AtticCare

99% of the homes in this country with garages, have shared attic space. There is NOTHING wrong with it being open. Also, since it is a garage, try your best to seal the door, seal any leaks you can find, this will help your insulation perform better. I have filled garage walls and insulated the ceiling to R-30, for less than 500$, and 3 hours.

mudmixer

I hope the garage ceiling is a fire-rated constructioon since the attic above is connected to the home attic.

A well insulated attached garage can be a problem in the summer when you pull in two hot cars in the summer. In the winter, it can be nice, but not really needed because the cars would have no problem starting in the cold since even a minimal car will still start wherever it is parked during the day or between errands.

The good thing about an attached garage is that it can melt the ice chunks (if the floor is sloped and drasined), but I prefer to kick them off at the store parking lot and let Mother Nature and the sun do the disposal.

I have heated garages in cold climates, but can be a problem when you wheel the car out into the cold.

Dick