Insulation/ventilation in a small space (Ed.)

[rob4diy]

Just had the glazing done.... above the new glazing (bay c.8''x9') is a very thin wall to outside
1st; pvcu foamboard that is attached to framing above the window
2nd; next 4'' gap,
3rd; lathe/render
The interior wall above the window is close to it so I am making a 3'' projection into the room so I can screw blinds up under it and will insulate this to plug the significant thermal bridge.

Q. I am therefore making an sealed box almost next to the outside. Can I simply fill it with foam (inc expanding to take in every gap) or will moisture problems result? Do I need to make some sort of vent to the 'box'.
Thanks!

 

[Wayners]

Yes.
Id stick kingspan in which is covered in foil which acts as a vapour barrier.
Moisture can't get past it.

Your probably ok with just foam. I don't think moisture can get past that either tbh