Spray Foam vs. Fiberglass Insulation: Cost & R-Value
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Get free quotesSpray foam vs. fiberglass
| Spray foam | Fiberglass | |
|---|---|---|
| R-value per inch | ~3.5–4 (open-cell), ~6–7 (closed-cell) | ~2.2–2.7 (batts), ~2.2–2.7 (blown-in) |
| Air-sealing | Yes — seals as it expands, filling gaps | No — must be paired with separate air-sealing |
| Cost per sq ft (material + install) | $1.50 – $2 (open-cell), $2.25 – $4+ (closed-cell) | $0.50 – $1.50 |
| Best for | Rim joists, irregular cavities, air-leak-prone areas | Standard wall/attic cavities, budget-conscious projects |
| DIY feasibility | Low — typically professional installation only | Moderate — batts are a common DIY project |
Why air-sealing matters as much as R-value
R-value measures resistance to heat conduction, but a lot of real-world energy loss in a home happens through air leakage — gaps, cracks, and penetrations that let conditioned air escape and outside air infiltrate, regardless of how much insulation is nearby. Fiberglass batts installed around a leaky gap don't stop that air movement; they just sit next to it. Spray foam's expanding application fills irregular gaps and cavities as it cures, which is why it performs so well in notoriously leak-prone areas like rim joists, around plumbing and electrical penetrations, and in irregularly shaped spaces where fitting a fiberglass batt cleanly is difficult.
Pro tip from our network contractors
You don't have to choose one material for the whole house — a common, cost-effective approach is spray foam in the highest-leak-risk areas (rim joists, irregular cavities, around penetrations) combined with fiberglass batts or blown-in insulation in standard, regular-shaped cavities where air-sealing is less of a factor.
Where this decision comes up most
This choice most often comes up during an attic insulation upgrade, a basement rim joist project, or when re-insulating walls during a siding or window replacement (since the wall cavity is briefly more accessible). See is it your windows or insulation if you're trying to diagnose a drafty room before deciding where to invest, and why air-sealing matters when re-siding if a siding project has already opened up that opportunity.
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Get free quotesFrequently asked questions
Is spray foam insulation better than fiberglass?
It performs better in leak-prone or irregular spaces because it air-seals as it insulates, but it costs significantly more. Fiberglass is a solid, budget-friendly choice for standard cavities when paired with separate air-sealing.
Does fiberglass insulation stop air leaks?
No — fiberglass resists heat conduction but doesn't block air movement on its own. It needs to be paired with separate air-sealing (caulk, foam sealant at penetrations) to perform at its full potential.
Can I install spray foam insulation myself?
Professional-grade spray foam installation typically requires specialized equipment and safety precautions, making it mostly a professional job. Small DIY spray foam kits exist for minor gaps but aren't a substitute for a full professional application.
Can I mix spray foam and fiberglass insulation in the same house?
Yes, and it's a common cost-effective strategy — using spray foam in high-leak-risk areas like rim joists and irregular cavities, and fiberglass in standard, regular-shaped cavities elsewhere.
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