Garage Insulation Calculator — Bags for Walls, Ceiling & the House Wall
Enter the garage size and its doors to get the net wall and ceiling area, how many bags of batts to buy for each surface, and the drywall to cover them. It takes the garage door out of the wall area and treats the wall you share with the house separately, because that wall and a ceiling under a bedroom are the two surfaces that actually change your energy bill.
Which Garage Surfaces Are Worth Insulating
A garage is unheated space, so insulating it only pays where heat is crossing from the house into the garage, or where you plan to heat the garage yourself. That splits the surfaces into two groups.
| Surface | Worth it when | Typical batt |
|---|---|---|
| Wall shared with the house | Always, if it is empty. It is part of the house's insulated shell. | R-13 (2x4) or R-19 (2x6) |
| Ceiling under a bedroom or bonus room | Always. The room above sits on top of an unheated box. | R-19 to R-38 by climate |
| Ceiling with an attic above | When you heat the garage or the attic connects to the house's attic. | R-30 or R-38 |
| Outside walls | Only when you heat or cool the garage, or use it as a workshop. | R-13 or R-19 |
| Garage door | Same as the outside walls; a door kit costs little next to the wall work. | Foam panel kit |
Before buying anything for the shared wall, check whether it is already insulated. In most houses it was done at construction because it counts as an exterior wall of the home. Turn off the breaker, take the cover off an outlet on that wall, and look into the gap beside the box with a flashlight. If you see insulation, skip that wall and put the money into the ceiling.
For a room above the garage, the floor levels in the 2021 energy code run from R-13 in the warmest zones, to R-19 in zones 3 and 4, R-30 in zones 5 and 6, and R-38 in the coldest. That ceiling is also where most cold-bonus-room complaints come from: batts that fall short of the rim joist or drop away from the subfloor above leave a cold gap that no R-value fixes. Fill the joist bays all the way to the outside edge and hold the batts tight to the floor above with wire supports.
Why the Bag Count Is Lower Than Area Divided by Coverage
The coverage printed on a bag of batts is the area of the batts themselves. Wall batts are 15 inches wide so they friction-fit between studs set 16 inches apart, which means each batt fills 16 inches of wall even though it is only 15 inches wide. The calculator divides the wall area by the printed coverage multiplied by 16/15, about 7% more wall per bag. Ceiling batts for 16-inch framing are sold 16 inches wide, so no adjustment applies there.
Coverage per bag also varies more than people expect. The same R-13 15x93-inch batt is sold in bags of 11 pieces (about 106 sq ft) and 13 pieces (about 126 sq ft), depending on the store. The two coverage boxes above are filled with common Owens Corning figures; type in the number from the bag you are actually buying.
| Garage | Outside walls (net) | R-13 bags | Ceiling | R-30 bags | R-38 bags |
|---|---|---|---|---|---|
| One-car, 12x20 | 413 sq ft | 4 | 240 sq ft | 5 | 6 |
| 16x16 | 413 sq ft | 4 | 256 sq ft | 5 | 6 |
| Two-car, 20x20 | 492 sq ft | 5 | 400 sq ft | 8 | 10 |
| Two-car, 24x24 | 620 sq ft | 6 | 576 sq ft | 11 | 14 |
| Three-car, 32x24 | 734 sq ft | 7 | 768 sq ft | 15 | 18 |
Detached, 8 ft walls, one outside door and one window; a 9x7 door on the one-car, 16x16 and three-car (two doors), a 16x7 door on the two-car sizes. Bag coverage as above.
Faced or Unfaced Batts in a Garage
The kraft paper on faced batts is a vapor retarder and belongs on the side of the wall that is warm in winter. In an unheated garage there is no warm side on the outside walls, so unfaced batts are the simpler choice there. On the shared wall the warm side is the house, but you are installing from the garage; unfaced batts avoid the question entirely. If you plan to heat the garage, faced batts with the paper toward the garage are correct.
Whichever you use, exposed kraft paper is flammable and building codes do not allow it to be left uncovered. Plan on drywall, or use unfaced batts behind plywood or OSB if you want to hang tools on the walls.
Drywall: What Code Requires on the Garage Side
An attached garage has to be separated from the house by gypsum board on the garage side. Under the residential code that means at least 1/2-inch drywall on the shared wall, and 5/8-inch Type X on the ceiling when there is a habitable room above. The calculator shows the sheet count for every surface you insulate and flags the Type X ceiling when you choose the room-above option. The differences between the two thicknesses are covered in 1/2 vs 5/8 drywall. Keep the door from the garage into the house solid-core or fire-rated as well; insulating around it does not change that requirement.
Frequently Asked Questions
How many bags of insulation do I need for a 2-car garage?
For a 20x20 two-car garage with 8-foot walls and a 16x7 door, about 5 bags of R-13 batts for the outside walls and 8 bags of R-30 for the ceiling, using 106 sq ft and 53 sq ft bags. A 24x24 garage needs about 6 and 11. Check the coverage on the bag you buy, since pack sizes vary.
How many bags of insulation for a 16x16 garage?
About 4 bags of R-13 for the walls, with a 9x7 door, one entry door and one window taken out, and 5 bags of R-30 (or 6 of R-38) for the ceiling.
Is it worth insulating garage walls?
The wall shared with the house and a ceiling under a living space are worth it every time. Outside walls only pay off if you heat or cool the garage, because otherwise the garage door and slab lose heat far faster than insulated walls can save.
Do I need drywall over garage insulation?
On an attached garage, yes: at least 1/2-inch gypsum on the house wall and 5/8-inch Type X on a ceiling with a room above. Kraft-faced batts cannot be left exposed anywhere, so they need a covering even in a detached garage.