ᐅ Insulating a garage roof – yes or no?

Hello everyone,

Our double garage shares a common wall (filled porous clay brick) with the house. Otherwise, it is outside the building’s thermal envelope, with insulated separation in the ceiling and in the floor slab. The flat roof of the garage will be greened. Since we plan to waterproof the flat roof soon, we are wondering: to insulate or not? The energy consultant advises NOT to insulate, as the garage is not within the thermal envelope of the house. The roofer who will do the waterproofing says most builders don’t insulate in such cases and he doesn’t see any need to do so either...
How have you executed your garage roof?
What are the arguments for and against insulation?

Since the roof will be greened/is required to be greened, I would of course like to avoid retrofitting insulation if possible.

There is a significant lack of information...

Is the garage heated, or is there a plan to heat it in the future? If so, for what purpose? (Hobby/workshop space?)
What is the wall construction and insulation like for the other walls? How is the garage door designed?

The greenery provides some insulation effect, depending on the substrate depth. Otherwise, insulation only makes sense if the rest of the garage is also properly insulated and heated.

For us, the ground slab for the house and garage was built at the same level. To avoid a step from the house into the garage, we also installed screed and rigid insulation in the garage. Therefore, it made sense to insulate the ceiling as well. The garage door is Hörmann with LPU 65 mm (2.6 inches). The garage windows and exterior door have triple-glazed insulating glass (like the house). The space is used as a hobby room or workshop.

i_b_n_a_n schrieb:

There is quite a lot of missing information...

Is the garage heated or is it planned to be heated in the future? If so, for what purpose? (Hobby/workshop space?)
What is the wall construction and insulation like for the other walls? How is the garage door designed.

- The garage is not heated (no workshop, just storage space at the back of about 2x6m (6.5x20 feet) for tires, lawn mower, bicycles...)
- Sectional door, 42mm (1.65 inches) thick
- Back door leading to the garden
- Door between garage and hallway: aluminum with thermal insulation
- Wall between house and garage (as I already mentioned): 36cm (14 inches) Poroton MZ70 filled with mineral wool.

Simple decision. You can install 100cm (40 inches) thick insulation on the roof to achieve an acceptable average thermal transmittance (U-value). But in practice, you are always heating the outside environment (if you have heating), and especially now, you really should avoid doing that. It doesn’t help either.

If there is no heating, what’s the point of insulation? Just get yourself an infrared panel for a future workbench...