Insulation
Wall Insulation Calculator
Works out wall insulation, where the cavity depth is fixed and that limits what you can achieve.
Result
Wall cavities cap what is achievable: a 2x4 stud bay is 3.5 inches deep, which holds about R-13 of fibreglass or R-21 of closed cell foam. Reaching higher means continuous exterior insulation, not a denser batt squeezed into the same space. 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 needed is the target R less existing R divided by R per inch. Walls differ from attics in one important respect: the cavity depth is fixed by the framing, so unlike an attic you cannot simply add more. A nominal two by four wall gives a shallow cavity and a two by six a deeper one, and the best available cavity material sets a ceiling on what the wall can reach. Getting beyond it means continuous exterior insulation over the framing, which is a different and larger job.
What moves the number
- Framing depth, which caps what any cavity material can achieve.
- Whether the walls are open or finished, since dense packing a closed wall is a different operation.
- Thermal bridging through the studs, which reduces the whole-wall performance below the cavity figure.
- Climate zone, which sets the target and may call for continuous exterior insulation.
Where this one goes wrong
The R-value of the insulation is not the R-value of the wall. Studs conduct heat around the insulation, and framing is a meaningful share of a wall's area once you count plates, headers and corners, so whole-wall performance is noticeably below the cavity rating. That is why energy codes increasingly call for continuous exterior insulation: it covers the framing rather than sitting between it. On a finished wall, dense packed cellulose or injected material is the practical retrofit, and the installer's density matters, since under-filling leaves voids and settling while the wall looks finished from outside.
Before you quote
- Confirm the framing depth before specifying a target the cavity cannot reach.
- Account for framing area when claiming whole-wall performance.
- Verify fill density on a dense pack retrofit rather than assuming.
Where the rules come from
Questions
About this calculator
Why can't I just add more insulation to my walls?
Because the cavity depth is fixed by the framing. Unlike an attic where you can add depth freely, a wall can only hold what fits between the studs, so the best cavity material sets a ceiling. Going beyond it needs continuous insulation on the outside of the framing.
Is my wall's R-value the same as the insulation's?
No, it is lower. Studs, plates, headers and corners conduct heat around the insulation, and framing is a significant share of a wall's area. That gap between cavity rating and whole-wall performance is why codes increasingly require continuous exterior insulation.
Can finished walls be insulated without opening them?
Yes, by dense packing cellulose or injecting material through small holes that are then patched. The critical variable is density: an under-filled cavity leaves voids and will settle, and you cannot see it afterwards. It is worth asking how density is verified.
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 120 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-19, walls or floors.
- Material. Enter your own figure; the page starts at Fibreglass batt (R-3.2 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 $1.1.
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) | 120 ft |
| Width (ft) | 9 ft |
| Target R-value | R-19, walls or floors |
| Material | Fibreglass batt (R-3.2 per inch) |
| Existing R-value in place | 0 |
| Cost per square foot at that depth | $1.1 |
What comes out
| Result | Value |
|---|---|
| Thickness needed main result | 5.9 |
| Area | 1,080.0 |
| R-value still to add | 19.0 |
| Estimated material cost | $1,188 |
| Bags if blown | 18 |
You need 5.9 inches to add R-19.0 across 1,080.0 square feet, roughly $1,188 in material.
How sensitive the answer is
Holding everything else steady and changing only cost per square foot at that depth, this is what happens to thickness needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per square foot at that depth ($) | Thickness needed |
|---|---|
| $0.55 | 5.9 |
| $0.83 | 5.9 |
| $1.1 example above | 5.9 |
| $1.65 | 5.9 |
| $2.2 | 5.9 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 120 ft.
- Width (ft) (ft)
- Measured in ft, starting at 9 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 0.
- Cost per square foot at that depth ($)
- A money amount, starting at $1.1.
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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