Rooms above garages are often done improperly. It's amazing, because it really isn't all that difficult, but a lot of people seem to mess it up.
Unfortunately, there's no "easy" way: You're going to have to remove some drywall to inspect and see how the insulation in the ceiling is done. There may or may not be a gap in the space, depending on how the ceiling was done. Even just taking the temperature above the insulation in that space will tell you a lot (above the insulation, it should be close to room temperature).
Drafts in that space are absolutely killer, it means there is a direct source of outside air. Black on the insulation is a good sign of airflow, indicating drafts.
Basically, if the ceiling space isn't totally sealed, the only thing you can do is turn the garage into a conditioned space (very expensive, in terms of both construction costs and ongoing energy costs), and even then, if the problem is between the insulation and sub-floor, it won't help.
What should be there at a minimum is a continuous vapour barrier, with a layer of insulation on top. The vapour barrier should be sealed to the subfloor or the vapour barrier from the walls above. The insulation should go all the way around the edge (in the headers), so the insulation is continuous from the floors up to the subfloor. There should absolutely be no drafts, exposed concrete, or anything directly connected to the outside that is uninsulated.
The best way (in my opinion) to seal this space is to use closed-cell spray foam, and form a continuous seal across the bottom of the entire subfloor. This gives you great insulation, no drafts, and acts as a vapour barrier as well (vapour barrier is not needed in this case -- though some building inspectors still don't understand this, so check your local codes). It also is better at ensuring fumes from vehicles in the garage can't get into your living space (in theory, vapour barrier prevents this as well, but 6mil vapour barrier is easier to puncture than a couple inches of hardened foam).
Though you may be able to find something obvious and fix it, be prepared that there's a possibility that the only "fix" is going to be to completely tear down the ceiling of the garage and re-do it properly.
If you're going to spend money and time on this, do it right.
I'm in a temp range about 5C more in each direction. Attached garages tend to be about half as cold as outside so a thermal break is less important here. It wouldn't be a huge loss if you just screwed the gypsum board directly to the underside of the floor joists. However, it's easy to get a thermal break without much more effort, and it's almost as easy as applying directly to floor joists.
After applying the main insulation, sheath the bottom of the joists with 1/2" polyiso (polyisocyanurate) insulation board -- your thermal break. Then using longer screws, apply the gypsum directly to the joists through the polyiso.
Highest possible ceiling, thermal break if you want one, level application, no faffing about with hangers. Wins all around.
Best Answer
There are different profiles of duct available. In height restricted areas you can switch to a rectangular flat profile which might give you the ability to better insulate.