ᐅ Airtightness in the area of collar beams

Hello dear forum members,
I’m Mirco from Neumünster.
We have bought an old building that we are currently renovating ourselves (with help from some craftsmen such as carpenters, plumbers, etc.).
Now I have a question...
I am in the process of connecting the vapor retarder. So far it’s working. The only thing puzzling me is how to connect the airtight layer around the area of the last or first truss.
There is a large air gap there (see pictures) and I’m uncertain how to properly seal it.
I already tried injecting Premur, but it leaked out again overnight.
I also briefly considered using PU foam, but unfortunately, that is not approved as a vapor retarder or vapor barrier.

I would be very grateful for any ideas on how to handle this.

Best regards from the far north

Depending on the size of the gap, apply a narrow strip of vapor barrier film or a wide corresponding adhesive tape along the beam and plaster it onto the gable wall.
You also need to seal and plaster the transition from the slope to the gable wall, by the way.

...and while I’m at it: old beams like these should be sanded and/or primed before applying the adhesive tape.