Crawl Space Insulation: Fiberglass, Spray Foam, or Rigid Board?

The insulation that came with your Nashville home was almost certainly fiberglass batts stapled between the floor joists. There’s a good chance it’s the wrong product in the wrong place.

The Question Insulation Has to Answer

Crawl space insulation has to do three things: control heat transfer between the conditioned house and the unconditioned crawl, resist the moisture that's typical of crawl space environments, and stay where it was installed for decades.

Different products do those three things very differently.

Fiberglass Batts Between Floor Joists

The default for older Nashville homes. Cheap and easy to install, which is why builders chose it. Long-term performance is the problem.

• The good: Low upfront cost, R-value matches what was sold
• The bad: Absorbs moisture from humid crawl air, loses R-value when wet, sags out of the cavity, becomes home to rodents and insects, traps moisture against the subfloor

Fiberglass in a vented crawl space is essentially a long-term failure mode. By year 10, much of it is on the ground.

Closed-Cell Spray Foam

Sprayed directly onto the crawl space walls (not the floor joists above) as part of an encapsulation approach.

• The good: Air-seals and insulates simultaneously, doesn’t absorb water, doesn’t sag, doesn’t feed pests
• The bad: Higher upfront cost, makes future foundation inspection harder, requires proper installer technique

Spray foam on the walls is the gold standard for encapsulated crawl spaces. It transforms the crawl into a conditioned, insulated extension of the house envelope.

Rigid Foam Board on Walls

Sheets of EPS or XPS attached to the crawl space walls inside an encapsulation.

• The good: Lower cost than spray foam, predictable R-value, doesn’t absorb moisture
• The bad: Requires careful sealing at edges and seams to avoid air bypass, more labor-intensive than spray foam in some applications

Rigid board is a strong choice when budget is a factor or when the geometry of the crawl makes spray foam impractical.

Which Approach Belongs Where

The current building science consensus for Middle Tennessee:

• Vented crawl space (legacy approach): Insulate the floor joists above. Use a vapor-permeable mineral wool or kraft-faced fiberglass with mechanical retention. This is a fallback, not a recommendation.
• Encapsulated crawl space (current best practice): Seal the vents, insulate the walls (not the floor), control humidity with a dehumidifier or supply air. Spray foam or rigid board on the walls.

Mixing approaches — encapsulating but leaving fiberglass in the floor — is usually a transition step, not an end state.

R-Value vs. Air Sealing

Homeowners obsess about R-value. The bigger heat loss in most crawl spaces is air leakage, not conductive loss through insulation. A perfectly air-sealed R-13 crawl will outperform a leaky R-30 crawl every season.

Spray foam wins on this because it does both jobs at once. Rigid board done well wins because the air sealing is at the perimeter and tape joints. Fiberglass loses because it can’t air-seal anything.

Frequently Asked Questions

Should I remove old fiberglass before encapsulating?

Generally yes. Wet, sagging, or moldy fiberglass should come out before encapsulation. Clean and intact fiberglass can sometimes be left, but the new system will work better without it.

How much R-value does a Nashville crawl need?

Code generally calls for R-13 to R-19 on crawl space walls in our climate zone. Higher is better, but the gains diminish above R-19.

Does insulating the walls instead of the floor really work in Nashville?

Yes — and it’s the current standard in our climate zone. The crawl becomes a buffered, conditioned space rather than a hostile environment under the home.

Will encapsulation alone reduce energy bills?

Usually yes, modestly. The bigger savings come from the combined encapsulation + wall insulation + humidity control system rather than any single piece.