Roof & Attic Insulation Calculator — ft, R-value, cost
How to use this calculator
Enter your attic floor area (length x width, or a custom square footage for irregular layouts), pick the insulation product and the target R-value for your IECC climate zone. If insulation is already in place, enter its depth and type — its R-value is subtracted so you only buy what is needed to top up. The diagram shows the existing layer plus the new layer on the joists.
Insulation types — blown fiberglass (R-2.5/in), blown cellulose (R-3.7/in), batt fiberglass (R-3.2/in), batt mineral wool (R-3.3/in).
Cost — price per bag or roll plus optional vapor barrier and ventilation baffles.
Saved Calculations
| Time | Type | Area ft² | Depth | R-Value | Qty | Cost |
|---|
Attic Insulation: Sizing for Target R-Value
Attic floors are the cheapest place to add R-value, so this estimator works in loose-fill bags or batt rolls and a waste allowance of 5–10% for joist obstructions, baffles and overlap trimming. R-values used per inch are R-2.5 blown fiberglass, R-3.7 dense-pack cellulose, R-3.2 fiberglass batt and R-3.3 mineral-wool batt — typical settled values, not freshly-installed peaks. Coverage per bag falls as you target more depth, which is why a 1100 ft² attic at R-49 needs far more bags than the same attic at R-30. Pair this with the wall insulation calculator to size the whole thermal envelope.
Formulas
Rneed = max(0, Rtarget − Rexisting); depth to add = Rneed / R-per-inch. For blown products, bags = area × (1 + waste) ÷ (bag coverage at that depth); for batts, rolls = area × (1 + waste) ÷ roll coverage. Existing R = existing depth × R-per-inch of the in-place material — layers are additive regardless of type.
Climate Zone Guide (IECC 2021)
Zone 1-3: R-30 minimum. Zone 4: R-38. Zone 5-6: R-49. Zone 7-8: R-60. For a high-performance roof deck instead of a vented attic, compare spray foam at the rafters, and add a ceiling vapor barrier on the warm side in cold zones.
FAQ
Can I mix insulation types? Yes. Blown insulation can be added over existing batts; R-values add together regardless of type, so leave serviceable batts in place and top up. Do I need a vapor barrier? In cold climates (zones 5+) a vapor retarder on the warm-in-winter side is recommended; in mixed climates consult local code. For roof-to-wall continuity also see the house wrap calculator and the carpet calculator if you are finishing a conditioned attic floor. Why does coverage per bag change? Loose-fill bags are rated by net coverage at a stated depth; deeper installs cover less floor area per bag, so always size by the target R-value.
On install day
Installed 2025 pricing in the US runs about $1.00–1.80/ft² for blown fiberglass at R-49, $1.20–2.20/ft² for dense-pack cellulose, and $1.80–3.50/ft² for batt mineral wool — labor is typically half of that on a DIY-vs-pro split. The #1 estimator mistake on attic top-ups is forgetting to install baffles at every soffit bay before blowing — without them the new insulation chokes the soffit vents and the roof deck rots, and IRC §R806 requires that vent path stay clear. Spec the depth as final settled (cellulose drops ~15–20% in the first year) and mark the joists with depth rulers before the truck arrives so the installer hits the target R, not a guess.
