We are currently building our double garage. Does it make sense to insulate the concrete ceiling?
The 6x6.7m double garage with partition is not connected to the house and stands alone in the front garden.
Floor construction; approximately 65 cm of gravel, strip foundation, approximately 15 cm deep concrete, no insulation, approximately 6 cm of screed. Brickwork; 17.5 lime sandstone, lime-cement plaster with insulation components. Flat single-pitched roof with a slope of 6%. There will be parking for cars and something for the garden, etc.
No do-it-yourself or hobby classes are planned. No heating is planned. Do you really need insulation on a concrete ceiling in these conditions?
An airtight garage is very, very bad. Moisture stays on the car and it rusts much faster. It's not good for electronics either.
Since you don't plan to heat the garage, make sure it has air exchange. This also makes the question of insulation unnecessary.
Especially during the winter months, you will encounter the problem of mold on the garage ceiling:
A large amount of moisture enters the garage with the cars and evaporates there.
The indoor air in the garage, while unheated, is colder but warmer than the outdoor air, especially at night. So where are the coldest parts of the garage? The walls and especially the ceiling. If the wall adjacent to the house is not the north wall of the garage, it will be especially cold.
Consequently, room air will condense on that cold ceiling (a heat or cold bridge) and mold will form.
Now you can argue about how much the ceiling insulation will help. If I were you, I would immediately tighten any existing screws:
Insulate the ceiling and, if possible, the exterior walls of the garage (you've already written about the components of insulation).
The garage door is relatively airtight, which is good in principle. However, you need to vent the moist air outside. I recommend thinking about core drilling for supply and exhaust air in conjunction with fans and a dew point controlled ventilation system (look it up online).
I would paint the ceiling and interior walls with antifungal paint, such as paint containing silver.
Finally, I would make provisions to run at least one heating pipe in the garage. If all of the above is not enough, you can install a small radiator there later. This is sub-optimal from an energy standpoint, but the garage should be at least somewhat insulated, and it should not be heated to 20 degrees. So if the radiator runs three weeks out of the year, I'd rather have that than mold. Even if you want to furnish a work nook in the garage, you'll appreciate it if the temperature there is 12 degrees instead of 2 degrees...
I believe that an unheated garage doesn't make sense to insulate just under the ceiling.
Insulation makes no sense in my opinion, the garage is cold anyway. You'll only provoke condensation.