Attic insulation calculator
Measure the insulation you already have and choose a target R-value. The calculator tells you how much more to add.
Add
11.1 inches of insulation
- Current R-value
- R-10 (estimated)
- R-value to add
- R-39
- Volume to add
- 1,114 cubic feet
- Attic floor area
- 1,200 sq ft
How it works
R-value measures resistance to heat flow, and each type of insulation provides roughly a fixed R-value per inch. The calculator estimates your current R-value (depth × the R-per-inch of what you have), subtracts it from your target, and divides by the R-per-inch of what you’re adding.
For blown-in insulation, the bag’s coverage chart tells you how many bags give that depth over your attic area. Use it for the bag count, since densities vary by product.
Example
With attic length 40 ft, attic width 30 ft, target R-value: r-49, existing insulation depth 4 in, existing insulation type: loose-fill fiberglass (about R-2.5/in), insulation you’re adding: loose-fill cellulose (about R-3.5/in), the answer is 11.1 inches of insulation.
Tips before you buy
- ENERGY STAR publishes recommended attic R-values by climate zone. Most of the US falls in the R-49 to R-60 range.
- Seal air leaks around light fixtures, pipes and the attic hatch before adding insulation. Leaks undo a lot of insulation.
- Keep insulation clear of soffit vents with baffles, and away from recessed lights not rated for contact.
- Old, compressed insulation performs below its nominal R-value, so the estimate here may be optimistic.
Common questions
How many inches of blown-in insulation for R-49?
About 14 inches of cellulose, or roughly 20 inches of loose-fill fiberglass.
How much attic insulation do I need?
Most US homes should aim for R-49 to R-60 in the attic, or R-30 to R-49 in the warmest areas.
Can I put new insulation over old insulation?
Usually yes, if the old insulation is dry and not moldy. Unfaced insulation over the top avoids trapping moisture.