The old-fashioned way to build and treat crawl spaces, opening the vents and lining the floor with plastic sheets, was proven to be not only inefficient, but also disastrous. Modern building science concluded that this type of treatment was either causing or aggravating the common crawl space problems so prevalent across the U.S.

Crawl spaces get moisture from two sources: groundwater and outside air. The typical plastic floor cover doesn’t completely isolate the crawl space from the ground, so groundwater is still evaporating into the space. To make matters worse, it tends to rip, tear, and crumble over time.

Crawl space ventilation, once thought to help dry things out, is actually allowing humid air from the outside in. That is especially bad during the summer, because as humid air enters the cool crawl space, relative humidity levels reach above 100%, at which point condensation starts to occur. Wood beams and joists soak up the water, and the moist, softwoods invites mold, dry rot, and termites.