Spray Foam vs Fiberglass Insulation

Spray foam and fiberglass insulation both work — but for very different applications and at very different price points. The right choice depends on where you're insulating and your goals.

Quick comparison

Factor	Closed-Cell Spray Foam	Open-Cell Spray Foam	Fiberglass Batt	Blown-In Fiberglass
R-value per inch	R-6 to R-7	R-3.5 to R-4	R-3.1 to R-3.8	R-2.2 to R-2.5
Air sealing	Excellent	Good	Poor	Poor
Vapor barrier	Yes (at 2”+)	No	No	No
Installed cost	$1.50–$3.50/sq ft	$0.75–$1.50/sq ft	$0.90–$1.50/sq ft	$0.80–$1.50/sq ft
DIY-friendly	No	No	Yes	Yes (with rented blower)
Moisture resistance	Excellent	Moderate	Poor if wet	Poor if wet

Cost comparison per 1,000 sq ft

Option	Installed cost	R-value achieved
Blown-in fiberglass (R-49)	$800–$1,500	R-49
Fiberglass batt (R-19, walls)	$900–$1,500	R-19
Open-cell spray foam (3”)	$2,250–$4,500	R-11
Closed-cell spray foam (3”)	$4,500–$10,500	R-20
Closed-cell spray foam (2”)	$3,000–$7,000	R-13

Spray foam achieves high performance per inch but not per dollar for large areas.

Where each wins

Use spray foam for:

Rim joists — closed-cell spray foam seals the most air-leaky part of most homes in one step
Crawl space walls — closed-cell creates a vapor and air barrier together
Unvented conditioned attics (spraying the roof deck) — spray foam is the only practical option here
Basement walls — closed-cell handles moisture and insulates; rigid foam is an alternative
Sealing penetrations — canned spray foam is the best product for sealing around pipes, wires, and gaps

Use fiberglass for:

Attic floors (vented attic) — blown-in fiberglass at R-49 is the highest-ROI insulation project available
Wall cavities (new construction) — batt insulation is standard; spray foam costs 5–10× more for marginal improvement
Retrofit walls (old homes) — blown-in cellulose or fiberglass is blown through small holes, far cheaper than spray foam
Large attic areas on a budget — blown-in DIY is 50–70% cheaper than professional installation

Air sealing: the most important consideration

The biggest difference between spray foam and fiberglass isn’t R-value — it’s air sealing. Fiberglass batt insulates but does not seal air gaps. Spray foam seals both.

In an average US home, air infiltration accounts for 25–40% of heating and cooling costs. A well-air-sealed house with R-38 blown-in fiberglass will outperform a poorly air-sealed house with closed-cell spray foam in many cases.

The practical solution: Air-seal the critical bypasses (rim joists, top plates, penetrations) with spray foam or caulk, then fill the rest of the attic with blown-in fiberglass. This hybrid approach delivers 80–90% of the air sealing benefit at 30–40% of the cost of spray foam everywhere.

Verdict by situation

Situation	Best choice
Attic floor (adding R-value)	Blown-in fiberglass
Rim joists	Closed-cell spray foam
Crawl space walls	Closed-cell spray foam or rigid foam
Wall cavities (new construction)	Fiberglass batt
Retrofit walls (existing)	Blown-in cellulose
Roof deck (conditioned attic)	Open or closed-cell spray foam
Tight spaces, irregular shapes	Spray foam

Frequently Asked Questions

Is spray foam insulation worth the cost?

For rim joists, crawl spaces, and attic roof decks, closed-cell spray foam provides significant air sealing benefits that fiberglass alone cannot match. The payback period is typically 5–10 years in energy savings. For attic floors, blown-in fiberglass is far more cost-effective.

Can I apply spray foam insulation myself?

Small cracks and gaps: yes, with canned spray foam (Great Stuff). Full-cavity or roof deck spray foam requires professional equipment — DIY spray foam kits exist but are costly ($600–$900 for 200 sq ft) and tricky to apply evenly. Most homeowners hire pros for spray foam.

What R-value does spray foam have?

Closed-cell spray foam delivers R-6 to R-7 per inch. Open-cell spray foam delivers R-3.5 to R-4 per inch. Fiberglass batt is R-3.1 to R-3.8 per inch. Blown-in fiberglass is R-2.2 to R-2.5 per inch.

Is spray foam better than fiberglass for attic insulation?

Depends on where in the attic. For the attic floor (open attic): blown-in fiberglass at R-49 is the most cost-effective option and performs nearly as well as spray foam at a fraction of the cost. For an unvented conditioned attic (spraying the roof deck): spray foam is the right choice because it seals the envelope.

How long does spray foam insulation last?

Spray foam insulation lasts 80+ years and does not settle, sag, or absorb moisture (closed-cell). Fiberglass batt lasts 80–100 years if kept dry and uncompressed. Both are effectively lifetime products under ideal conditions.