Insulation Calculator
Estimate how many bags of batt insulation you need for an area, and the thickness a target R-value takes.
How it works
bags = ceil(area ÷ coverage per bag) · R-value ÷ ~3.2 per inch ≈ fiberglass batt thickness
Divide the area you are insulating by the square footage one bag covers and round up. The R-value is the resistance to heat flow: fiberglass batts add roughly R-3.2 per inch, so a target R-value implies a thickness — and a cavity depth. Fitting the batt to the cavity depth without compressing it is what actually delivers the rated R-value.
Worked example
500 sq ft of attic floor at 40 sq ft per bag needs 500 ÷ 40 = 12.5, rounded up to 13 bags. Hitting R-30 there takes roughly 9.5–10 in of batt.
Frequently asked questions
What R-value do I need?
It depends on climate and location: attics commonly call for R-38 to R-60, 2×6 walls around R-19 to R-21, and floors R-25 to R-30. The US is split into climate zones with recommended values — check the zone your home is in.
Why can’t I compress insulation to fit?
R-value comes from the trapped air in the fibers. Squashing an R-30 batt into a shallow cavity can drop it to R-20 or less. Use a batt sized for the cavity depth, and split it around wiring rather than crushing it.
Do I need a vapor barrier?
In colder climates, faced batts with the kraft or foil facing toward the heated (interior) side help control moisture. Requirements vary by climate zone and assembly, so confirm against local code before closing the wall.
Related calculators
Estimates only — add for waste and confirm against local building codes and supplier specs.