Thank you for all the information! For that rotted board, we'll remove the other panels and see how far we need to go to add new support. Appreciate all the suggestions.
For perspective, this is a pole barn - wood "structure" with insulation and then the metal shell. This issue is on the back side. The idea of a window to replace the AC was intended to make it "easy" (insert a sad chuckle...nothing has been simple lately) to deal with the opening. I'm wondering if I can get a panel to replace that section as I would be fine with no window on that wall (already have the three windows in that room that show on the picture). This would allow me to repair the inside and then make sure of no future leaks. Not sure how to do a "patch" otherwise that would be nice and waterproof?
