Attaching wall plate to plaster ceiling

[Perry15]

Hello,

I'm wondering if any knowledgeable person has tips on how to best solve this problem. The idea is to frame drywall around a stairwell, but I've encountered a problem as I can't fasten the top plate to the joists on 2 of the 3 walls since they run parallel to these walls, and of course, luck wasn't on my side. Molly bolts seem like a bad idea considering the vapor barrier.
The question is how to solve this? Does a part of the ceiling need to be opened up to install blocking between the joists to screw into? Could it be enough to attach the top plate to the vertical stud that is screwed into the existing wall where there is a stud behind it, and into one of the rafters (visible under the ceiling) and then screw into the end of the top plate that goes in the other direction?

If it's unclear what I mean, I can arrange a picture.

Thanks in advance.

 

[jonaserik]

Images from various angles are the only thing that can provide information about what you have and want.

 

[Hennke85]

The vapor barrier is usually placed between the spars and studs and not between the drywall and the spars, so a molly could potentially work. Another solution is to drill with a hole saw around 80 mm in the drywall where the stud is supposed to be, insert a piece of spars between the drywall and the vapor barrier. Secure it on both sides of the hole. Screw the cut-out piece back in and then attach the ledger board to the spars piece.

 

[jonaserik]

Plaster and stairwells or other variations are not good to attach to; it should be rigid where the mounts are placed.