Garage wall & ceiling insulation calculator
Add the wall area and the ceiling area of the garage and get the batt bundles to buy. This sizes the walls and ceiling only — the garage door and its kit are a separate job.
Calculator
400 ft² walls + 400 ft² ceiling = 800 ft² — about 10 bundles. This insulates garage WALLS/CEILING only — the garage DOOR and its insulation kit are out of scope (garagedoorcalcs).
Formula
total_area = wall_area_ft² + ceiling_area_ft²
bundles = ceil( total_area ÷ coverage_per_bundle )
Add the two surfaces you are insulating and divide by what one bundle covers, rounded up to whole bundles. Set the ceiling to 0 if you are only doing the walls (a detached garage) or the walls to 0 if it is only the ceiling below a bonus room.
Worked example
A garage with 400 ft² of walls and 400 ft² of ceiling, R-13 batts covering 88 ft²/bundle:
Total: 400 + 400 = 800 ft². Bundles: ceil(800 ÷ 88) = ceil(9.09) = 10 bundles.
So order 10 bundles. If the garage ceiling is under a heated room above, insulate it to the attic/floor target for your zone; the shared wall to the house matters more than the exterior walls if the garage stays unconditioned.
What to insulate first
Prioritize the surfaces that touch conditioned space. If the garage is unheated, the walls and ceiling that separate it from the house (the common wall and the floor of a room above) do the real work — insulate those to the same target as any interior/attic assembly. The exterior garage walls matter only if you plan to heat or cool the space.
Match the batt to the framing, do not compress. Garage walls are usually 2×4 — use R-13 or R-15. A garage ceiling under a bonus room is often 2×10 or 2×12, so use a deep batt or blow it. Do not cram a thick batt into a shallow bay; sized-to-fit is what holds the R (see the wall calculator for the cavity-R math).
Air-seal the common wall and any fire-rated details. The wall and ceiling between the garage and the living space usually need to be air-sealed and, by code, fire-rated (typically 5/8" Type X drywall on the garage side). Insulation goes in the cavity; the drywall and sealing are separate work that keeps fumes and fire out of the house — confirm with local code.
This is the walls and ceiling — not the door. The garage door and its insulation kit are a different product and a different calculation entirely (that is garagedoorcalcs). This tool sizes cavity batts for the framed walls and ceiling only.
Reference table
Labeled batt coverage per bundle by R-value — use R-13/R-15 for 2×4 garage walls, a deeper batt for a ceiling under a room.
| Batt R-value | Coverage per bundle |
|---|---|
| R-13 | 88 ft²/bundle |
| R-15 | 67 ft²/bundle |
| R-19 | 75 ft²/bundle |
| R-21 | 58 ft²/bundle |
| R-30 | 58 ft²/bundle |
| R-38 | 40 ft²/bundle |
Higher R = thicker batt = fewer ft² per bundle. Labeled planning typicals — read your bag.