Estimate how many rigid foam board sheets you need for an area, or find the R-value of a foam thickness using typical XPS, EPS or polyiso R per inch.
Rigid Foam Board Formula
The calculator works in three directions. To find how many boards cover an area, it adds a waste factor, divides by the area of one board, and rounds up to a whole sheet:
To find the R-value of a foam layer, it multiplies the total thickness by the R-value per inch of the foam type:
To find the thickness needed to reach a target R-value, it rearranges the same relationship and then converts that thickness into whole board layers:
Variables:
- A is the area to cover, in square feet
- W is the waste factor as a decimal, so 10 percent is 0.10
- B is the area of one board in square feet, such as 32 for a 4 by 8 ft sheet
- Sheets is the number of boards, always rounded up to a whole board
- R is the total thermal resistance, or R-value, of the layer
- T is the total insulation thickness, in inches
- Rpi is the R-value per inch of the foam type
- Rtarget is the R-value you want to reach
- tboard is the thickness of one board, in inches
- Layers is the number of stacked boards, rounded up
Choose boards, R-value or target thickness. Quantity mode accepts area or dimensions, standard or custom sheet size, whole layers, waste and optional price. The thermal tasks use a stated planning/product rate or an explicitly entered design R-value per inch. US R-values are not metric RSI values. For a specific board, divide its documented total R by that board’s thickness in inches before entering its product rate.
Published ratings may already include conditioning or long-term thermal-resistance testing. Actual performance depends on the product, thickness, facing, temperature and application. No universal aging/cold reduction is applied here. Use current product documentation or a supported project-specific design value; board material R is not a whole-wall or roof rating.
Rigid Foam R-Value by Type
Ratings are commonly reported at a specified mean test temperature, often 75°F. This table summarizes approximate planning ranges, not guarantees. The calculator’s EPS4, XPS5 and polyiso6.5 R/in examples are assumptions, and a product table can be nonlinear with thickness. Product-specific approval governs below-grade and other applications.
| Foam type | Approximate R per inch | Design R per inch | Below grade |
|---|---|---|---|
| EPS (white beadboard) | 3.6 to 4.2 | Use supported product data | Check product approval |
| XPS (pink or blue) | about 5.0 | Use supported product data | Check product approval |
| Polyiso (foil faced) | 6.0 to 6.8 | Use supported product data | Check product approval |
This second table is linear arithmetic using the example rates EPS4, XPS5 and polyiso6.5 R/in, rounded to one decimal. It is not a product-label table. Use the actual board’s documented R at its purchased thickness; two thinner boards need not have the same combined rating as one thicker board.
| Thickness | EPS | XPS | Polyiso |
|---|---|---|---|
| 0.5 in | R-2.0 | R-2.5 | R-3.3 |
| 1 in | R-4.0 | R-5.0 | R-6.5 |
| 1.5 in | R-6.0 | R-7.5 | R-9.8 |
| 2 in | R-8.0 | R-10.0 | R-13.0 |
| 3 in | R-12.0 | R-15.0 | R-19.5 |
Example Problems
Example 1: Boards for a basement wall.
For a32ft by8ft wall, the area is256ft². With10% waste, allow281.6ft²; ceil(281.6/32)=9 sheets of4×8ft material per layer. At the assumed XPS5 R/in, a2in sheet estimates R-10. That rate does not establish installation or below-grade suitability.
Example 2: Thickness for a target R-value.
With the illustrative polyiso6.5 R/in, targetR13 has a linear equivalent thickness13/6.5=2in and needs two1in sheets. For a specific1in board documented asR6, enter6R/in instead: ceil(13/6)=3 layers, totaling3in and modeledR18. Select a supported design rate when needed; no automatic extra layer is prescribed merely because the climate is cold.
Frequently Asked Questions
How many rigid foam boards do I need?
Measure the area to cover in square feet, add a waste factor for cuts and offcuts, then divide by the area of one board and round up. A 4 by 8 ft sheet covers 32 sq ft, so 640 sq ft with 10 percent waste needs ceil(704 / 32) = 22 sheets. Odd shapes and many openings raise the waste factor, so 10 to 15 percent is common.
Which rigid foam board has the highest R-value?
Foil-faced polyiso commonly has a high nominal R-value per inch, but compare actual product, thickness and design-condition data. Rated values may already include aging. Temperature effects vary: a single family-wide “effective” value cannot represent all EPS, XPS or polyiso products.
Can I stack two layers of foam board?
Compatible material R-values add for layers in series under the same rating/design basis. Equal total thickness does not universally imply the same rating: the Johns Manville AP Foil guide lists1in asR6 and2in asR13, so two listed1in layers sum toR12. Follow the selected product’s seam, fastening, moisture, fire-protection and covering instructions.
