Insulation Thickness Calculator (Required R)
How deep does the insulation have to be to hit your target R? Divide the target by the material R per inch.
Calculator
To reach R-49 at 3.50 R per inch you need about 14.0 inches of material. R/inch is a labeled typical — confirm on your product’s data sheet.
Codes and ENERGY STAR talk in R-values, but the crew installs inches. This tool converts one to the other: give it the R you are chasing and the R per inch of the material, and it returns the depth you have to build. It is the single most useful number for an attic top-up — it tells you how deep to set the depth rulers before you start blowing.
The default targets a common cold-climate attic (R-49) with cellulose at R-3.5/in.
Formula
required_thickness_in = target_R ÷ R_per_inch
Higher R per inch = fewer inches. That is why closed-cell foam fits an R-value into a shallow cavity that batts never could.
Worked example
Target R-49 attic, three ways. Cellulose at R-3.5/in: 49 ÷ 3.5 = 14.0 in. Blown fiberglass at R-2.5/in: 49 ÷ 2.5 = 19.6 in. Closed-cell foam at R-6.5/in: 49 ÷ 6.5 = 7.5 in. Same R, wildly different depth — which is exactly how you decide whether the material fits the space you have.
Background & practice
Blow it a little deep. Loose-fill settles, so set your depth cards ~1–2″ over the calculated number for blown fiberglass and cellulose. The bags are sold by coverage at settled depth, so a shallow-looking finished job is usually an under-filled one.
Cavity depth is the hard limit. A 2×10 rafter gives you 9.25″ of usable cavity. If your target needs more inches than the cavity holds, you either switch to a higher-R material (foam), fur the framing out, or accept the lower R the cavity can hold.
What to measure first: the depth you actually have to fill, and the R per inch printed on the product — not a generic average. For the bag/kit count once you know the depth, jump to the blown-in or spray-foam calculators.
Reference table
| Material | R-30 | R-38 | R-49 | R-60 |
|---|---|---|---|---|
| Fiberglass batt | 9.2 in | 11.7 in | 15.1 in | 18.5 in |
| Blown fiberglass | 12.2 in | 15.5 in | 20.0 in | 24.5 in |
| Cellulose | 8.6 in | 10.9 in | 14.0 in | 17.1 in |
| Mineral wool (Rockwool) | 9.5 in | 12.1 in | 15.6 in | 19.0 in |
| Open-cell spray foam | 8.3 in | 10.6 in | 13.6 in | 16.7 in |
| Closed-cell spray foam | 4.6 in | 5.8 in | 7.5 in | 9.2 in |
| Rigid EPS | 7.7 in | 9.7 in | 12.6 in | 15.4 in |
| Rigid XPS | 6.3 in | 8.0 in | 10.3 in | 12.6 in |
| Rigid polyiso | 5.0 in | 6.3 in | 8.1 in | 9.9 in |
Inches of material to reach each target R, at the labeled band midpoint. A shallow rafter or joist bay can cap what fits — measure the cavity before you order.