Buying foam board? Enter the area and pick the board. You get the sheets to buy and the R-value you reach.
Your shopping list
21Sheets to buy
10 RTotal R-value you reach
600 sq ftArea after openings
- Area with extra660 sq ft
R-values are nominal and vary by product. Extra of 10% is included.
How to use the Rigid Foam Board Calculator
- Measure the area and take out the openings.
- Pick the foam type, the thickness and the sheet size.
- Read the sheets to buy and the total R-value.
The math behind it
Net area = area – openings
Sheets = net area x (1 + extra %) / sheet area, rounded up
Total R = R-value per inch x thickness
A worked example
What we typed in
- Area to cover: 900 sq ft
- Windows and doors (total area): 60 sq ft
- Foam board type: Polyiso foam board (about R-6 per in)
- Board thickness: 3 in
- Sheet size: 4 x 8 ft sheet (32 sq ft)
- Extra for mistakes and cuts: 10%
What we got
- Sheets to buy: 29
- Total R-value you reach: 18 R
- Area after openings: 840 sq ft
- Area with extra: 924 sq ft
Tips from the job site
- Stagger the seams between two layers for fewer air leaks.
- Polyiso loses some R-value in cold weather, so check the label for your climate.
Common questions
Which foam board has the highest R-value?
Polyiso is the highest per inch, then XPS, then EPS. The values here are typical.
Can I use foam board on its own?
Some foam board needs a fire barrier like drywall on the inside. Check your local code.
See all Insulation Calculators.
Last checked: (worked example and form display).