ᐅ Vapor barrier in knee wall – challenging penetration due to beams

Good evening, home building community.

I have already watched numerous videos and read many posts but still haven’t found any advice that fits my situation. So, I would like to ask for your guidance.

I wanted to install the membrane on the concrete knee wall. The problem is the raised rafter on the right side. This makes it extremely complicated. I had planned to cut the membrane completely at that spot, position it against the wood, and then fix it in place using adhesive and tape. This will probably be difficult because there is very little space. The strip on the wall and wood has been primed, and I sanded the rough-sawn wood a bit at the joint.

I wonder if there is a better solution.

The plan is also to build a stud wall on top of the screed. This wall should be covered with drywall panels. Would it be allowed to install the stud wall first and then attach the membrane there?

Are there any suggestions on what to install behind the stud wall (since it will create a cavity)? > Mineral wool insulation? Or would it be better to use a material less susceptible to moisture? XPS boards?

Thanks in advance!

Mike_Ef schrieb:

The plan is to install a stud wall on top of the screed. This should be covered with drywall panels. Would it be acceptable to first apply the drywall and then fix the vapor barrier there?

Acceptable by whom? Who would prohibit that? The important thing is to prevent moisture according to the principles of building physics, which means the vapor barrier must be on the warm side facing the living area. Of course, you can extend the vapor barrier over the stud frame. That’s assuming I understand your plan correctly. Otherwise, I wouldn’t worry too much. In any case, make sure to include insulation, whether wood wool or mineral-based – as long as the vapor barrier is in front of it and the exterior is fully sealed, you shouldn’t have any moisture problems.

Good evening, thank you for your feedback.
By "permissible," I meant whether the execution is justifiable. Of course, no one can actually forbid it ;-)

I understand your explanation like this:

When I install the stud frame (= knee wall), I should run the vapor barrier from underneath along the frame down to the floor and insulate the knee wall behind it accordingly.

What would not be correct:

I run the vapor barrier along the rafters up to the concrete wall and connect it at the top. Then insulation in front of the concrete wall and then the stud frame.
--> That would be incorrect because the insulation would be behind the stud frame but in front of the vapor barrier.

--> Have I understood this correctly?

Thank you very much in advance.