My intentions from the start was to removed all interior fiberglass panels and insulate myself with polyiso foam board. However, as I got into it a bit today, I removed a window trim piece to take a look at what was behind the wall panel. Much to my surprise, it was 1" foam of some sort. Now I am rethinking my plan. The roof section for sure will be insulated, but if I leave the wall as it is, I can then add 1/2" Polyiso and then secure tongue and groove wood paneling directly to over the poly and screw to the factory fiberglass wall.
Are there any apparent flaws in my thinking that I am overlooking? This will provide me with 1.5" of insulation AND I will not have to to scrape the factory insulation out which might me a great time saver...that said, my priority is what is BEST, not what is easy. Below is a photo of the window trim: