Flashing for siding with no air barrier?
I want to renovate the bump out on my 1916 house to make it a usable breakfast nook. It is roughly 8x12, and I will be replacing 4 windows, adding an exterior door to a new deck, and insulating so it can be used year round. The outside siding is 1"x8" redwood shiplap, nailed directly to the studs. It is generally in good shape, except where the crappy windows leaked. I want to know 1) how to flash the new windows and doors so I avoid future leaks and 2) whether I should try and put an air/water barrier on the inside before insulation. I am not planning on removing all the siding, just some around the windows and doors, and replacing some rotten pieces.
My one thought is to directly spray isocyanate foam on the inside of the siding, to serve as both an air and water barrier. Anyone see problems with this?