What Is a Closed (Conditioned) Crawl Space?
A closed crawl space is one with no foundation vents. Its ground and often its walls are covered with a sealed vapor retarder, and a mechanical system keeps the air dry, so the space acts more like part of the house than part of the outdoors.
Explanation
Traditional crawl spaces have wall vents meant to let moisture escape. In a humid climate like North Carolina's, summer air coming through those vents can carry moisture in, not out. It can condense on cool ductwork and framing. A closed crawl space takes the opposite approach and keeps outdoor air out.
North Carolina has its own closed crawl space section in the Residential Code, Section R409. Its main requirements:
- Air-sealed walls (R409.1): Foundation vents are not allowed.
- Ground vapor retarder (R409.2.1): At least 6-mil polyethylene with 12-inch laps. It can extend up the walls as a sealed liner, with a termite inspection gap left below the sill.
- Mechanical drying (R409.5): At least one of four methods: a permanent dehumidifier rated for at least 15 pints a day, conditioned supply air, house air blown in by a fan, or an exhaust fan.
- Combustion safety: Fuel-burning appliances in the space must draw combustion air from outdoors.
"Encapsulation" is the industry's word for this kind of conversion. "Conditioned" usually means the space gets some heating or cooling, typically through the supply-air method. The NC Office of State Fire Marshal has said converting an existing vented crawl space to a closed one requires a permit, whatever the cost.
When to seek help
- Humidity under the house stays high, or you notice musty smells, cupped floors or mold on joists.
- Your ducts or pipes sweat in summer.
- The crawl space has standing water. Drainage has to be fixed before sealing. See sump pump.
- Your home is in a mapped flood zone. Read flood zones and coastal crawl spaces first.
Learn more in NC closed crawl space code provisions and vented vs. closed crawl spaces, or compare encapsulation contractors.