Crawl space encapsulation
Also called: sealed crawl space, conditioned crawl space, closed crawl space
Sealing a crawl space off from the ground and the outdoors: a heavy reinforced vapor barrier over the floor and up the walls, sealed foundation vents, insulated perimeter walls, and usually a dehumidifier. It turns a vented crawl space into a dry, semi-conditioned space. Costs $3,000–$15,000.
A traditional vented crawl space relies on outdoor air to keep it dry, which works in dry climates and fails in humid ones — summer air condenses on cool surfaces under the house. Encapsulation solves that by treating the crawl space as part of the house: keep the moisture out, insulate the walls instead of the floor, and control humidity mechanically.
A typical job: 12–20-mil liner on the floor and walls with sealed seams, foam board or spray foam on the walls, sealed vents and access door, a rim-joist air seal, and a dehumidifier or a small supply of conditioned air. Ducts and pipes in the space end up inside the thermal envelope, which is a large part of the energy benefit.
It’s the right answer for many homes in the Southeast and for cold-climate homes with HVAC in the crawl space. It’s also heavily marketed, sometimes to homeowners whose dry, well-vented crawl space didn’t need it. An energy audit ($200–$500) before committing is money well spent.