Ok sorry for the giant image I originally posted above. The edited version should be easier to look at.
Originally Posted by GBR in WA
If the new wood box frame is against a metal frame, add some electricians tape to the contact edge against vapor wicking.
The inside of the box should have 1/4" reveal to the window frame edge (inside). Add some 2x2 or deep enough wood under the box frame, on the outside edge, for apron backing to attach. As Joe said, really hard to guess without pictures...
So far the plan is to use 2x3 furring strips over the 2" XPS foam. I guess the first question is how close should I frame around the window? I originally just assumed that I would put the foam and the furring strips right over the flat metal edge of the window frame that is set into the concrete like this.
Looking at the picture now and reading what you're saying, maybe what I should actually be doing is leaving some extra room, something more like this:
Grey is concrete, Pink is the 2" foam insulation, Brown is the 2x3s, White is the windowframe and sheetrock. In this diagram I could fill the half inch between the top furring strip and the metal window frame with spray foam, cover most of that with the sheetrock and seal the rest of the gap from the sheetrock to the windowframe with caulk? Would that look odd with the sort of stairstep edge the window has?