How much insulation for a 400 sq ft attic?
A 400 sq ft attic floor insulated to R-38 with blown-in insulation needs about 10 bags, at roughly 48 sq ft per bag, with a 10% allowance for settling. Your required R-value is set by local code — this sizes the quantity, not the R-value.
- Area
- 400 sq ft
- Area
- 400 sq ft
- With allowance
- 440 sq ft
- Purchased
- 480 sq ft
- Leftover (est.)
- 80 sq ft
Based on ~48 sq ft per bag of blown-in at R-38 and a 10% settling allowance. Coverage per bag changes with R-value — read your bag’s chart. The required R-value is set by your local code.
Change the numbers in the insulation calculator →How this is worked out
Insulation calculator uses the same method as every tool here: measure the area, add a visible allowance, divide by what one bag covers, and round up. The number above is the honest planning quantity — not an exact cut list. Confirm coverage on your product before buying.
buy = net area × (1 + allowance) ÷ per-bag coverage → round up
Other sizes
Related
- Insulation calculator — change any number for your job
- Room material planner — every material on one shopping list
- Working on the drywall below?
Frequently asked questions
How many bags of insulation for a 400 sq ft attic?
About 10 bags of blown-in at R-38 (~48 sq ft per bag) with a 10% settling allowance. At a higher R-value a bag covers less, so you need more — use your bag's coverage chart.
Does this tell me what R-value to use?
No. The required R-value is set by your local building code and climate zone. This sizes the number of bags once you know your target R-value.