Working on a large project….climate zone 6 with 7600 HDD…..
We have an aluminum curtain wall assembly that is hung off of the steel super structure with clips. The not glass faces are covered with a 1/2″ aluminum plate rain screen attached to hat channels. To close the curtain wall jamb off, the plate extends out past the curtain wall frame. So the 1/2″ alum plate, which is fully exposed to the exterior, is in full contact with the curtain wall frame jamb which is 10″ deep (so 10″ of exterior contact).
One architect thinks that by simply caulking and installing expanding foam in the shim space will create an adequate thermal break so the curtain frame does not frost up. I completely disagree and detailed it so I have my 2″ of exterior XPS continuing out with the plate between the plate and the curtain wall jamb. So it would go aluminum plate, 2″ XPS (wrapping exposed end in alum flashing) and then the curtain wall jamb frame.

Are there any thoughts on this? Hopefully the description is enough to know what is going on.
Is the 1/2″ space of expanding foam enough thermal break to prevent the curtain frame from frosting in the winter? is 2″ overkill? Would 1″ be enough?