Insulation in a crawl space is one of those things that looks correct from a distance and turns out to be the cause of the problem on closer inspection. Five mistakes we find most often in Western NC crawl spaces.
1. Fiberglass batts in a vented crawl
Fiberglass is a moisture sponge. In a humid, vented crawl space it absorbs water, sags, and holds dampness against the subfloor — the worst possible outcome. Remove it.
2. Vapor retarder facing the wrong direction
Faced insulation has a vapor retarder, and the side it faces matters and varies by climate. In WNC, if faced insulation must be used, the vapor barrier should face the warm-in-winter side — but in an encapsulated crawl, faced batts are not needed at all.
3. Insulating the floor when the walls should be insulated
In an encapsulated crawl, the foundation walls should be insulated (typically with 2” closed-cell foam board), not the floor joists. Insulating the joists in an encapsulated space is double work for half the result.
4. Spray foam without a code-compliant ignition barrier
Open-cell spray foam in a crawl space requires an ignition or thermal barrier in most jurisdictions. Skipping it is a code violation and a fire risk.
5. Insulating before fixing the moisture problem
Insulation is a multiplier for whatever conditions exist behind it. Insulating over a wet, moldy crawl insulates the mold and makes it harder to detect. Always fix the moisture first.