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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
Most garage insulation advice ignores the two things that decide whether it works: which walls are shared with the house, and the fact that the door is a large uninsulated panel that nothing else compensates for.
With these inputs:
| Result | Value |
|---|---|
| Ceiling area | 528 sq ft |
| Shared wall with the house | 216 sq ft |
| Exterior wall area | 500 sq ft |
| Garage door area | 112 sq ft |
| Order with waste | 1,368 sq ft |
| R-value at this depth | R-11.2 |
| Zone target for walls | R-20 |
| Biggest weak point | Ceiling or shared wall |
Ceiling area = length x width · Gross wall area = 2 x (length + width) x height · Shared wall = length x height (one wall) or (length + width) x height (two) · Exterior wall = gross wall - shared wall - door area
It depends on whether the garage is attached. On an attached garage the shared wall and the ceiling under living space are genuinely worth doing, because they separate conditioned space from unconditioned and your heating already pays for that difference. On a detached garage you are heating a new building, which is a comfort decision rather than an efficiency one.
The one shared with the house, then the ceiling if there is living space above. Those two are where heat is currently leaving rooms you pay to warm. Exterior garage walls only matter once you intend to heat the garage itself, and the calculator separates them for exactly this reason.
If you are heating the garage, yes — the door is often a fifth or more of the total wall area, and leaving it bare wastes much of what the walls gained. The calculator shows the door as a percentage of your wall area so the trade-off is visible. Bear in mind an insulated door is heavier and may need the opener springs adjusted.
The same zone recommendation as any exterior wall applies if you are conditioning the space, and a 2x4 wall limits you to what 3.5 inches of your chosen material gives. Where garages differ is that the ceiling under living space should be treated as a floor assembly and generally wants more, not less, than the walls.
Be careful here. A garage holds a car that brings in snow, rain and exhaust, and sealing it tightly without ventilation traps both moisture and combustion products. Air sealing between the garage and the house is important and worth doing properly; making the garage itself airtight is not the same goal and can be actively unsafe.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.