Crawl Space Insulation: Types, Costs & Which Is Best

Insulation is a critical but often misunderstood component of crawl space work. The wrong insulation in a crawl space can actually cause more problems than having none at all — trapping moisture, harboring mold, and falling from between joists in soggy clumps.

This guide compares the three main insulation types used in crawl spaces, with real cost data, performance specs, and clear guidance on which one fits your situation.

Why Crawl Space Insulation Matters

An uninsulated crawl space allows heat to transfer freely between your home and the ground below. In winter, cold air chills your floors and increases heating costs. In summer, heat radiates upward and makes your AC work harder.

The Department of Energy estimates that proper crawl space insulation can reduce energy costs by 10–20% — a savings of $200–$500 per year for the average home. But more importantly, the right insulation in a crawl space also serves as part of your moisture control strategy.

The key question isn’t whether to insulate — it’s where and with what.

Insulation Placement: Joists vs. Walls

Before comparing materials, you need to understand the two insulation strategies:

Between Floor Joists (Traditional)

Insulation is installed between the floor joists in the ceiling of the crawl space, directly beneath your living area. This was the standard approach for decades in vented crawl spaces.

Pros: Keeps heat in the living space, works with vented crawl spaces Cons: Doesn’t condition the crawl space itself, prone to falling down, exposes pipes to freezing temperatures

On Foundation Walls (Modern Best Practice)

Insulation is applied to the interior of the foundation walls, making the crawl space part of the conditioned building envelope. This is the preferred approach for encapsulated crawl spaces.

Pros: Protects plumbing from freezing, conditions the entire space, more energy efficient overall Cons: Requires encapsulation to work properly, higher upfront cost

Bottom line: If you’re encapsulating, insulate the walls. If you have a vented crawl space that will remain vented, insulate the joists. Never insulate both — it creates a moisture trap.

Fiberglass Batts

The pink or yellow rolls you see at every hardware store. Fiberglass has been the default insulation for decades, but it’s the worst choice for most crawl space applications.

Specifications

• R-value: R-3.0 to R-3.8 per inch
• Cost installed: $0.50–$1.50 per square foot
• Typical thickness: 3.5” (R-11) to 6” (R-19) in joist bays
• Moisture resistance: None — absorbs water like a sponge

Advantages

• Cheapest upfront cost
• Widely available
• Easy to install (initially)
• Good for temperature-only applications in dry conditions

Disadvantages

• Absorbs moisture — wet fiberglass loses essentially all insulating value and becomes a mold incubator
• Falls down — gravity and moisture cause batts to sag and eventually drop from joist bays. Drive through any neighborhood and look under the older homes — fallen fiberglass insulation is epidemic
• Pest habitat — rodents love nesting in fiberglass
• Not suitable for wall application in crawl spaces

When Fiberglass Makes Sense

Honestly, rarely in a crawl space. If your crawl space is in an extremely dry climate like Arizona or Nevada, stays consistently below 50% humidity, and you need basic joist bay insulation on a tight budget, fiberglass can work. In every other scenario, the other options are better investments.

Closed-Cell Spray Foam

The premium option and the best performer for crawl space applications. Closed-cell spray foam is applied as a liquid that expands and hardens into a rigid, air-tight insulation layer.

Specifications

• R-value: R-6.0 to R-7.0 per inch (highest of any common insulation)
• Cost installed: $1.50–$3.50 per square foot (1” thickness)
• Typical application: 2–3 inches on foundation walls
• Moisture resistance: Excellent — acts as its own vapor barrier

Advantages

• Highest R-value per inch — 2” of closed-cell spray foam provides R-13, matching 3.5” of fiberglass
• Vapor barrier built in — closed-cell foam at 2” thickness has a perm rating below 1.0, qualifying as a vapor retarder
• Air sealing — expands to fill every crack and gap, eliminating air infiltration
• Structural reinforcement — adds racking strength to foundation walls
• Won’t absorb water — closed cells are impervious to moisture
• Pest deterrent — provides no food source or nesting material

