Seal insulated ventilation duct against air gap and vapor barrier

[no_brakes]

Hi,
I'm wondering about the best way to seal an insulated ventilation duct against the masonite in the air gap as well as against the plastic in the vapor barrier? The roof will then be insulated with loose fill insulation.

Or is it perhaps better to construct a shaft all the way and solve the sealing against that instead?

 

[Gabbe1]

I know that for ready-made chimneys (like premodul) there is a special collar used to seal against the vapor barrier. Now these chimneys have a metal casing so it might be considerably easier than a duct with insulation that lacks an outer casing.