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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.
Polyisocyanurate (polyiso) is the rigid foam with the highest R-value per inch of the common boards, but the figure you can count on depends on how it is used. Roof boards are rated by their long-term thermal resistance (LTTR); wall sheathing is rated per inch on its actual thickness, and its R per inch rises as the board gets thicker. Pick the use, enter the thickness and layers, and get the R-value, the 4 × 8 sheets for your area and the thickness needed for a target R.
With these inputs:
| Result | Value |
|---|---|
| R-value | R-11.4 |
| 4 × 8 sheets | 16 |
| Same thickness of XPS (R-5/in) | R-10 |
Why polyiso has more than one R-value. Polyiso's cells hold a blowing agent that conducts heat less than air, and that gas slowly diffuses out. Roof boards are therefore rated by long-term thermal resistance (LTTR) under ASTM C1289, a 15-year time-weighted figure; the design value for roof board is R-5.7 per inch. The roofing contractors' association (NRCA) goes further and suggests designing with a flat R-5.0 per inch, which the polyiso makers' association (PIMA) disputes — use it if you want a margin. Wall boards are rated on their actual thickness. PIMA puts foil-faced polyiso at R-5.6 to 6.9 per inch, rising with thickness, and glass-faced board at R-5.6 to 5.9. Standard XPS, by contrast, is R-5.0 per inch regardless of thickness. Polyiso in cold weather. Like most foams, polyiso's R-value is measured at a mean temperature of 75 °F, and polyiso is known to perform below its label when the board itself is very cold. In cold climates, designers often pair it with a layer of mineral wool or XPS, or simply size it with the conservative per-inch figure.
R-value = thickness (in) × layers × R per inch · Roof board (LTTR, ASTM C1289): R-5.7 per inch; NRCA conservative design: R-5.0 per inch · Foil-faced wall board: R-5.6 to 6.9 per inch; glass-faced: R-5.6 to 5.9 per inch (PIMA) · Sheets = ceil(area ÷ 32 sq ft) × layers; thickness for a target = target R ÷ R per inch
| Thickness | Roof LTTR (R-5.7/in) | Foil-faced wall (R-5.6–6.9/in) | XPS for comparison |
|---|---|---|---|
| 0.5 in | R-2.9 | R-2.8 – 3.5 | R-2.5 |
| 1 in | R-5.7 | R-5.6 – 6.9 | R-5 |
| 1.5 in | R-8.6 | R-8.4 – 10.4 | R-7.5 |
| 2 in | R-11.4 | R-11.2 – 13.8 | R-10 |
| 2.5 in | R-14.3 | R-14 – 17.3 | R-12.5 |
| 3 in | R-17.1 | R-16.8 – 20.7 | R-15 |
| 4 in | R-22.8 | R-22.4 – 27.6 | R-20 |
Computed from the per-inch figures above. The labelled R-value of the board you buy wins: polyiso's R per inch rises with thickness, so a thick wall board is near the top of its range.
| Material | R per inch | Inches for R-13 | Inches for R-30 |
|---|---|---|---|
| Polyiso roof board (LTTR) | R-5.7 | 2.3 in | 5.3 in |
| Polyiso foil-faced wall board (low end) | R-5.6 | 2.3 in | 5.4 in |
| Blown cellulose (loose-fill) | R-3.5 | 3.7 in | 8.6 in |
| Blown fiberglass (loose-fill) | R-2.5 | 5.2 in | 12 in |
| XPS rigid foam board | R-5 | 2.6 in | 6 in |
Polyiso from the LTTR and PIMA figures above; loose-fill cellulose and fiberglass at the midpoints of the DOE Energy Saver ranges; XPS at the ASTM C578 minimum. Inches = target R ÷ R per inch. Fiberglass and mineral wool batts are sold in fixed thicknesses for each R-value — see the R-13, R-19 and R-30 insulation calculators for theirs.
R-5.7 per inch for roof board, the ASTM C1289 long-term (LTTR) design value; R-5.6 to 6.9 per inch for foil-faced wall board depending on thickness; R-5.6 to 5.9 per inch for glass-faced board (PIMA).
About R-11.4 as a roof board at R-5.7 per inch, and roughly R-11 to R-14 as foil-faced wall board — check the label, which gives the exact rated figure for that thickness.
Per inch, yes: standard XPS is R-5.0 per inch, so 2 in of XPS is R-10 against about R-11.4 to R-13.8 for polyiso. XPS handles ground contact and wet locations better; polyiso is the usual choice above grade and on roofs.
About 5.3 inches of roof board at R-5.7 per inch (30 ÷ 5.7), usually installed as two or three layers with staggered joints. At NRCA's conservative R-5.0 per inch it is 6 inches.
Divide the area by 32 sq ft per 4 × 8 sheet, round up, and multiply by the number of layers. 500 sq ft in one layer is 16 sheets.
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.