Everyday
Attic Insulation Calculator
Pick your IECC climate zone for the target R-value, enter your existing insulation, and the page figures how many inches to add, in blown-in or batt form, with DIY vs. contractor costs.
How it works
Behind the numbers
The target comes from IECC 2021 Table R402.1.3: R-30 in climate zone 1, R-49 in zones 2 and 3, R-60 in zones 4 through 8. These are code minimums for new construction, not comfort maximums. Your county's zone is on the DOE climate zone map.
Existing insulation counts toward the target. Six inches of fiberglass batts at R-3.1 per inch is about R-19, so a zone 4 attic needs roughly R-41 more. The calculator divides the remaining R by the new material's R-value per inch: blown cellulose at R-3.5 per inch needs fewer inches than blown fiberglass at R-2.5.
The cost section compares two paths. DIY is material per square foot plus a blower rental, which runs about $75 a day at most home centers. Contractor pricing is a single per-square-foot installed figure. Both are editable estimates; local quotes decide the real number.
FAQ
Questions, answered
What R-value should my attic have?
Under IECC 2021, ceilings need R-30 in climate zone 1, R-49 in zones 2-3, and R-60 in zones 4-8. These are code minimums. Many energy advisors suggest going beyond code in cold climates because the extra inches are cheap relative to the labor.
How many inches of blown cellulose is R-49?
About 14 inches at R-3.5 per inch. Blown fiberglass at R-2.5 per inch needs about 19.6 inches for the same R-49. Check your product's bag chart; per-inch values vary by brand.
Can I add new insulation over old insulation?
Yes, if the old insulation is dry and in decent shape. Air-seal gaps and penetrations first, since insulation slows heat flow but does not stop air leaks. Do not trap moisture between layers.
Is blown-in or batt insulation better for attics?
Blown-in covers irregular joist bays and goes around obstructions without cutting, which suits attics. Batts work where bays are open and regular. Installed performance depends more on coverage and air sealing than on the material choice.