Disadvantages

• Highest cost — 2–3x more expensive than other options
• Professional installation required — specialized equipment and training needed
• Difficult to inspect — once applied, you can’t see the wall behind it (termite inspection concerns)
• Off-gassing during installation — requires ventilation and temporary evacuation

When Spray Foam Makes Sense

Spray foam is ideal for encapsulated crawl spaces where you’re insulating the foundation walls. It’s particularly valuable in cold climates like Minnesota, Michigan, and New York where maximum R-value matters most. The combined insulation + vapor barrier function means you may save on the vapor barrier component of encapsulation.

Open-Cell vs. Closed-Cell

Open-cell spray foam (R-3.5 per inch, $0.75–$1.50/sq ft) is cheaper but absorbs moisture and should not be used below grade or on crawl space walls. Open-cell is only appropriate for joist bay applications in dry environments. Always specify closed-cell for crawl space wall insulation.

Rigid Foam Board

The middle ground between fiberglass and spray foam. Rigid foam boards (XPS or polyiso) are installed against foundation walls with adhesive or mechanical fasteners.

Specifications

• R-value: R-5.0 per inch (XPS) or R-6.5 per inch (polyiso)
• Cost installed: $1.00–$2.50 per square foot
• Typical thickness: 1.5–2 inches
• Moisture resistance: Good to excellent (XPS is better below grade)

Advantages

• Good moisture resistance — XPS (extruded polystyrene) doesn’t absorb water
• Consistent R-value — maintains performance even in humid conditions
• DIY-friendly — can be cut and installed without specialized equipment
• Moderate cost — less expensive than spray foam with comparable performance

Disadvantages

• Seams must be sealed — unlike spray foam, boards have joints that allow air passage
• Doesn’t fill irregular surfaces — gaps behind boards can harbor moisture and pests
• Adhesive compatibility — must use foam-compatible adhesive (solvent-based products dissolve foam)
• Termite pathway concern — insects can tunnel behind or through foam boards in some climates

Which Foam Board Type?

• XPS (Extruded Polystyrene) — pink or blue boards, R-5/inch. Best for below-grade applications because it absorbs almost no water. This is the right choice for crawl space walls.
• EPS (Expanded Polystyrene) — white beadboard, R-3.8/inch. Cheaper but absorbs more moisture. Acceptable but not ideal for crawl spaces.
• Polyiso — foil-faced, R-6.5/inch. Highest R-value but performance drops in cold temperatures and it absorbs moisture at the edges. Best for above-grade use; not recommended for crawl space walls.

Cost Comparison Summary

For a 1,000 sq ft crawl space with foundation wall insulation:

Type	Material Cost	Installed Cost	R-value (2”)	Lifespan
Fiberglass batts (joists)	$300–$600	$500–$1,500	R-6 to R-7	10–15 years*
Rigid foam board (walls)	$600–$1,200	$1,000–$2,500	R-10	25+ years
Closed-cell spray foam (walls)	—	$1,500–$3,500	R-12 to R-14	25+ years

*Fiberglass in crawl spaces often fails within 5 years due to moisture and gravity, even though the material itself can last longer in dry conditions.

Our Recommendation

For most homeowners doing crawl space encapsulation:

Best value: 2” rigid XPS foam board on foundation walls, properly sealed at joints with foam-compatible tape. Provides R-10, excellent moisture resistance, and costs roughly half of spray foam.

Best performance: 2” closed-cell spray foam on foundation walls. Provides R-13+, acts as its own vapor barrier, and seals every gap. Worth the premium in very cold climates or homes with high energy costs.

Skip it: Fiberglass batts in the joist bays of an encapsulated crawl space. If you’re encapsulating, the walls should be insulated instead.

The right choice depends on your climate, budget, and whether you’re doing a full encapsulation. A good contractor will recommend the appropriate insulation type for your specific situation. Get free quotes to compare recommendations and pricing from local professionals.