ᐅ Insulation of the Top Floor Ceiling

I want to insulate the top floor ceiling of our house, which does not need to be walkable, with glass wool. I have encountered a somewhat complex problem regarding the sealing with the vapor retarder foil and would appreciate any expert advice on the best way to proceed.

As can be seen in the photos, there are pairs of approximately 45° boards spaced at regular intervals in front of the wall plate, which apparently serve a load-bearing function. These make it difficult for me to seal the vapor retarder as airtight as possible to the wall plate. I consider the effort required to achieve a tight connection of the vapor retarder to these boards and the wall plate to be disproportionately large, and I also assume that this connection would likely not be airtight even with high effort.

Therefore, I have considered nailing a continuous horizontal batten to the 45° boards and connecting the vapor retarder/glass wool there, leaving the floor along a roughly 20cm (8 inches) wide longitudinal strip up to the wall plate uninsulated.

However, would this solution also be acceptable from a building physics point of view, especially regarding potential rising moisture from the living space below?

Is there perhaps another, better solution to the sealing problem with the 45° support boards?

I would be grateful for any helpful advice.

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?

Also, from the other side of the ceiling, it is not possible. It is covered. The collar ties were probably poured with a rather coarse screed. Air likely enters the room through gaps between the roof tiles.

If these cross beams attached to the collar ties could be removed, it would be considerably easier. I have never seen them in photos of other rafter roofs.

So I wouldn’t remove them. We are facing a similar problem as well. It’s all a difficult job.