Attic Insulation Calculator

About this attic insulation calculator

Insulation is rated by R-value — resistance to heat flow — and attic targets run R-30 to R-49 depending on climate (R-38 is the common middle target; cold climates push toward R-49). This calculator takes your attic's area, subtracts what you already have, and converts the gap into inches to add for your chosen product: blown fiberglass at R-2.5 per inch, blown cellulose at R-3.7, or fiberglass batts at R-3.2.

To find your current R-value, measure the depth of existing insulation and multiply by its R-per-inch — or read it off the product if you know it. Old, compressed insulation insulates worse than its label depth suggests; if yours is flattened or patchy, treat the current value as the lower end of your estimate.

Bag counts for blown insulation are the softest number here, because manufacturers' coverage per bag changes with target depth. This calculator assumes about 25 cubic feet of blown material per bag and says so on the result — always confirm against the coverage chart printed on the bag you actually buy, since that chart is the authority.

How to use

1. Measure your attic length and width in feet (the insulated floor area).
2. Pick your current R-value — measure existing depth × R-per-inch if you are unsure, or 0 for no insulation.
3. Pick your target R-value (R-38 is a common target; R-49 for cold climates).
4. Choose your product, then press Calculate for inches to add and quantity.

Good to know

• Bag estimate assumes ~25 cu ft per bag of blown material — real coverage varies by brand and target depth; the product's coverage chart governs.
• Assumes a flat attic floor; vaulted ceilings, dormers, and kneewalls need separate measurements.
• This is a quantity tool, not an energy audit. Ventilation (soffit/ridge balance), vapor control, and local code all matter — and air sealing comes first.