How Much Insulation for a 20x24 Garage?
Bags or rolls
6 bags
The four walls of a 20 by 24 garage at 8 foot plate height, 88 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 area704 sq ft
- Stud or joist spacing16 in
- Coverage per bag or roll106.8 sq ft
Bags or rolls6 bags
| Gross area | 704 sq ft |
|---|---|
| Framing factor at 16 in oc | 90% |
| Cavity area to insulate | 634 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) | 704 |
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 | 16 bags | 640 sq ft |
| 50 sq ft | 13 bags | 650 sq ft |
| 75 sq ft | 9 bags | 675 sq ft |
| 90 sq ft | 8 bags | 720 sq ft |
| 106.8 sq ft | 6 bags | 641 sq ft |
What else matters here
Eighty eight feet of wall at eight feet is 704 square feet gross, 634 of cavity, six bags. Six bags buys 641 square feet, so there are seven square feet spare, which means buy seven bags.
The bigger issue on a garage of this size is that the largest single surface is the one the batts do not go in. A 16 by 7 foot overhead door is 112 square feet, and an uninsulated steel door is close to R-2 no matter how much fiberglass is in the walls around it.
Door insulation kits exist and they are worth doing if the garage is heated, but the honest order of value is: seal the perimeter weatherstrip and the bottom seal first, then insulate the door, then worry about the walls. Air moving through a bad door seal beats any R value argument.
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 20x24 garage?
Six bags covers 641 square feet against 634 of cavity, which is tight. Seven bags is the safer order, and the ceiling is separate.
Should I insulate the garage door?
If the garage is heated, yes, but seal the weatherstrip and the bottom seal first. Air leakage around the door costs more than the door's R value does.
What R value is an uninsulated garage door?
Around R-2 for bare steel. It is the weakest surface in the room by a wide margin, and it is often the largest one too.
The insulation calculator takes any dimensions you like and hands back the same list.