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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.
Foam board is sold by the sheet, usually 4×8. Enter the area you're covering and this returns sheets, layers, R-value and cost for each foam type.
With these inputs:
| Result | Value |
|---|---|
| Coverage to buy | 880 sq ft |
| Sheets required | 28 sheets |
| R-value per layer | R-12 |
| Seam tape needed | 336 ft |
| Material cost | $845 |
| Cost per sq ft | $0.96 |
Coverage = area × (1 + waste %) · Sheets = coverage ÷ (sheet width × sheet length), rounded up · R per layer = R-per-inch × board thickness
Polyiso gives the most R per inch (about R-6) and suits above-grade walls and roofs. XPS (R-5) handles moisture best and is the usual pick below grade. EPS (R-3.9) is cheapest and fine under slabs.
Yes if the foam doubles as your air barrier. Tape every seam and seal the edges with canned foam — untaped board can lose most of its air-sealing benefit.
Polyiso has the highest R per inch and suits interior applications and above-deck roofing. XPS sits in the middle and handles moisture and ground contact well. EPS is the cheapest per R and is the usual choice below grade and under slabs. Depth available, moisture exposure and budget decide it more than the R rating alone.
If the board is doing any air-sealing work, yes, and that is usually the point of installing it. Untaped seams leave a continuous air path straight through the layer, which undoes much of the benefit of going continuous in the first place. Use tape rated for the board rather than general-purpose tape.
Inside a building, almost always. Rigid foam is combustible and codes generally require it to be covered by a thermal barrier — half-inch drywall is the common answer. This applies in basements and garages, which is exactly where people are most tempted to leave it exposed. Check your local requirement before finishing.
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.