Insulation of the top floor ceiling

I would like to insulate the top floor ceiling of our house, which does not need to be walkable, with glass wool. I have encountered a problem that seems quite complicated to me regarding the sealing with the vapor barrier foil and would be grateful for any wise advice on how to proceed best.

As can be seen in the photos, there are pairs of approximately 45° boards at regular intervals in front of the foot purlin, which apparently serve a load-bearing function. These make it unfavorable and difficult for me to seal the vapor barrier as airtight as possible to the foot purlin. I estimate the effort to make the connection of the vapor barrier to these boards as well as the purlin tight to be disproportionately large, and I also assume that this connection would probably not be airtight even with great effort.

Therefore, I have considered nailing a continuous horizontal batten to the 45° boards and connecting the vapor barrier/glass wool there, leaving the floor along a longitudinal strip approximately 20 cm wide uninsulated up to the purlin.

However: would this solution also be acceptable from a building physics standpoint, especially with regard to rising moisture from the living floor below?

Might there be another, better solution for the sealing problem with the 45° support boards?

I would appreciate any helpful advice on this.

Hello.

Can you install the vapor barrier from below? Is the attic ventilated, and if so, how? And what material is between the collar beams?

So it is not possible from the other side of the ceiling. It is clad. The collar beams were probably poured with a quite coarser screed. Air probably comes into the room through the gaps between the roof tiles.

If these crossbeams on the collar beams could be removed, it would be significantly easier. I have never seen them in pictures of other rafter roofs.

So I wouldn't remove them. We are facing a similar problem. It's all a lousy job.