Crawl space encapsulation

What it is

Researchers use "closed", "sealed", "unvented" and "conditioned" for the same idea: a crawl space with no ventilation to outside air (Advanced Energy).

The 2024 International Residential Code sets the floor for an unvented crawl space: exposed earth covered with a continuous Class I vapor retarder, joints overlapped 6 inches and sealed or taped, and edges run at least 6 inches up the stem wall and sealed to it (ICC, 2024 IRC chapter 4). The same code lets a dehumidifier sized to the manufacturer's specifications serve as the drying method (2024 IRC).

A vented crawl space under the same code needs at least 1 square foot of net vent area for every 150 square feet of floor, with a vent within 3 feet of each corner (2024 IRC). EPA treats a non-vented crawl space like a basement, except that a plastic or rubber membrane can stand in for the concrete slab (U.S. Environmental Protection Agency).

Does it work

In Advanced Energy's field study of 12 houses in the humid 2003 summer, the closed crawl spaces sat above 70% relative humidity only 5% of the time. The wall-vented ones sat above 70% for 98% of the time and above 80% for 86% (Advanced Energy).

Climate decides how much it matters. In the dry West and the marine Pacific Northwest, DOE researchers found vented and closed crawl spaces both performed adequately, as long as the ducts weren't in the crawl space (DOE Building America).

Where the ground is wettest

Oak Ridge National Laboratory's modeling found a sealed crawl space depends on an effective vapor retarder on all ground and wall surfaces; without one, moisture can build up, "especially when the ground water table level is high" (Oak Ridge National Laboratory). These counties with 20,000 or more homes have the largest share of land where USDA rates a shallow water table a limit for basements:

What it costs

Published ranges, newest first. Forbes Home's average sits far above the others, and Fixr gives two different ranges on two pages, so compare more than one.