Insulating the garage from the inside (is it sensible)?

Good evening,

we are currently building and I have a question regarding our garage.
Short background.
Garage 9x5x3m, adjacent to the residential building (fire protection door), flat roof made of wooden construction (bitumen on OSB board), 18cm rafters with about 60cm spacing, outside 4cm render carrier board + plaster, inside lime-cement light plaster.
There is a car lift in the garage and it is planned to heat the garage with an infrared heater at cold temperatures (above the work area).
Does it make sense to equip the garage ceiling with 180mm insulation, vapor barrier/retarder (including sealing), counter battens, Fermacell boards?
I have already heard mixed opinions and would like to get your feedback on this. Some say it makes sense because it is heated a little, others say, for example, that the OSB boards will start to rot (although I do not understand where the moisture should come from, maybe someone can explain that to me).

My basic idea is simply that I want to hide the beam construction and install spotlights. I want to provide the garage with a nice ceiling from the inside (preferably something sturdy). What would you recommend? I am open to suggestions.

I hope this was not too much text. If necessary, I can gladly post some pictures.

Thank you in advance for your help.

Best regards
Kevin Ritter

The moisture comes from the air.

Whether and how you can insulate this should be calculated by a professional, otherwise you will have mold.

It is important to have an air layer with circulation between the roof membrane and insulation, otherwise you will wet the insulation in summer. (Outside air warm, garage inside cool -> condensation on vapor barrier on insulation side) Additionally, do not use a vapor barrier, but a humidity-variable vapor retarder. Simply put, it opens the pores on the warm side (roof) in summer and allows moisture to pass through to the cool side, the interior of the garage. There are also special products with flat roof approval for this. With this, you should be on the safe side.

Hello Evo89

What you can do to be absolutely sure is to get a professional to give you a quote (you don’t have to accept this quote), then you buy the materials yourself and do it on your own. Caution: no warranty on your work.

Mold forms when a thermal bridge can develop and the ventilation is not adequate. That’s why insulation is always recommended from the outside (for walls); when insulating from the inside, for example if you want to insulate the roof, it must be ensured that the interior area is completely separated from the exterior, in this case the insulation from the heated area so that the moisture created cannot penetrate the insulation and cause mold.

Whether it makes sense to insulate there always depends on what, how often, and how long you use that area. Are you just there to park your car, or do you work in the room often? If you are not in there very often, it is enough to hang a ceiling and install two ventilation slots (opposite each other for cross ventilation) with manual flaps you can open and close. If you have more planned for the rooms, e.g., parties or working on the car, insulation definitely pays off (provided that walls, windows, and doors are also insulated). For bigger events like parties, you can also put a patio heater that will simultaneously heat the pavilion in front of the garage (to cover every aspect that might possibly come up).

If you have any further questions, I am very happy to assist.

Best regards Ricco Neugebauer