Spray Foam vs Blown-In Insulation: Cost & R-Value

⚖️ The Four Materials at a Glance

Prices last reviewed July 2026. Installed insulation cost swings with assembly, access, and how much material a crew applies — treat these as wide relative ranges, not quotes. For a size-specific estimate, use the insulation cost guide.

💵 Cost & the Board-Foot Trap

On a vented attic floor the math isn't close: blown cellulose or fiberglass reaches R-49 for roughly $1–$3 per square foot installed, while spraying the roof deck to insulate a whole conditioned attic runs about $2–$6 per square foot — typically 2–4× more. Foam's premium buys air-sealing and vapor control you often don't need on an already-air-sealed ceiling plane.

🌬️ Air-Sealing & Vapor — the Real Reason to Pay for Foam

This is what the money buys. Spray foam is an air barrier — closed-cell qualifies at ≥1.5", open-cell at ≥5.5" — so it insulates and air-seals in one pass. Closed-cell is also a Class II vapor retarder at ≥1.5", which is why it's the go-to for rim joists, crawlspaces, and below-grade walls where you want to stop both air and moisture.

Blown-in does neither on its own: loose-fill on an attic floor gives minimal air-sealing (dense-pack in walls does reduce infiltration, but it's still not a code air barrier), and both cellulose and fiberglass are vapor-open. That's not a flaw — on a vented attic you air-seal the ceiling separately and then bury it in cheap R. Per ENERGY STAR, air-sealing plus insulation saves about 15% on heating and cooling; skipping the air-seal step is the single biggest way to waste a blown-in job.

🧩 Match the Material to the Assembly

There is no single winner — the right answer changes by location:

• Vented attic floor → blown-in (best $/R by far; air-seal the ceiling first).
• Unvented / conditioned attic or cathedral ceiling → spray foam at the roof deck (moves the thermal boundary; governed by IRC R806.5).
• Closed existing walls (retrofit) → dense-pack cellulose (drill-and-fill to ~3.5 lb/ft³).
• Rim / band joists → closed-cell foam (the building-science "critical seal" — air, vapor, and R in one step).
• Crawlspace / below-grade walls → closed-cell foam (resists bulk water; stays put if wetted).
• Thin cavity, high R needed (2×4 walls, steel studs) → closed-cell foam (R-6.5/inch).

📋 Code: R-Targets, Unvented Attics & Fire Barriers

• Attic R-target (2021 IECC). R-30 in the hottest zones (0–1), R-49 in zones 2–3, and R-60 in zones 4–8. Code is material-neutral — you comply by achieved R-value.
• Unvented attic (IRC R806.5). Air-impermeable foam can go directly under the roof sheathing, but a minimum foam R (by climate zone) is required for condensation control, and in zones 5–8 the foam must be — or be coated by — a Class II vapor retarder.
• Ignition & thermal barrier over foam (IRC R316). Foam plastic generally needs a 15-minute thermal barrier (e.g. ½" gypsum) toward living space, or an ignition barrier in attics/crawlspaces — unless the specific product is ICC-ES-listed for exposed use. Blown-in has no such requirement (borate cellulose is Class A).
• Vapor retarder (IRC R702.7). Colder zones (5–8, Marine 4) need a Class I or II interior retarder on frame walls; closed-cell foam at ≤1.5 perms can serve as that retarder.

🎯 Which Should You Use?

• Attic-floor top-up on a budget or DIY → blown-in.
• Conditioned attic / ducts in the attic → spray foam at the roofline.
• Air-sealing is the priority in a leaky, complex assembly → spray foam.
• Rim joist, crawlspace, or below-grade → closed-cell foam.
• Wall retrofit through closed cavities → dense-pack cellulose.
• Lowest cost per R, vapor-open drying wanted → blown-in (then choose between cellulose and fiberglass).