Attic Expansion - Insulating the Pitched Roof from Inside - Totally Confused

If there were a climate membrane under your roof tiles, the insulation could be placed flush.

As I said, at that time we did not have something like a climate membrane or it was called differently (roofing membrane :D)
Not that this wouldn't be correct or good, but back then we had exactly the same setup.
Tiles - roofing membrane - 24mm wooden boards - full Isover insulation - PE foil - wooden paneling/Drywall in the living area.

I don't mean to say that you should do it this way or that it would be better, but simply that this exact setup was professionally recommended to me back then and, looking back, I can say that it made sense to me (and the carpenters) at the time and I never had any problems with it.

If a KFW renovation is carried out with an energy consultant, is she then the primary contact person who should assess whether a ventilation cavity is necessary?