Insulation Calculator
R-value measures resistance to heat flow, and for a single material it adds up evenly with thickness. Enter your target R-value and the material's R-per-inch for the thickness you need.
Insulation thickness
14in
- Target R-value
- 49 R
How it’s calculated
Thickness = target R-value ÷ R-per-inch. R-value is additive and (for a uniform material) linear in thickness, so doubling the thickness doubles the R. R-per-inch is a property of the specific insulation — use the figure for your product; typical values run from about 2.5 (blown fiberglass) to 6+ (closed-cell spray foam). This sizes the insulation itself — its nominal, center-of-cavity R; the whole assembly performs a bit lower once framing is counted (see the FAQ).
Worked example
Targeting R-49 in an attic with cellulose at ~3.5 R per inch: 49 ÷ 3.5 = 14 inches of cellulose.
FAQs
- What R-value do I need?
- It depends on the location and your climate zone. The IECC recommends roughly R-49 to R-60 for attics, R-13 to R-21 for walls, and R-25 to R-30 for floors, with the higher numbers in colder zones. Check your local code and zone for the figure to target.
- What's the R-per-inch of common insulation?
- Approximately: fiberglass batt 3.1–3.4, blown (loose-fill) fiberglass ~2.5, cellulose ~3.5, EPS rigid foam ~3.9, XPS ~5.0, and closed-cell spray foam ~6.0–6.5 per inch. These vary by product and density, so use your material's published figure — that's why it's an input here, not baked in.
- What R-value does a given thickness give me?
- Multiply the other way: R-value = thickness × R-per-inch. So 6 inches of XPS at R-5/in is about R-30. Note that compressing batt insulation to fit a thinner cavity lowers its R-value — don't cram a thick batt into a shallow space.
- Will my wall actually perform at this R-value?
- Usually a bit less. This sizes the insulation to a nominal (center-of-cavity) R, but in a real wall or roof the studs and joists conduct heat around it — wood is only about R-1.25 per inch — so the whole-assembly R runs meaningfully lower unless you add continuous exterior insulation over the framing. And blown or loose-fill insulation settles over time, so install it deeper than the rated depth: the bag lists an installed thickness and a smaller settled thickness, and the R you want is the settled one.
Sources
- Thickness = target R ÷ R-per-inch (R-value is additive and linear in thickness — building science). R-per-inch is a cited, editable material property; typical ranges per ASHRAE/manufacturer data.