ᐅ How to close the gap between the gable wall and rafters

Hello everyone,
I haven’t found an answer yet. Please help.
Expanding foam could be an option, but is that the right application and does it provide a lasting solution?
Is it not possible to plaster the 26 cm (10 inches) depth afterwards where there is a 1–2 cm (0.4–0.8 inches) large gap?
Thank you, Jörg

I have now attached clear photos again :-)

The gap is 2 cm (1 inch) wide on one side and about 4-5 cm (2-2 inches) wide on the side shown in the picture

Hello, what are you using to insulate the other rafter bays? I would use the same material, for example, glass wool. Then, of course, apply a vapor barrier with a loop and the necessary adhesive from a tube to seal it to the wall. You should be able to bridge the 2-4cm (1-1.5 inches) gap with the substructure that you use to hang your ceiling.

Regards, Achim