How Much Insulation for a 20x20 Garage?
Bags or rolls
6 bags
The four walls of a 20 by 20 garage at 8 foot plate height, 80 linear feet of wall, framed at 16 inches on center.
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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 area640 sq ft
- Stud or joist spacing16 in
- Coverage per bag or roll106.8 sq ft
Bags or rolls6 bags
| Gross area | 640 sq ft |
|---|---|
| Framing factor at 16 in oc | 90% |
| Cavity area to insulate | 576 sq ft |
| Bags at 106.8 sq ft | 6 |
| Coverage bought | 641 sq ft |
The material list this produces
| Insulation batts, bags | 6 |
|---|---|
| Vapor retarder, sq ft (if required) | 640 |
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 | 15 bags | 600 sq ft |
| 50 sq ft | 12 bags | 600 sq ft |
| 75 sq ft | 8 bags | 600 sq ft |
| 90 sq ft | 7 bags | 630 sq ft |
| 106.8 sq ft | 6 bags | 641 sq ft |
What else matters here
Eighty feet of wall at eight feet is 640 square feet gross, 576 of cavity, six bags for the walls. The ceiling is a separate order.
Before buying any of it, decide whether the garage is being conditioned. Insulating the walls of a garage that has no heat does not make it warm, it makes it lag: slower to cool down and slower to warm up, which is worth something for a workshop and almost nothing for parking.
If only one thing gets insulated, it should be the wall and ceiling shared with the house, and those are usually already required to be. That is the boundary that is costing money on the heating bill. An attached garage should also be air sealed at that boundary for a second reason, which is keeping exhaust fumes out of the living space.
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 20x20 garage?
Six bags for the four walls at 8 foot plates, based on 640 square feet of wall and 576 of cavity. The ceiling is extra.
Is it worth insulating an unheated garage?
It slows the temperature swings rather than making it warm. Worth it for a workshop, not for parking, unless you are adding heat.
Which garage wall matters most?
The one shared with the house, along with the ceiling under living space. That is the boundary on the heating bill, and it usually has code requirements of its own.
The insulation calculator takes any dimensions you like and hands back the same list.