Flat roof insulation - 5 craftsmen / 5 opinions

Exposed roof truss = visible rafters? No, they are supposed to be clad from below.

There is now another approach: insulation goes between our 20 cm rafters and wood fiber boards are installed under the rafters for soundproofing; on the rafters, normal roofing and no additional full-surface sheathing.

But we are still in the exploratory phase.

But you already have the shell almost finished, right? So everything regarding the roof should be settled by now?

So if there is no exposed roof truss, then I am a bit puzzled by the idea of insulating with rigid foam. Isn't a soft material that you don't have to cut so precisely and can just stuff in much easier?
I think in this case I would use wood fiber boards for insulation completely. 20cm between the rafters and then another 4 or 5cm over the rafters. That should be sufficient in terms of thermal insulation and soundproofing (at least as far as the roof area is concerned).
But as I already wrote, I am just an amateur and can only judge this vaguely.

An exposed roof truss is rather visible rafters and sheathing. Although visible rafters probably also count as exposed roof truss.