How Much Insulation for 300 Square Feet?
Bags or rolls
3 bags
300 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 area300 sq ft
- Stud or joist spacing16 in
- Coverage per bag or roll106.8 sq ft
Bags or rolls3 bags
| Gross area | 300 sq ft |
|---|---|
| Framing factor at 16 in oc | 90% |
| Cavity area to insulate | 270 sq ft |
| Bags at 106.8 sq ft | 3 |
| Coverage bought | 320 sq ft |
The material list this produces
| Insulation batts, bags | 3 |
|---|---|
| Vapor retarder, sq ft (if required) | 300 |
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 | 7 bags | 280 sq ft |
| 50 sq ft | 6 bags | 300 sq ft |
| 75 sq ft | 4 bags | 300 sq ft |
| 90 sq ft | 3 bags | 270 sq ft |
| 106.8 sq ft | 3 bags | 320 sq ft |
What else matters here
Three hundred square feet gross is 270 square feet of cavity and three bags, which buys 320 square feet. A small job, and often the most valuable three bags in the house.
Three hundred square feet is roughly the rim joist band around an average house: the strip of framing at the top of the foundation wall where the floor joists land. It is thin, it is directly exposed to outside air, and in most older houses it is either empty or stuffed with a loose batt that air walks straight past.
Batts are the wrong material there. The band joist wants an air seal first, which means rigid foam cut to fit each bay and sealed around the edges with canned foam, or two part spray foam. A fiberglass batt alone lets warm indoor air reach a cold surface, and that is how you get condensation and eventually rot on the sill plate.
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 300 square feet?
Three bags at 106.8 square feet per bag, covering the 270 square feet of cavity left after framing.
What is the best insulation for a rim joist?
Rigid foam cut to fit and sealed at the edges, or closed cell spray foam. A plain fiberglass batt does not stop the air movement that causes condensation there.
Why is the cavity area less than the wall area?
Because studs and plates take up part of the wall and they are not insulated. Framing runs about 10 percent of a wall at 16 inch on center and about 7 percent at 24 inch, so the calculator takes that off before dividing.
The insulation calculator takes any dimensions you like and hands back the same list.