Attic insulation calculator: how much insulation do I need?
Measure the attic floor, check how deep the insulation is now, and pick your climate zone. You get how many inches to add and about how many bags of blown-in fiberglass or cellulose.
- ENERGY STAR targets
- Fiberglass or cellulose
- Bags from the chart
Bag coverage changes a lot between products. Find the R-38 row on your bag's coverage chart and enter its square feet per bag. Batts list their R-value on the facing.
Before you insulate
How it works
What you have = current depth × R per inch. Fiberglass defaults to 2.5 and cellulose to 3.5, the middle of the settled ranges in the Department of Energy's Building America guide (2.2 to 2.7 for fiberglass, 3.2 to 3.8 for cellulose).
Target comes from ENERGY STAR's table for an uninsulated attic: R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8. Pick your zone or type your own number.
Inches to add = (target − what you have) ÷ new R per inch. The defaults come from maker coverage charts: Owens Corning AttiCat fiberglass reaches R-38 at 13 inches (about 2.9 per inch), and GreenFiber cellulose at 10.77 inches settled (about 3.5 per inch).
Bags = area × R-value to add ÷ (38 × square feet per bag at R-38), rounded up. Defaults are 51.4 square feet per bag for AttiCat and 16.4 for GreenFiber at R-38. Always check the chart on your bag.
About this tool
Who it's for
For homeowners adding blown-in insulation on top of what is already in the attic.
Example
You enterA 40x25 attic with 4 inches of fiberglass, zones 4 to 8, blown fiberglass
You getAbout 17 inches to add and 26 bags.
What's different
- Counts the insulation you already have.
- Uses ENERGY STAR targets by climate zone.
- Takes bag coverage from your product's chart.
Gotchas and tips
- Compressed or wet insulation is worth less than its depth suggests.
- Don't cover soffit vents. Use baffles.
- Old vermiculite (gray-brown, pebbly) may contain asbestos. Leave it alone and call a pro.
Privacy: Runs entirely in your browser. Nothing you type is sent to us, and anything it remembers is saved on this device only.
Built and maintained by the GrabTheHammer team, using published figures from manufacturers and agencies, listed below. Updated November 2026. Spot a problem? Tell us.
Insulation questions
How much attic insulation do I need?
ENERGY STAR recommends R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8 for an attic that has none. With blown fiberglass, R-60 is roughly 20 inches.
Can I put new insulation over old?
Yes, as long as the old insulation is dry and not moldy. Loose-fill can go right on top of batts or older loose-fill.
How many bags of blown-in insulation do I need?
It depends on the product. Owens Corning lists 19.5 bags of AttiCat per 1,000 square feet for R-38. Cellulose bags are smaller and cover less, so you need more of them.
Sources: ENERGY STAR: recommended R-values; DOE Building America: loose-fill R per inch; Owens Corning AttiCat coverage chart; GreenFiber DIY coverage chart.