ᐅ Airtight cavity insulation as a base layer for a vapor retarder

Hello everyone,

I am currently insulating the roof of my house, which was built in 1930.
As shown in the pictures, there are cavities that I need to fill in order to attach the vapor barrier properly.
Do you have any suggestions on what I could use to fill these cavities?
I was thinking about expanding foam, but I’m concerned that the adhesive might not stick to the cured foam.

Thank you very much in advance,
Vince