Wall Insulation Calculator
Estimate the insulation thickness and area to bring a wall up to a target R-value.
Calculator
Free, runs entirely in your browser, and your numbers never leave your device. Results are estimates for planning only.
What this calculator does
Wall cavities have limited depth, so the R-per-inch of your insulation decides whether you can hit a target within the studs. This tool estimates the additional R-value you need, the thickness to achieve it, and the wall area to cover.
How to use it
- Enter the wall area you are insulating.
- Enter the current R-value (0 if the cavity is empty) and your target.
- Enter the product's R-per-inch, then compare the required thickness to your cavity depth.
The formula
Additional R = max(target − current, 0). Thickness = additional R ÷ R-per-inch. Material area = area × (1 + waste%).
Example calculation
An empty 500 sq ft of wall, target R-15, a product rated R-3.6 per inch, 10% waste:
- Additional R: 15 − 0 = 15
- Thickness: 15 ÷ 3.6 ≈ 4.2 in
- Material area with 10% waste: 500 × 1.10 = 550 sq ft
Result: About 4.2 in across roughly 550 sq ft — check this fits your stud cavity depth.
Buying and planning tips
- Compare the required thickness to your actual cavity depth (a 2×4 wall gives about 3.5 in).
- If the target won't fit, a higher R-per-inch product reaches it in less depth.
- Fill the cavity fully and avoid gaps and compression around wiring and boxes.
Common mistakes to avoid
- Specifying a thickness that won't fit the stud cavity.
- Compressing batts to fit, which lowers their R-value.
- Ignoring thermal bridging through the studs when judging real performance.
Assumptions and limits
- Thickness assumes the product performs at the R-per-inch you enter — confirm it on the label.
- Cavity depth limits thickness; a 2×4 wall is about 3.5 in deep, a 2×6 about 5.5 in.
- This is a planning estimate only, not engineering or building-code advice; check local requirements.
Frequently asked questions
How thick should wall insulation be?
Divide the extra R-value you need by the product's R-per-inch. Reaching R-15 with an R-3.6 per inch product needs about 4.2 inches — which fits a 2×6 wall but not a 2×4.
Will the insulation fit my wall cavity?
Check the required thickness against your cavity depth: roughly 3.5 in for a 2×4 wall and 5.5 in for a 2×6. If it won't fit, use a higher R-per-inch product.
Can I compress insulation to make it fit?
Avoid it. Compressing insulation reduces its R-value, so you lose performance. Choose a product sized for the cavity instead.
Does this account for studs?
No. Heat travels through the wood studs (thermal bridging), so the whole-wall R-value is somewhat lower than the cavity insulation's rating.
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