Crawl Space Insulation
Reviewed by Michael Peterson· Licensed Home Inspector
Proper crawl space insulation keeps floors warmer, reduces energy bills, and improves comfort. Costs range from $1,000–$4,000 depending on size and insulation type.
Key Takeaways
- Crawl space insulation costs $1,000–$4,000 for most homes, depending on material and square footage.
- Sealed/encapsulated crawl spaces should be insulated at the walls (rigid foam or closed-cell spray); vented crawl spaces at the underside of the floor (batts with support).
- Air-sealing the rim joist is the highest-payback single step — it is the largest leak point in most crawl spaces.
- Wet or rodent-contaminated insulation must be removed, not covered — wet fiberglass loses R-value and feeds mold.
Insulation Types & Costs
Fiberglass Batt (Floor Joists)
$1.00–$1.50/sq ftR-Value: R-11 to R-30
Rigid Foam Board (Walls)
$1.50–$2.50/sq ftR-Value: R-10 to R-25
Spray Foam (Walls/Floor)
$2.50–$5.00/sq ftR-Value: R-6 to R-7 per inch
Where to Insulate: Floor vs. Walls
Vented Crawl Space → Insulate the Floor
In a vented crawl space, insulate between the floor joists (the subfloor). Use fiberglass batts or rigid foam boards. The crawl space itself remains unconditioned.
Sealed Crawl Space → Insulate the Walls
In a sealed/encapsulated crawl space, insulate the foundation walls instead. This conditions the entire crawl space and is more energy-efficient overall.
Frequently Asked Questions
What is the best insulation for a crawl space?
For sealed crawl spaces, rigid foam board (XPS or polyiso) on the foundation walls is the best option. It resists moisture, provides high R-value per inch, and does not sag. For vented crawl spaces, closed-cell spray foam on the subfloor is ideal but fiberglass batts are a budget alternative.
Should I insulate the floor or the walls of my crawl space?
If your crawl space is vented, insulate the floor (between joists). If it is sealed or encapsulated, insulate the foundation walls instead. Wall insulation in a sealed crawl space is 15 to 20 percent more energy efficient than floor insulation in a vented one.
How much does crawl space insulation cost?
Crawl space insulation costs $1,000 to $4,000 depending on type and size. Fiberglass batts run $1.00 to $1.50 per square foot, rigid foam $1.50 to $2.50, and spray foam $2.50 to $5.00 per square foot installed.
Should crawl space insulation be replaced if it gets wet?
Yes. Wet fiberglass compresses, loses most of its R-value, and becomes a mold and pest harbor — it does not recover by drying in place. Remove it, fix the moisture source, then reinstall dry insulation (or switch to rigid foam on the walls, which tolerates humidity far better).
What R-value do I need for a crawl space?
It depends on climate zone. As a rule of thumb from DOE guidance: R-11 to R-19 for floors over vented crawl spaces in moderate climates (up to R-25/R-30 in cold ones), and roughly R-10 to R-15 continuous insulation on the walls of sealed crawl spaces. Your local energy code sets the minimum.
Does crawl space insulation actually lower energy bills?
Yes — typically 10–20% off heating/cooling when combined with air sealing, because uninsulated crawl spaces leak conditioned air and chill first-story floors. Insulation alone without rim-joist air sealing captures only part of the saving.
Sources & References
- Insulation — Energy Saver — U.S. Department of Energy
- Seal and Insulate Your Home — ENERGY STAR (U.S. EPA)
- Moisture Control — Indoor Air Quality — U.S. Environmental Protection Agency
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