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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 is sold in board feet — one square foot at one inch thick. Enter your area and target thickness to size the job, count kits, and compare DIY against a contractor.
With these inputs:
| Result | Value |
|---|---|
| Board feet required | 1,980 bd ft |
| R-value achieved | R-19.5 |
| DIY kits needed | 4 × 600 bd ft kits |
| DIY kit cost | $3,600 |
| Contractor estimate | $2,673 |
Board feet = area × thickness in inches × (1 + waste %) · R-value = R-per-inch × thickness · Kits = board feet ÷ kit rating, rounded up
Closed-cell gives about R-6.5 per inch, adds structural rigidity and blocks vapor — best for rim joists, crawlspaces and thin cavities. Open-cell is about R-3.7 per inch, much cheaper, and fine for interior walls and attics with room to spare.
For under about 600 board feet, yes. Past that, contractor pricing per board foot usually beats kit pricing and you get proper ratio control and cleanup.
One square foot at one inch thick. A job needing 1,000 sq ft covered at 2 inches is 2,000 board feet. Contractors quote in board feet and kits are sold by board feet, so it is the unit to get quotes on — giving a contractor your floor area instead invites quotes that are not comparable.
Closed-cell has a much higher R per inch, adds structural rigidity and is its own vapour retarder, which makes it the choice where depth is limited or moisture must be controlled. Open-cell is cheaper per board foot, expands far more so it fills irregular cavities well, and stays vapour-open, which some assemblies need in order to dry.
Small two-part kits exist and work for rim joists, gaps and modest areas. Whole-house work is a different matter: it needs the right substrate temperature, correct mix ratio and proper respiratory protection, and foam that goes off wrong does not come off the framing easily. The failure mode for DIY foam is expensive and permanent, which is why most whole-house jobs are contracted.
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.