How Much Insulation for 400 Square Feet?
Bags or rolls
4 bags
400 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 area400 sq ft
- Stud or joist spacing16 in
- Coverage per bag or roll106.8 sq ft
Bags or rolls4 bags
| Gross area | 400 sq ft |
|---|---|
| Framing factor at 16 in oc | 90% |
| Cavity area to insulate | 360 sq ft |
| Bags at 106.8 sq ft | 4 |
| Coverage bought | 427 sq ft |
The material list this produces
| Insulation batts, bags | 4 |
|---|---|
| Vapor retarder, sq ft (if required) | 400 |
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 | 9 bags | 360 sq ft |
| 50 sq ft | 8 bags | 400 sq ft |
| 75 sq ft | 5 bags | 375 sq ft |
| 90 sq ft | 4 bags | 360 sq ft |
| 106.8 sq ft | 4 bags | 427 sq ft |
What else matters here
Four hundred square feet gross, 360 of cavity, four bags, and 427 square feet bought. That is the size of one interior partition wall run in a normal house, which is where sound batts usually go.
Understand what a batt does for sound, because it is easy to expect too much. Filling a stud cavity absorbs the sound bouncing inside it, which cuts the hollow drum effect and takes a standard partition from around STC 33 to the high 30s. That is a real improvement and it is not soundproofing.
The bigger gains come from mass and decoupling: a second layer of drywall, a viscoelastic damping compound between layers, resilient channel, or staggered studs. And none of it works if the wall has a gap. Sound moves through an unsealed outlet box or an undercut door far more easily than through the wall itself.
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 400 square feet?
Four bags at 106.8 square feet per bag, covering the 360 square feet of cavity left after framing.
Do insulation batts soundproof a wall?
They help. A filled cavity takes a standard partition from about STC 33 to the high 30s. Going further needs mass, damping or decoupled framing.
What ruins a sound wall?
Gaps. An unsealed outlet box, a gap at the top plate or an undercut door lets more sound through than the whole wall assembly does.
The insulation calculator takes any dimensions you like and hands back the same list.