Head-to-Head Comparison · 2026

Spray Foam Insulation vs. Fiberglass Batt Insulation

Our Verdict

Spray foam insulation is the clear winner for crawl space applications. Fiberglass batts in crawl spaces are considered outdated by building scientists because they absorb moisture, sag, and can accelerate the problems they are meant to prevent. If budget is a constraint, a vapor barrier combined with rigid foam board on crawl space walls outperforms fiberglass batts between joists. The only scenario where fiberglass is reasonable is in a fully sealed, dehumidified crawl space where moisture exposure is minimal.

Quick Comparison

Factor Spray Foam Insulation Fiberglass Batt Insulation
Cost Range $1,500 – $7,000 $500 – $3,000
Average Cost $3,500 $1,200
Duration 1-2 days 4-8 hours
Longevity Lifetime (25+ years) 10-15 years in crawl spaces
Best For Sealed or encapsulated crawl spaces, moisture-prone regions, and homeowners seeking maximum energy efficiency Budget-focused homeowners with dry, encapsulated crawl spaces, or as a temporary measure before a full insulation upgrade
Warranty Lifetime material warranty from most manufacturers 10-15 year material warranty

Spray Foam Insulation: Pros & Cons

Highest R-value per inch — R-6 to R-7 for closed-cell
Doubles as an air barrier and vapor retarder, eliminating drafts
Will not absorb water, sag, or support mold growth
Adheres permanently to surfaces and never shifts or falls
Closed-cell spray foam adds structural rigidity to floor joists
Delivers 15-25% energy savings compared to fiberglass
2-3x more expensive than fiberglass batts
Must be installed by a licensed professional with specialized equipment
Off-gassing during curing requires 24-hour vacancy
Makes future joist and subfloor inspections more difficult
Removal is labor-intensive if repairs are needed underneath

Fiberglass Batt Insulation: Pros & Cons

Significantly lower upfront cost — 60-70% less than spray foam
Simple enough for a confident DIY homeowner to install
Easily removed for plumbing, electrical, or structural access
Available at every home improvement store nationwide
Provides R-13 to R-19 in standard batt thicknesses
Absorbs moisture readily and loses insulating value when damp
Gravity and moisture cause batts to sag and fall from joist bays
Wet fiberglass becomes a breeding ground for mold, mildew, and pests
Provides zero air sealing — air moves through it freely
Needs replacement every 10-15 years in crawl space conditions
Widely regarded by building scientists as inappropriate for crawl spaces

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Frequently Asked Questions

Should I insulate crawl space walls or the subfloor between joists?
In a sealed or encapsulated crawl space, insulate the walls. This brings the crawl space into the home's conditioned envelope and is more energy-efficient. In a vented crawl space, insulation goes between the floor joists to separate conditioned living space from unconditioned air below.
Is open-cell or closed-cell spray foam better for crawl spaces?
Closed-cell spray foam is strongly preferred for crawl spaces. It has a higher R-value per inch (R-6 vs. R-3.5), acts as its own vapor barrier, and does not absorb water. Open-cell foam absorbs moisture and requires a separate vapor retarder, adding cost and complexity in an environment where moisture control is critical.
Can I install spray foam over old fiberglass insulation?
No. The old fiberglass must be removed first. Spraying foam over damp or moldy fiberglass traps the moisture and mold against the wood, creating hidden decay. Remove the fiberglass, remediate any mold, let the surfaces dry, and then apply spray foam to clean substrates.
How much will I save on energy bills with spray foam vs. fiberglass?
Homeowners typically see 15-25% heating and cooling savings with spray foam crawl space insulation, compared to 5-10% with fiberglass. The air-sealing properties of spray foam account for most of the additional savings, since air leakage through fiberglass is a major source of energy loss.

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Crawl Space Dehumidifier vs. Ventilation Fan

$1,800 vs. $400 avg.

Encapsulation (Sealed Crawl Space) vs. Traditional Ventilation (Vented Crawl Space)

$5,500 vs. $200 avg.

Full Encapsulation vs. Vapor Barrier Only

$5,500 vs. $2,100 avg.

Interior Drainage System vs. Exterior Drainage System

$3,500 vs. $8,500 avg.

Closed (Sealed) Crawl Space vs. Open (Vented) Crawl Space

$5,500 vs. $200 avg.

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