Create an insulated technical room for the gas boiler in the attic

And nothing would mold either if I glued the foil to the stripwood...

I would be afraid of mold if a lot of warm and thus moist air from the upper floor reached the unventilated attic. However, I do not see that here now.

No, exactly. The ceiling is insulated and airtight. Therefore, the attic is "autonomous".

Let's summarize. Klemmfilz 160, climate membrane, roof battens and formwork boards, adhesive tape for the overlaps, some cartridges of foil adhesive, two men, two days.

That's how it is! Thanks for your help.

Pictures and experience report when it is finished.