Insulation boards - flush or set back?

[Harrier23]

Building a timber frame with 50mm PIR insulation boards in between.

Should these be flush against the front of the stud work or set back slightly?

I'm thinking if I set them back I can fill what gaps I have with expanding foam more easily, but might this then cause cold bridging in the gap between insulation and plaster board?

 

[^woody^]

You should not have any air cavities within the frame.

 

[ETAF]

this might be worth a watch

 

[^woody^]

Looks like a roof.