Before anything else, answer this: what do you want the garage to do?
It sounds like a soft question and it is not. There are three common answers, they lead to three different jobs, and most people insulate the wrong surface because nobody asked them first.
The Three Jobs
You want the room above the garage to stop being freezing. The surface that matters is the garage ceiling. Insulating the garage walls will do almost nothing for it.
You want to use the garage itself as a workshop or gym. Now the walls, the ceiling and the garage door all matter, and you need a heat source, because insulation does not create warmth. It only slows the loss of warmth that something else provides.
You want the house to stop smelling of exhaust, or you are worried about fumes. That is the shared wall between the garage and the house, and it is a different job again, governed by the Building Code rather than by comfort.
Work out which of those you are actually solving before anyone quotes you. Paying to insulate exterior garage walls when your problem is the bedroom above is the single most common way money gets wasted here, and it is worth knowing what garage work involves for each of the three before you commit to any of them.
If the Room Above Is Cold
This is the most frequent reason people call about a garage, and it is worth understanding why the room is so bad.
That floor has a heated room on top and an unheated, uninsulated, often draughty space directly underneath. Unlike a floor over a basement, the garage is effectively outdoors: the door opens regularly and the space is ventilated by design. So the floor is a surface between a warm room and outside temperatures, and in most houses it is the least insulated surface in the building.
The fix is insulating the garage ceiling, ideally with air sealing first, because the joist cavities between garage and bedroom frequently connect to exterior wall cavities and leak air continuously.
It is also the point where a cold room upstairs stops being a mystery. If the coldest room in your house sits above the garage, you have already found your answer, and it is a smaller, cheaper job than most people assume once they realise only one surface needs doing.
One thing to check first: whether there is already insulation up there. Many houses have some, badly installed, sagging out of the joist bays, which performs close to nothing while looking like a finished job from below. Batts stapled up decades ago tend to come loose over time, and a batt hanging half out of its cavity is doing almost no work. If your garage ceiling is unfinished you can see this yourself in about a minute.
There is a second thing worth looking for, and it catches people out. Rooms above garages frequently extend slightly beyond the garage wall below, overhanging into open air at the front or side. That cantilevered section is exposed on the underside, the ends of the joist bays are often open, and it is routinely the coldest strip of floor in the room. Insulating the main ceiling and leaving the overhang untouched fixes most of the problem and leaves a cold band along one wall, which is why it is worth identifying before the work rather than after.
The Wall You Must Not Get Wrong
This is the part almost no article about garage insulation covers, and it is the one with rules attached.
Where a garage is attached to or built into a house, the Ontario Building Code requires an air barrier system between the garage and the rest of the building, and the Code states its purpose plainly: to provide an effective barrier to gas and exhaust fumes. Where membrane materials are used, all joints have to be sealed and structurally supported.
Read that again, because it reframes the whole thing. That wall's job under the Code is not thermal. It is keeping combustion products out of the place you sleep.
A point of confusion worth clearing up: a full fire separation with a rated assembly is generally not required where the garage serves only the dwelling it is attached to. Requirements are heavier for garages serving multiple units. But the air barrier requirement applies either way, and it is the one that gets compromised during renovations, when someone cuts into that wall for wiring, shelving, pot lights or a new door and never properly re-seals it.
Related and current: since 1 January 2026, carbon monoxide alarms are required in bedrooms sharing a wall, floor or ceiling with an attached garage, along with every floor containing an attached garage or fuel-burning appliance. If the cold room you are about to insulate is a bedroom over the garage, that alarm requirement applies to it.
If anyone insulates or finishes that shared wall for you, the air barrier has to be maintained. Ask the question before the work starts.
If You Want to Use the Space
Different job, bigger scope.
Walls, ceiling and the garage door all need doing, and the door is usually the weakest element by a wide margin. A standard uninsulated steel door is close to a hole in the wall thermally, and no amount of work on the surrounding walls compensates for it. Insulated door panels or a replacement door generally does more than anything else you could spend the same money on.
The other requirement is heat. An insulated garage with no heat source is simply a garage that takes longer to reach outside temperature. It will be less unpleasant than before and it will not be warm.
Worth knowing that garage walls are often built to a lighter standard than exterior wall work on the house itself, sometimes with shallower framing, which limits how much insulation will physically fit.
Finally, ventilation. If you run a car, a heater or any fuel-burning equipment in there, sealing the space up tight changes how it ventilates. That needs thinking about rather than assuming.
Older Attached Garages
Attached garages became standard in postwar subdivisions, which is most of North York and a good deal of Etobicoke's postwar streets. Houses from that era frequently have the garage ceiling uninsulated entirely, and the bedroom above is the room everyone in the family avoids.
Detached garages are a different conversation. There is no shared wall, no room above, and no code requirement about fumes, so the only question is whether you want the space usable. That is purely a comfort decision.
What It Costs
A garage ceiling on its own is one of the smaller insulation jobs there is, and considerably cheaper than most people expect, because you are treating one surface rather than a whole envelope.
A full garage, walls and ceiling and door, is a larger project and the door is often the single biggest line on it.
What insulation costs generally is a reasonable guide to how quotes are built, though the rebate position differs: the Ontario programme is aimed at the heated envelope of the house, so an unheated garage typically will not qualify. Ask before assuming it does.