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

No. Not that I know of.

Well, that is of course mandatory!

Well, that is of course mandatory!

Yes, the owner has been quite sloppy here in many ways. But it was never meant to be a rental property, always just for personal use, and you can tell. I'm only here until the end of the year, and then hopefully moving into the house.

Something like that is the worst case and the boiler in the utility room on the ground floor is then clearly better. About the foil. Stick it with a thick bead of this foil adhesive onto the boards. The bead should be continuous. Then it is tight enough.

But if I just stick the foil onto the tongue and groove boards, air can still get behind the foil... It wouldn’t be completely airtight then...

From where? Not from the inside, probably. From the outside, air always reaches the wool. We seal between warm and colder.

Don't you think that air or moisture can get between the [Rauspund] and then into the insulation?

Basically yes. However, if a climate membrane is really used together with the vapor-permeable underlay, it is rather uncritical. I would still glue it to the masonry though. Or rather, so that no cold air can flow behind it. (with or without h?) But seriously... It's just an attic with a heater... It's only important that nothing molds.