Best Insulation for Crawl Spaces

Vented vs. encapsulated

Old building code required crawl space vents. Modern building science has reversed that — vents pull humid outdoor air into the crawl, condense on cold ducts and floor joists, and cause rot.

Encapsulation (vapor barrier on the ground, sealed walls, dehumidifier) plus insulation on the perimeter walls outperforms vented + batt insulation almost everywhere except the driest desert climates.

Where to put the insulation

Encapsulated crawl space: insulate the foundation WALLS, not the floor above. Closed-cell spray foam or rigid foam board work best.

Vented crawl space: insulate the floor above (between joists). Use fiberglass batts with kraft facing UP (toward the heated rooms) or mineral wool. Replace any sagging insulation.

Why closed-cell spray foam dominates here

Tight, water-resistant air seal on the foundation wall

Won't fall down like batts inevitably do in a crawl

Acts as a vapor retarder

R-13 to R-19 in just 2-3 inches

Lasts the life of the home

Typical scope and cost

Encapsulation alone: $3,000-$7,000

Encapsulation + perimeter spray foam: $5,000-$12,000

Batt installation in a vented crawl: $1,500-$3,500

Most homeowners notice warmer floors within 24 hours of completion.