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Insulation Thickness for Target R-Value Calculator

Find how thick a layer of your chosen insulation must be to bring an attic, wall or floor up to a target R-value.

Method reviewed October 26, 2026. Version 1.0.0. How it works

This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.

Your inputs

Unit system

From the package chart or fact sheet (FTC rules require it). For loose fill, use the chart row closest to your target instead.

Result

Thickness to add
15.5in
R-value to add
49h·ft²·°F/BTU

About 15.5 in of this product adds R-49 (2 layers of the R-30 product).

  • Some products lose R-value per inch as they get thicker, and compressed batts lose R-value (an R-19 batt squeezed into a 3½ in cavity is rated about R-13). Check the product chart at your target R-value.
Full layers of this product
2
Implied R-value per inch of this product
3.16per in
Show the working
  1. R-value to add = target − existing
    49 − 0 = 49
  2. Implied R per inch = label R ÷ label thickness
    30 ÷ 9.5 in = 3.158
  3. Thickness = R to add ÷ R per inch
    49 ÷ 3.158 = 15.52 in

How it works

Enter the R-value and thickness printed on the product’s package or fact sheet. The tool works out the product’s R-value per inch and the thickness needed to add the difference between your target and any existing insulation:

  • R to add = target − existing
  • Thickness = R to add ÷ (label R ÷ label thickness)

It also shows how many full layers of the product that takes.

Use the label, not a generic table

The Federal Trade Commission’s R-value rule requires insulation packages to show the R-value and thickness (for batts) or the settled thickness and coverage at R-13 to R-49 (for loose fill), and limits per-inch claims because some products lose R-value per inch as they get thicker. That is why no generic R-per-inch table is built in. For loose fill, read the thickness straight from the bag’s chart at your target.

Compression and gaps

Squeezing a batt into a shallower cavity lowers its R-value: an R-19 batt in a 3½ inch stud cavity is rated about R-13. Gaps, voids and settling also reduce the installed R-value.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Product label R-value
30 h·ft²·°F/BTU
Thickness for that label R-value
9.5 in
Existing R-value to keep
0 h·ft²·°F/BTU
Target total R-value
49 h·ft²·°F/BTU

Result

Thickness to add
15.5 in
R-value to add
49 h·ft²·°F/BTU
Full layers of this product
2
Implied R-value per inch of this product
3.16 per in

About 15.5 in of this product adds R-49 (2 layers of the R-30 product).

Assumptions

  • Thickness scales in proportion to R-value for the product you choose, at the R-value per inch implied by its label.

Limitations

  • No material R-per-inch table is built in; FTC rules restrict per-inch claims, so use the product’s own label or fact sheet.
  • Settling, compression, gaps and framing reduce installed R-value.

Frequently asked questions

How thick is R-49 fibreglass?

Using an R-30 batt labelled 9.5 inches, R-49 works out to about 15.5 inches, which matches the standard thickness listed for R-49 batts in the Seattle Energy Code tables.

Can I just add the R-values of two layers?

Yes. R-values in series add, so new insulation laid over existing insulation adds its R-value to the existing one.

Why does the tool round layers up?

Batts come in fixed thicknesses, so reaching the target with one product usually means a whole number of layers.

Sources

  1. 16 CFR Part 460 — Labeling and Advertising of Home Insulation (§ 460.12 labels, § 460.20 R-value per inch) — U.S. Federal Trade Commission, via eCFR. Batt and blanket labels must show R-value, length, width, thickness and coverage; loose-fill labels show minimum settled thickness and coverage at R-13 to R-49; R-value-per-inch claims are not allowed unless tests show the per-inch value does not drop with thickness.
  2. 2021 Seattle Energy Code, Appendix A (A101.3 air films; Table A101.4 compressed batts) — City of Seattle, Department of Construction and Inspections. Default air-film R-values: 0.17 all exterior surfaces, 0.61 interior horizontal heat flow up, 0.92 interior horizontal heat flow down, 0.68 interior vertical; fibreglass batts R-30 at 9.5 in and R-49 at 15.5 in standard thickness; an R-19 batt in a 3.5 in cavity is rated R-13.

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