Insulation
Rigid Foam Insulation Calculator
Works out rigid foam board, where the value is covering the framing rather than filling between it.
Result
Required R-values are set by climate zone and by local energy code, and they differ for attics, walls and floors. The figures offered here are common code minimums rather than recommendations, and a minimum is a floor, not a target. Air sealing comes first: insulation slows heat conduction but does nothing about air leaking through gaps, and an uninsulated but sealed attic often outperforms an insulated leaky one.
Get a Free Business AuditHow the calculation works
Thickness from R target divided by R per inch, and board count from area divided by sheet size. The three common boards differ meaningfully: expanded polystyrene is cheapest per R and most vapour permeable, extruded polystyrene sits in the middle with higher water resistance, and polyisocyanurate has the highest R per inch and is usually foil faced. Rigid board is used as continuous insulation over framing or sheathing, which is where it does something cavity insulation cannot: it interrupts thermal bridging through the studs.
What moves the number
- Which foam type, since R per inch, moisture behaviour and cost all differ.
- Whether it is continuous over framing or in a cavity, as continuous is where the benefit is largest.
- Taped and sealed joints, because unsealed board loses much of its air sealing value.
- Thickness relative to the assembly's drying direction and condensation risk.
Where this one goes wrong
Continuous exterior foam changes where the dew point falls in a wall, and that is both why it works and how it can go wrong. Enough exterior foam keeps the sheathing warm and dry; too little can leave the sheathing cold while also slowing its ability to dry outward, which concentrates moisture in it. Codes and building science guidance give minimum ratios of exterior to cavity R by climate zone for exactly that reason, so this is not a case where a little is simply better than none. Note also that polyisocyanurate's R per inch falls at low temperatures, which matters in a cold climate, and that exposed foam needs a thermal barrier.
Before you quote
- Check the minimum exterior-to-cavity R ratio for your climate zone.
- Tape and seal the joints so the board also air seals.
- Account for polyiso's reduced performance at low temperature in cold climates.
- Cover exposed foam with the required thermal barrier.
Where the rules come from
Questions
About this calculator
What is continuous insulation and why does it matter?
Insulation applied over the framing rather than between it, usually rigid board on the exterior. It matters because studs conduct heat around cavity insulation, and continuous board interrupts that path. It is why energy codes increasingly require it, since it raises whole-wall performance rather than just cavity R.
Can too little exterior foam cause problems?
Yes, which is counterintuitive. Exterior foam slows the sheathing's ability to dry outward while also keeping it warmer. Enough foam keeps it warm and dry; too little leaves it cold and less able to dry, concentrating moisture there. Minimum ratios of exterior to cavity R exist by climate zone for this reason.
Which rigid foam should I use?
Expanded polystyrene is cheapest per R and most permeable, extruded polystyrene resists water better, and polyisocyanurate gives the most R per inch but loses some performance when cold. The choice depends on the assembly, the climate and whether the board needs to let moisture through.
The formula
thickness = (target R - existing R) / R-value per inch
Put this calculator on your site
Free to embed. Paste this where you want it to appear. It stays up to date automatically because it loads from BDEVY.
Please keep the attribution line. It is the only thing we ask in return.
How to use this calculator
- Length (ft) (ft). Enter your own figure; the page starts at 40 ft.
- Width (ft) (ft). Enter your own figure; the page starts at 9 ft.
- Target R-value. Enter your own figure; the page starts at R-13, walls, mild.
- Material. Enter your own figure; the page starts at Rigid foam board (R-5 per inch).
- Existing R-value in place. Enter your own figure; the page starts at 0.
- Cost per square foot at that depth ($). Enter your own figure; the page starts at $1.75.
The result updates as you type, so you can try a figure, see what it does to thickness needed, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Length (ft) | 40 ft |
| Width (ft) | 9 ft |
| Target R-value | R-13, walls, mild |
| Material | Rigid foam board (R-5 per inch) |
| Existing R-value in place | 0 |
| Cost per square foot at that depth | $1.75 |
What comes out
| Result | Value |
|---|---|
| Thickness needed main result | 2.6 |
| Area | 360.0 |
| R-value still to add | 13.0 |
| Estimated material cost | $630 |
| Bags if blown | 5 |
You need 2.6 inches to add R-13.0 across 360.0 square feet, roughly $630 in material.
How sensitive the answer is
Holding everything else steady and changing only cost per square foot at that depth, this is what happens to thickness needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per square foot at that depth ($) | Thickness needed |
|---|---|
| $0.88 | 2.6 |
| $1.31 | 2.6 |
| $1.75 example above | 2.6 |
| $2.63 | 2.6 |
| $3.5 | 2.6 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 40 ft.
- Width (ft) (ft)
- Measured in ft, starting at 9 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 0.
- Cost per square foot at that depth ($)
- A money amount, starting at $1.75.
What the results tell you
| Result | What it is |
|---|---|
| Thickness needed | The headline result. A decimal figure. |
| Area | A decimal figure. |
| R-value still to add | A decimal figure. |
| Estimated material cost | A money figure. |
| Bags if blown | A number. |
Keep going
Related calculators
Estimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.