Sealing gable wall during attic conversion

Hello and good evening!

In our detached house, the upper floor was insulated and sealed with the vapor barrier foil last week. Since the house is built with Poroton bricks, the exterior wall sealing (hopefully airtight) is done through the interior plaster.

Now the attic is also going to be converted by ourselves, and we want to start with the rough-sawn boards (flooring) tomorrow.

The foil has been laid by the drywall installers from the upper floor down to the middle purlin at the floor, so I can later attach "my" foil directly overlapping there.

Now the problem: The gable will probably also be plastered in the attic, but at the height of the ceiling between the upper floor and the attic, a strip of the exterior wall will remain unplastered. Basically, below the rough-sawn boards installed by us and above the foil at the ceiling of the upper floor.

Does this strip of the exterior wall need to be sealed additionally? If "yes," how?

Now the attic is also supposed to be converted by ourselves, and we want to start tomorrow with the rough boarding (flooring).

Great that you’re already asking today!
Forget the project and get professional help from an expert for it!
Otherwise, it’s a shame about the material and the work done as well as everything that has already been built!

The foil has been laid by the drywallers down to the floor at the middle purlin, so that I can later attach “my” foil directly overlapping it.

Where “on the floor” and with what was the foil connected?

Now the problem: The gable will probably also be plastered in the attic, but at the height of the ceiling between the upper floor and attic, a strip of the exterior wall will remain unplastered. Basically below the rough boarding laid by us and above the foil on the ceiling of the upper floor.

You have described the problem perfectly! Unfortunately, there is no patent solution for it and these connections, which are hardly airtight to create in practice, subsequently lead to considerable moisture problems!!!

Does this strip of the exterior wall have to be sealed additionally? If “yes”, how?

3 x “yes”!

Your planner must tell you the “how”! Everything else you can throw in the bin, sorry!

Except for the planner, of course the drywallers are also liable for the airtight installation...!

The only help is: install a blower door system, perform differential pressure leakage testing... until no air exchange is detected! At the same time, care must be taken to ensure the “durability” of the airtightness layer’s function...

... and only then should you start your DIY project!