How Much Insulation for a 1,000 Square Foot Attic?
Bags or rolls
9 bags
A 1,000 square foot attic floor, joists at 24 inches on center, with 106.8 square foot bags.
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How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- Wall or ceiling area1000 sq ft
- Stud or joist spacing24 in
- Coverage per bag or roll106.8 sq ft
Bags or rolls9 bags
| Gross area | 1,000 sq ft |
|---|---|
| Framing factor at 24 in oc | 93% |
| Cavity area to insulate | 930 sq ft |
| Bags at 106.8 sq ft | 9 |
| Coverage bought | 961 sq ft |
The material list this produces
| Insulation batts, bags | 9 |
|---|---|
| Vapor retarder, sq ft (if required) | 1,000 |
Coverage per bag compared
Bag coverage is the number that decides this, and it is printed on the bag. Here is the same area at five coverages, from 40 square feet a bag up to the 106.8 default.
| Coverage per bag | Bags or rolls | Coverage bought |
|---|---|---|
| 40 sq ft | 24 bags | 960 sq ft |
| 50 sq ft | 19 bags | 950 sq ft |
| 75 sq ft | 13 bags | 975 sq ft |
| 90 sq ft | 11 bags | 990 sq ft |
| 106.8 sq ft | 9 bags | 961 sq ft |
What else matters here
A thousand square feet of attic floor at 24 inch centers is 930 square feet of cavity and nine bags. The same area at 16 inch centers would be 900 square feet of cavity and nine bags as well, but the framing factor is genuinely different and on a bigger attic it shows.
Framing at 16 inches on center takes about 10 percent of the surface and 24 inch framing takes about 7 percent, because there is simply less wood per square foot. So an attic framed at 24 inches and worked out with the 16 inch factor comes out slightly low.
Wood conducts heat far better than fiberglass, so every stud and joist is a small thermal bridge, and a wall of R-13 batts does not perform at R-13. Continuous exterior insulation is the answer to that, and it is a separate calculation.
What this calculator assumes
- Framing takes up roughly 10 percent of a wall at 16 inch on center and 7 percent at 24. Cavity area is reduced accordingly, so you do not buy insulation for the studs.
- Bag coverage varies a lot by R value and width. The default is a common R-13 kraft faced batt bag; read your own bag and change it.
Questions people ask
How many bags of insulation for a 1,000 square foot attic?
Nine bags at 24 inch joist spacing, which leaves 930 square feet of cavity after the framing factor.
Why does joist spacing change the amount of insulation?
Because the framing is not insulated. Wood takes about 10 percent of the surface at 16 inch centers and about 7 percent at 24, so wider spacing means more cavity per square foot.
What is thermal bridging?
Heat taking the easy path through the framing rather than through the insulation. It is why a wall of R-13 batts does not perform at R-13, and continuous exterior insulation is the answer to it.
The insulation calculator takes any dimensions you like and hands back the same list.
Other common sizes
23 more sizes are answered on the insulation calculator page