How Much Insulation for 900 Square Feet?
Bags or rolls
8 bags
900 square feet of wall or ceiling, framed at 16 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 area900 sq ft
- Stud or joist spacing16 in
- Coverage per bag or roll106.8 sq ft
Bags or rolls8 bags
| Gross area | 900 sq ft |
|---|---|
| Framing factor at 16 in oc | 90% |
| Cavity area to insulate | 810 sq ft |
| Bags at 106.8 sq ft | 8 |
| Coverage bought | 854 sq ft |
The material list this produces
| Insulation batts, bags | 8 |
|---|---|
| Vapor retarder, sq ft (if required) | 900 |
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 common coverages.
| Coverage per bag | Bags or rolls | Coverage bought |
|---|---|---|
| 40 sq ft | 21 bags | 840 sq ft |
| 50 sq ft | 17 bags | 850 sq ft |
| 75 sq ft | 11 bags | 825 sq ft |
| 90 sq ft | 9 bags | 810 sq ft |
| 106.8 sq ft | 8 bags | 854 sq ft |
What else matters here
Nine hundred square feet gross, 810 of cavity, eight bags, 854 square feet bought.
Fiberglass works by trapping still air, which means how it is installed decides what it delivers. A batt that is compressed, gapped at the edges, folded over a wire or cut short at the top plate does not perform to its label, and the industry has a grading system that says so: Grade I is a proper fill, Grade III can lose a third of the rated R value.
The failures are all visible before the drywall goes on. Look for batts that stop short of the plate, batts pushed in behind wiring instead of split around it, gaps at the sides of the cavity, and voids behind electrical boxes. Every one of those is a few minutes to fix while the wall is open and impossible afterwards.
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 900 square feet?
Eight bags at 106.8 square feet per bag, covering the 810 square feet of cavity left after framing.
Does compressing insulation reduce its R value?
Yes. Fiberglass works by holding still air, so a squashed, gapped or folded batt can lose a third of its rated value.
How do I know insulation was installed properly?
Look before the drywall goes up: full contact at the top and bottom plates, no gaps at the sides, no batts stuffed behind wiring, and no voids behind electrical boxes.
The insulation calculator takes any dimensions you like and hands back the same list.