Insulation
Spray Foam Insulation Calculator
Works out spray foam, where open and closed cell are different products for different problems.
Result
Closed cell foam is roughly twice the R-value per inch of open cell and also acts as a vapour retarder and an air barrier, which is why it costs several times as much. Spray foam is installed by licensed applicators and has curing and ventilation requirements that are not optional. 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.
Get a Free Business AuditHow the calculation works
Thickness from R target divided by R per inch, with the material choice making a large difference. Closed cell foam has roughly double the R per inch of open cell, is a vapour retarder at sufficient thickness, and adds structural rigidity, which makes it the choice where depth is limited or moisture control is wanted. Open cell is cheaper per R, expands far more so it fills irregular cavities well, and remains vapour permeable, which lets an assembly dry. Both air seal as they cure, which is a large part of their value.
What moves the number
- Open against closed cell, which changes R per inch, vapour behaviour and cost substantially.
- Whether the assembly needs to dry, since closed cell restricts drying in one direction.
- Available depth, which is what makes closed cell's higher R per inch worth paying for.
- Ignition and thermal barrier requirements, since exposed foam generally needs covering.
Where this one goes wrong
Spray foam changes how an assembly dries, which is why it is the insulation most capable of causing damage when applied to the wrong assembly. Closed cell foam against a roof deck creates an unvented assembly, and that can be entirely correct when designed as such, or a trap when sprayed into a roof that was relying on ventilation to dry. Either design it as unvented or keep it vented, rather than converting one into the other by accident. Installation quality also matters more than with other materials: foam applied too thick in one pass, at the wrong temperature, or off ratio can cure badly, shrink, pull away or smell persistently, and remediation means removing it. Exposed foam generally needs an ignition or thermal barrier.
Before you quote
- Decide whether the assembly is vented or unvented and insulate consistently.
- Choose open or closed cell on drying and depth, not just R per inch.
- Confirm the installer's pass thickness, temperature and ratio practice.
- Provide the required ignition or thermal barrier over exposed foam.
Where the rules come from
Questions
About this calculator
Open cell or closed cell spray foam?
Closed cell has roughly double the R per inch, acts as a vapour retarder and adds rigidity, which suits shallow cavities and moisture control. Open cell is cheaper per R, fills irregular spaces better and stays vapour permeable so the assembly can dry. The right one depends on the assembly, not on which is better generally.
Can spray foam cause moisture problems?
It can, by changing how an assembly dries. Closed cell against a roof deck makes the assembly unvented, which is fine if designed that way and a problem if the roof was relying on ventilation. Converting a vented assembly to unvented by accident is the main way spray foam causes damage.
Does spray foam need covering?
Exposed foam plastic generally requires an ignition barrier or a thermal barrier depending on the space and the adopted code. It is a requirement that gets overlooked on retrofits in attics and crawl spaces. Worth confirming for your situation rather than leaving it bare.
Why do some spray foam jobs smell?
Usually because the foam was applied off ratio, too thick in a single pass, or outside its temperature window, so it did not cure properly. A badly cured job can smell persistently and the only real remedy is removal, which is expensive. Installer practice matters more with foam than with any other insulation.
The formula
thickness = (target R - existing R) / R-value per inch
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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 28 ft.
- Target R-value. Enter your own figure; the page starts at R-38, attic.
- Material. Enter your own figure; the page starts at Closed cell spray foam (R-6.5 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 $4.2.
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) | 28 ft |
| Target R-value | R-38, attic |
| Material | Closed cell spray foam (R-6.5 per inch) |
| Existing R-value in place | 0 |
| Cost per square foot at that depth | $4.2 |
What comes out
| Result | Value |
|---|---|
| Thickness needed main result | 5.8 |
| Area | 1,120.0 |
| R-value still to add | 38.0 |
| Estimated material cost | $4,704 |
| Bags if blown | 38 |
You need 5.8 inches to add R-38.0 across 1,120.0 square feet, roughly $4,704 in material.
How sensitive the answer is
Holding everything else steady and changing only cost per square foot at that depth, this is what happens to thickness needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per square foot at that depth ($) | Thickness needed |
|---|---|
| $2.1 | 5.8 |
| $3.15 | 5.8 |
| $4.2 example above | 5.8 |
| $6.3 | 5.8 |
| $8.4 | 5.8 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 40 ft.
- Width (ft) (ft)
- Measured in ft, starting at 28 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 0.
- Cost per square foot at that depth ($)
- A money amount, starting at $4.2.
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
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