Everyday ToolboxRoof Insulation Calculator
How much attic insulation do you actually need? Enter your attic area, what insulation you already have, and your climate zone — and get the extra R-value, inches, bags or rolls, and estimated cost to hit your target.
Frequently Asked Questions
How much insulation do I need in my attic?
The US Department of Energy recommends attic R-values of R-30 to R-49 in warm zones (1–2), R-38 to R-60 in mixed zones (3–4), and R-49 to R-60 in cold zones (5–8). This calculator uses R-38, R-49, and R-60 as targets for those three bands — or enter a custom target if your local code or energy audit says otherwise.
How many inches of insulation is R-49?
It depends on the material: blown cellulose ≈ 13.2 inches (R-3.7/inch), fiberglass batts ≈ 15.3 inches (R-3.2/inch), blown fiberglass ≈ 19.6 inches (R-2.5/inch), closed-cell spray foam ≈ 7.5 inches (R-6.5/inch). R-value per inch is what matters — the same R-value takes fewer inches with denser materials.
How many bags of insulation do I need?
For blown-in insulation, bags = attic area × added R-value ÷ coverage per bag. This calculator assumes a 25-lb bag of cellulose covers about 40 sq ft at R-19, and a bag of blown fiberglass about 52 sq ft at R-19 — so a 1,000 sq ft attic adding R-30 of cellulose needs about 40 bags. Always check your bag's own coverage chart; brands vary.
Should I remove old insulation before adding more?
Usually no — you can blow or lay new insulation right on top of old, dry insulation. Remove it only if it is water-damaged, moldy, or contaminated (animal waste, fire damage). If your attic has knob-and-tube wiring or recessed lights that are not rated IC, get those checked before insulating over them.
Is more insulation always better?
Up to the recommended R-value, yes — but with diminishing returns past it. Going from R-19 to R-49 saves far more than going from R-49 to R-60. Air sealing (gaps around pipes, wires, attic hatches) and soffit ventilation matter just as much: never block your soffit vents with insulation, or moisture and ice dams follow.
Is this roof insulation calculator free?
Yes — completely free, no signup, no downloads. It runs 100% in your browser, so your answers never leave your device. Costs are estimates from typical retail and installed prices — get a local quote before budgeting.