Insulation between rafters - what thickness should I choose?

Good evening,

the screed is currently drying in the new building.

The following question came up today:

I would like to insulate the roof structure, I have a rafter thickness of 20cm.
I would like to use Klemmrock, but in which thickness should I install it?
18 cm (so that air can still circulate), 20 cm, or 22 cm (the more, the denser it is, after all).

When I have installed the insulation, does the vapor barrier go on top and then I install a counter batten on the outside and then drywall over it?

Thank you

The thickness of the insulation to be installed depends on the thermal conductivity group (TCG) of the insulation material and the insulation thickness calculated in the thermal protection verification, which, together with the rest of the component structure, must result in a thermal transmittance coefficient that meets the requirements of the Energy Saving Ordinance: 0.24 (W/m2K), and for individual measures under KfW funding also 0.14.

The remaining questions can only be answered in connection with the already installed building materials and when the basic structure of the roof is known. In any case, the ventilation layer in new construction is not placed between the insulation and the underlay/waterproof board, but above the underlay/waterproof board, at the level of the counter battens and outside the insulated roof.

If you do not find any clear information about this in your planning documents, you should seek expert and independent advice – and by that I do not mean the hardware store. It is better not to act than to act incorrectly.

under no circumstances stronger than 20cm with a rafter depth of 20cm. The mineral wool should not be compressed because the air in between provides insulation.

you should look at your Wärmeschutznachweis. It was planned there which values must be achieved at least. And the person who calculated the proof is also liable for the correct execution. So who was that for you?