minimum of two vents, located no farther than 3 ft. from corners. The vents should be as high on the wall as possible to best capture breezes, and landscaping should be planned to prevent obstruction of the vents. The total free (open) area of all vents should be at least 1/1500 of the floor area. In the absence of a ground cover, the vent area should be increased to 1/150 of the floor area.

Ventilation alone should not be relied upon where soils are known to be moist.