Insulation Calculator

Insulation thickness to reach a target R-value with loose-fill or any product, or how many batt packages and layers you need.

ENERGY STAR (2021 IECC) for an uninsulated attic: R-30 in climate zone 1, R-49 in zones 2–3, R-60 in zones 4–8.
Estimate as existing depth × R per inch (e.g. 4 in of old loose fiberglass ≈ R-10).
Insulation depth to add
13.2
R-value to add
49.0 R
Installed volume
1,103.6
Bags of loose-fill
Use the coverage chart on the bag: find the row for R-49 and divide 1000 ft² by its "square feet per bag".

Estimate generated with SortedSums: https://sortedsums.com/insulation-calculator/. For informational purposes only; verify before relying on it.

Estimate only. This tool is for informational and educational purposes. Results depend on your inputs and simplifying assumptions, and are not a substitute for professional engineering, design or financial advice. Always verify with a qualified professional and applicable codes before purchasing, building or making decisions.

How it’s calculated

R-value adds up through layers and is roughly proportional to thickness, so the depth to add is the missing R-value divided by the material’s R per inch.

R to add = target R − existing R Depth (in) = R to add ÷ R per inch Batt layers = ⌈ R to add ÷ batt R ⌉ Packages = ⌈ area × layers × (1 + waste%) ÷ coverage per package ⌉

Example: a 1,000 ft² attic with no insulation, target R-49 with blown cellulose (R-3.7/in): 49 ÷ 3.7 = 13.2 in, about 1,104 ft³ installed. With R-19 batts at 48.96 ft² per package: ⌈49 ÷ 19⌉ = 3 layers (R-57), 1,000 × 3 × 1.05 = 3,150 ft² → 65 packages.

Material (DOE rule of thumb)R per inch
Loose-fill fiberglass2.5
Loose-fill rock wool2.8
Fiberglass batts3.2
Blown cellulose3.7

Loose-fill settles and its R per inch depends on installed density, so always follow the bag’s coverage chart for minimum thickness and bag count. Air-seal before insulating.

Frequently asked questions

How many inches of insulation is R-49?

About 13 in of blown cellulose (R-3.7/in), 15 in of fiberglass batts (R-3.2/in) or nearly 20 in of loose-fill fiberglass (R-2.5/in).

What R-value should my attic have?

ENERGY STAR, based on the 2021 IECC, recommends R-30 in climate zone 1, R-49 in zones 2–3 and R-60 in zones 4–8 for an uninsulated attic, or R-25 to R-49 added on top of 3–4 in of existing insulation.

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Sources

Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.

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