Spray Foam vs Fiberglass Attic Insulation
Direct comparison of R-value ratings, thermal sealing efficiency, and installation costs for attics.
Spray Foam Insulation, Spray Foam, 40 Sq.Ft. (2x27.1 oz.)
BONDURE
Owens Corning R-13 Pink Kraft Faced Fiberglass Insulation Roll 15 in. x 32 ft. (3)
Vigpil
Spray foam and fiberglass both insulate an attic, but they solve fundamentally different problems — and picking the wrong one means paying twice. Fiberglass is a cheap thermal barrier that slows heat transfer but does nothing to stop air moving through gaps. Spray foam is an air seal that happens to insulate. Before you buy a single batt or can, decide whether your attic’s problem is heat or airflow, because that answer determines which material is actually worth your money.
The Core Difference: Insulating vs. Air Sealing
Fiberglass works by trapping still air between glass fibers. It resists conductive heat flow, which is exactly what an R-value measures. But fiberglass is famously poor at stopping air leakage — a batt packed between studs still lets air whistle around its edges, and the R-value collapses when air moves through it.
Spray foam expands to fill the cavity completely, sealing every gap, crack, and penetration as it cures. That’s why closed-cell foam delivers roughly double the R-value per inch of fiberglass, and why it’s the only choice that doubles as an air and vapor barrier.
Head-to-Head
| R-value per inch | Spray foam ~6–7 (closed-cell) · Fiberglass ~3.2 (batts) |
| Air sealing | Foam seals gaps and cracks · Fiberglass does not |
| Vapor barrier | Closed-cell foam acts as one · Fiberglass needs a separate barrier |
| Install difficulty | Foam is best left to pros · Fiberglass is a DIY-friendly weekend job |
| Upfront cost | Foam costs significantly more per square foot · Fiberglass is cheap |
If Your Attic Leaks Air, Buy Spray Foam
An attic with recessed lights, plumbing vents, and wiring penetrations leaks air through a dozen paths a batt can’t touch. In that case, spray foam is the correct — and often the only — fix. It air-seals the thermal envelope, stops the stack effect that pulls conditioned air out of the house, and delivers the highest R-value per inch of any common insulation. The trade-off is cost and application: foam is a professional job, and a bad DIY foam job is a sticky, expensive mess.
Spray Foam Insulation, Spray Foam, 40 Sq.Ft. (2x27.1 oz.)
BONDURE
- SPRAY FOAM INSULATION (40 Sq.Ft. - 2 Pack x 27.1 oz.) HIGH R-VALUE THERMAL & SOUNDPROOF : BONDURE spray foam insulation…
- EXPANDING FOAM INSULATION SEALANT, FILLS BIG GAPS, CRACKS & VOIDS FAST : Our expanding foam insulation sealant expands c…
- FOAM GUN COMPATIBLE, CONTROLLED APPLICATION : Designed for cleaner, easier application: works with most foam guns or str…
- SPRAY FOAM INSULATION FOR ATTIC / BASEMENT / GARAGE / ENERGY SAVER : Use our spray foam insulation for home to improve e…
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If You’re On a Budget With a Standard Attic, Buy Fiberglass
For a vented attic with a clean, open floor and no air-sealing needs, fiberglass batts or rolls remain the rational choice. They’re inexpensive, safe to install yourself, and deliver solid thermal resistance for the dollar. Just understand the limitation: fiberglass insulates, it does not seal. You’ll still need to air-seal the attic floor and ceiling penetrations separately, and the material’s effective R-value drops if it gets compressed or damp.
Owens Corning R-13 Pink Kraft Faced Fiberglass Insulation Roll 15 in. x 32 ft. (3)
Vigpil
- R-13 faced insulation, easy exterior/interior 2x4 wall install
- Fiberglass insulation roll helps maintain temps and control noise
- Pink fiberglass insulation is formaldehyde free and soft to touch
- The greater R-value provides greater insulation performance.
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The Verdict
Air-seal first, then insulate. If your attic has leaks, gaps, and penetrations — and most do — spray foam is the winner because it does both jobs at once. If you have a straightforward, already-sealed attic and a tight budget, fiberglass gets you most of the way for a fraction of the cost. The mistake to avoid is treating them as interchangeable: fiberglass in a leaky attic is money spent on a thermal barrier that the moving air simply bypasses.
Frequently Asked Questions
Is spray foam safe to install myself? The two-component kits are DIY-available, but the failure mode is expensive: improper mixing or application produces foam that doesn’t cure right, and you can’t easily remove it. For anything beyond a small gap-fill, most people are better off hiring a pro. Fiberglass is the genuinely DIY-friendly option.
Does fiberglass insulation actually lose R-value over time? It doesn’t degrade on its own, but it loses effective R-value when compressed, damp, or blown by moving air. Packed or wet fiberglass insulates far less than its rated value — which is why air-sealing is the prerequisite for any fiberglass install.
Which is better for an older home with lots of penetrations? Spray foam, almost always. Recessed lights, plumbing vents, and wiring create air leaks a batt physically can’t cover, and foam is the only one that seals every gap while insulating.
How long does spray foam last? Closed-cell foam is effectively permanent — it doesn’t settle or sag and holds its R-value for the life of the structure. Fiberglass batts, by contrast, can settle over decades if not properly supported.
