I am now at the stages of starting the interior wall/ceiling surfaces of my three season project. Everything is insulated and have installed a vapor barrier under the floor, walls, and ceiling. I plan on using a plywood beadboard on the ceiling and lower portion of the walls. My struggle is what to use for the remainder of the walls? There isn't much wall space...really just between and above the windows. I was thinking of using plywood in the same thickness as the beadboard but the seams will show a bit. Any other options really? The room won't be heated or cooled, or at least no time soon. I want something smooth, as if it were sheet rock. I plain on priming both sides of the ply bead FYI.