Insulation Calculator
Inches of insulation by type to hit your target R-value, with cost. Updated for 2026.
Last reviewed July 2026 · Free · Nothing you enter is stored
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Planning estimate using standard formulas and 2026 national average prices — confirm quantities with your supplier and get local quotes.
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How this calculator works
Depth needed = target R-value ÷ R-per-inch for your material. Cost estimates use 2026 installed averages: fiberglass batts ~$0.90/sq ft, blown cellulose ~$1.45/sq ft (attic at R-49), open-cell spray foam ~$0.55 and closed-cell ~$1.25 per board foot (1 sq ft at 1 inch). DIY blown-in with a rented machine typically halves the cellulose figure.
Frequently asked questions
How many inches of insulation is R-49?
About 14 inches of blown cellulose (R-3.5/inch), 16 inches of fiberglass batts, 20 inches of blown fiberglass, or 7.5 inches of closed-cell spray foam. Mark the target depth on rafters before blowing.
What R-value do I need in my attic?
R-49 minimum for most of the US; R-60 in cold northern zones; R-38 can suffice in the Deep South. If your current insulation is below the joist tops, you almost certainly benefit from topping up.
Is spray foam worth the cost?
For rim joists, cathedral ceilings, and tight cavities where air sealing matters most — often yes, especially closed-cell. For open attic floors, blown cellulose delivers the same R-value for a quarter of the price.
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