great point. I wouldn't have thought of that. I finished getting the old luan panel off the styro, and cut the new 5mm panel to fit. It is slightly thicker than the luan panel it is replacing, so some bevels, etc were required. Fortunately the seams will be hidden by the window box, and the sofa (or entertainment center if we swap sides in the future.
Turns out that the exterior wall is of the same construction....foam inner core then luan panel glued on top, the the fiberglass bonded to the luan panel..Unfortunately that panel has dry rot too. No clue yet as to what I'll have to do there.