Foam board on outside of stairs
I was just thinking about how to better insulate the front of my bus and had a thought...why doesn't anyone glue foam board to the outside of their stairs with construction adhesive? Seems like it would be fairly cheap and durable enough, even without something covering the outside of the foam board. Only downside I can imagine is trapping water behind the foam board. My "solution" to that would be to apply the adhesive in vertical lines so water has a way to travel down and out from behind the foam board.
I don't really want to insulate the inside of the stairs as that will make them smaller and I'll definitely have issues with water getting tracked in and trapped behind my covering material.