Polyiso R-Value Calculator
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.
Your project
Gives the total thickness needed at the low R per inch.
The labelled R for one board overrides the per-inch figure.
Your results
- R-value
- R-11.4
- 2 in total at R-5.7/in
- 4 × 8 sheets
- 16
- Same thickness of XPS (R-5/in)
- R-10
Estimates use typical US material and labor pricing. Confirm quantities against the coverage chart on the bag before you buy.
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How this calculator works
- 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
Polyiso R-value chart by thickness
| 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.
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.
Frequently asked questions
What is the R-value of polyiso per inch?
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).
What is the R-value of 2 inch polyiso?
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.
Is polyiso better than XPS?
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.
How thick does polyiso need to be for R-30?
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.
How many sheets of polyiso do I need?
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.
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R-value
R-11.4