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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
Spray foam costs several times more than fiberglass for the same cavity, and people want to know whether it earns that back. Enter the area, the cavity depth and your climate, and this compares installed cost, R-value and the yearly heating cost of the heat that conducts through each option.
With these inputs:
| Result | Value |
|---|---|
| Fiberglass batts, installed | $875 – $1,400 |
| Open-cell spray foam, installed | $1,400 – $2,625 |
| Closed-cell spray foam, installed | $3,000 – $6,000 |
| Extra cost of open-cell over batts | $875 |
| Extra cost of closed-cell over batts | $3,363 |
| Heating degree days used | 5,800 (zone 5) |
Why the payback looks long — and when foam still wins. Measured by R-value alone, fiberglass batts are hard to beat: a 2×4 wall filled with batts is about R-11 to R-13 for a fraction of the price of foam, and open-cell foam in the same cavity is about the same R-value. So the conduction-only payback this calculator shows for foam over batts is often decades. That is honest, but it leaves out the reason people choose foam: air sealing. Batts do nothing to stop air moving through gaps, and a leaky wall or roof can lose as much heat to air leakage as to conduction. Spray foam seals as it goes. If you would otherwise air-seal carefully and install batts well, the gap narrows; if you would not, foam's real-world advantage is larger than the conduction figures here. Where each one makes sense. Fiberglass or mineral wool batts: open stud walls, floors over crawlspaces, anywhere budget matters and you will air-seal separately. Open-cell foam: cathedral ceilings and unvented roof decks where it fills the full rafter depth and seals at the same time. Closed-cell foam: rim joists, basement walls, metal buildings and anywhere you need a vapor retarder or the highest R per inch in a shallow space. Installed prices used here are the 2026 ranges published by consumer cost guides: about $0.40–$0.75 per board foot for open-cell and $1.00–$2.00 for closed-cell, with batts at $0.25–$0.40 per square foot per inch of cavity installed. Heating degree days by zone are this site's climate-zone table.
Batts installed = area × depth × $0.25–$0.40 per sq ft-inch · Open-cell = area × depth × $0.40–$0.75 per board foot · Closed-cell = area × min(depth, 3 in) × $1.00–$2.00 per board foot · Yearly loss = area × HDD × 24 ÷ R ÷ 100,000 ÷ efficiency × $/therm · Payback = extra cost over batts ÷ yearly saving (conduction only)
| Cavity | Fiberglass batts | Open-cell foam | Closed-cell foam |
|---|---|---|---|
| 3.5 in | $875 – $1,400 (R-11) | $1,400 – $2,625 (R-13) | $3,000 – $6,000 (R-20) |
| 5.5 in | $1,375 – $2,200 (R-18) | $2,200 – $4,125 (R-20) | $3,000 – $6,000 (R-20) |
| 7.25 in | $1,813 – $2,900 (R-23) | $2,900 – $5,438 (R-27) | $3,000 – $6,000 (R-20) |
| 9.25 in | $2,313 – $3,700 (R-30) | $3,700 – $6,938 (R-34) | $3,000 – $6,000 (R-20) |
Closed-cell is priced at up to 3 in, the usual sprayed depth. Computed with this calculator.
On conduction alone, rarely — the extra cost takes a long time to recover. It earns its price where air sealing, moisture control or R-value in a shallow cavity matter: rim joists, cathedral ceilings, basement walls and metal buildings.
For a 2×4 wall, open-cell foam runs about 1.5–2 times the price of installed batts, and closed-cell about 3–5 times, in this calculator. The gap widens in deeper cavities for open-cell because it is priced per board foot.
Per inch, closed-cell foam (about R-6.5) beats fiberglass batts (about R-3.2) and open-cell foam (about R-3.7). In a full 2×4 cavity, batts and open-cell end up close; closed-cell reaches the highest R in the least depth.
Yes — a 1–2 inch layer of closed-cell foam to air-seal and control condensation, with batts or blown insulation filling the rest (often called flash-and-batt), gets most of the benefit for much less than a full foam job.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.