Insulation
Insulation Thickness Calculator
Converts an R-value target into inches, which depends entirely on the material you choose.
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 is the R-value you need divided by the material's R per inch. That is the whole calculation, and its usefulness is in comparing materials: the same target takes very different depths depending on what you use, which matters when the space available is fixed. Closed cell spray foam achieves a given R in the least thickness, rigid foams vary by type, and loose fill and batts need the most. Where depth is unconstrained, as in an open attic, the cheaper material per R usually wins. Where it is constrained, the higher R per inch earns its price.
What moves the number
- The material's R per inch, which varies by a factor of roughly two across common products.
- Whether the available depth is fixed, which is what makes R per inch worth paying for.
- Existing R already in place, which reduces what you have to add.
- Whether the figure is aged R, since some foams lose a little over time.
Where this one goes wrong
Manufacturer R per inch figures are measured under test conditions and installed performance can be lower, particularly where the material is compressed, left with voids, or installed in an assembly that allows air movement around it. Compressing a batt into a shallower cavity does not give you its rated R: it gives you less. Settling is the equivalent issue for loose fill, which is why installed depth and installed density both matter. Note also that R-value describes resistance to conduction only, and says nothing about air leakage or radiant gain, so a high R assembly that leaks air will underperform a lower R one that is sealed.
Before you quote
- Use the material's rated R per inch, not a generic figure.
- Do not compress material into a shallower cavity and claim its rated R.
- Account for settling on loose fill by installing to the specified depth and density.
Where the rules come from
Questions
About this calculator
How many inches of insulation do I need?
Divide your target R-value by the material's R per inch. The answer differs substantially by material, so the same target can be a few inches of closed cell foam or considerably more of loose fill. Which matters most when the depth available is fixed.
Can I compress insulation into a shallower space?
You can fit it, but you do not get its rated R-value. Compressing a batt reduces its thickness and therefore its resistance, and it can also eliminate the air spaces the rating depends on. Using a material with a higher R per inch is the correct answer for a shallow cavity.
Is a higher R-value always better?
Up to the point of diminishing returns, and only if the assembly is also air sealed. R-value measures resistance to conduction and says nothing about air leakage, so a very well insulated wall that leaks air will underperform a more modestly insulated one that is sealed. Sealing first is the better spend.
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 Blown fibreglass (R-2.5 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 $1.35.
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 | Blown fibreglass (R-2.5 per inch) |
| Existing R-value in place | 0 |
| Cost per square foot at that depth | $1.35 |
What comes out
| Result | Value |
|---|---|
| Thickness needed main result | 15.2 |
| Area | 1,120.0 |
| R-value still to add | 38.0 |
| Estimated material cost | $1,512 |
| Bags if blown | 38 |
You need 15.2 inches to add R-38.0 across 1,120.0 square feet, roughly $1,512 in material.
How sensitive the answer is
Holding everything else steady and changing only cost per square foot at that depth, this is what happens to thickness needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per square foot at that depth ($) | Thickness needed |
|---|---|
| $0.68 | 15.2 |
| $1.01 | 15.2 |
| $1.35 example above | 15.2 |
| $2.03 | 15.2 |
| $2.7 | 15.2 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 40 ft.
- Width (ft) (ft)
- Measured in ft, starting at 28 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 0.
- Cost per square foot at that depth ($)
- A money amount, starting at $1.35.
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. |
Estimating is the easy part
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