How to Insulate a Crawl Space

The traditional approach insulates the floor above the crawl with batts and leaves the vents open. It works only in dry climates and only if the ductwork is not down there.

The modern approach — closed and conditioned — seals the vents, covers the ground with a 10–20 mil vapor barrier, insulates the stem walls with rigid foam or closed-cell spray foam, and adds a small amount of conditioned air or a dehumidifier. It performs better anywhere ducts run through the crawl.

Grade soil away from the house, extend downspouts, and add a sump if water pools. Insulating over a wet crawl space grows mold, and fiberglass batts in a damp crawl become sagging, moldy mats within a few years.

Clear debris, lay the vapor barrier up the walls 6–12 inches and mechanically fasten it, tape all seams and penetrations, seal the vents with foam board and sealant, then insulate the perimeter walls to R-10 to R-19 depending on climate zone.

Insulate and air seal the rim joist while you are down there — cut rigid foam blocks and foam the edges, or use closed-cell spray foam. It's the leakiest part of most crawl spaces.

Use unfaced batts held tight to the subfloor with support rods every 12–18 inches, insulate and seal the rim joist, and insulate any pipes and ducts in the crawl. Leave the ground vapor barrier in place regardless.