ᐅ Insulate a Concrete Garage Ceiling or Not?

Hello!

Our double garage is currently under construction.

I would like to hear your opinions on whether insulating the concrete ceiling is worthwhile, as there seem to be differing views on this.

Here is some information about our project to help you give more informed feedback:

The 6 x 6.7 m (20 x 22 ft) double garage with a dividing wall is not connected to the house and is located freestanding in the front garden area.

Floor structure: about 65 cm (26 inches) of gravel, strip foundation, approximately 15 cm (6 inches) thick waterproof concrete in the center, without insulation, about 6 cm (2.5 inches) screed
Masonry: 17.5 cm (7 inches) calcium silicate blocks, lime-cement plaster with insulating components
Iso 45 doors from Novoferm
Flat shed roof with a 6% slope

Cars will be parked there and some storage for garden items, etc.
No DIY work or hobby activities planned. No heating installed.

Under these conditions, is insulating the concrete ceiling really necessary?

In my opinion, insulation doesn’t make sense; the garage is cold anyway. You would only be causing condensation.

Especially during the winter months, you will have a problem with mold forming on the garage ceiling:

Cars bring a lot of moisture into the garage, which then evaporates inside.

The indoor air in the garage is cold due to being unheated but still warmer than the outside air, especially at night. Where are the coldest spots in the garage? The walls and especially the ceiling. If the wall adjacent to the house is not the north wall of the garage, then that wall tends to be particularly cold.
As a result, the indoor air will condense on this cold ceiling (thermal bridge), causing mold growth.

There can be debate about how effective ceiling insulation alone would be.

In your place, I would address all available options simultaneously:
Insulate the ceiling and, if possible, also the exterior walls of the garage (you mentioned having some insulation material already).

The garage door is relatively airtight, which is generally good. However, you need to vent the moist air outside. I recommend considering core drillings for supply and exhaust air, combined with fans and a dew point-controlled ventilation system (try searching for this online).

I would also paint the ceiling and interior walls with an antifungal paint, for example, one containing silver.

Finally, consider installing at least one heating pipe in the garage. If everything described above is not sufficient, a small radiator could be added later. While this is not energy optimal, the garage should at least be somewhat insulated, and it is not meant to be heated to 20°C (68°F). Running the radiator for about three weeks per year would be preferable to having mold. Also, if you want to set up a workshop area in the garage, you will appreciate having the temperature around 12°C (54°F) instead of 2°C (36°F).

An airtight garage is problematic. Moisture stays on the car, causing it to rust much faster. It’s also bad for the electronics.
Since you don’t plan to heat the garage, make sure there is proper ventilation to allow air exchange. This also makes the question of insulation unnecessary.

I also think it makes little sense to insulate only the ceiling of an unheated garage.

We will probably skip the insulation and simply apply a waterproof membrane on the roof right after pouring the concrete.

I will consider the ventilation options.

@MM1506zzzz Please also provide a Wi-Fi router for the smart home control of the radiator. It should be a panel radiator, as these are simply better for some car models!