Our seat rail is about1" deep. We are skinning over the windows with sheet metal with 1" R5 Owens-Corning (think pink) Foam sheathing glued to the metal. We will allow a 1/2" spacer (scrap foam) for "dead space". Then we are gluing sheathing over the existing interior metal panels with 1" R5 Owens-Corning Foam sheathing. This will end up flush with the metal seat rail. Then we will glue 1/4" underlayment (moisture resistant) plywood. The plywood will be painted with interior latex paint (with a mildewcide added to the paint).
What we are using:
BTW, Dow's website states
"STYROFOAM™ Brand Residential Sheathing is an extruded polystyrene foam board with plastic film facers on both sides to reduce job site damage to the insulation boards. In new construction and residing applications, it provides a level, even surface for the exterior sheathing and for new siding material, plus helps enhance energy efficiency, reduce condensation and create an attractive, long-lasting home exterior."
and it also has an R-value of 5 per inch..... it's BLUE.
To make things even more confusing DOW also make extruded Polyisocyanurate under the names :