Can you use greenboard as a tile backer board? It's for a tub surround, all three walls will be like 6x5 tile. The long wall and the wet wall have cementboard as a backer, but I'd rather not add another half inch (as in cementboard) to that last wall. I'd read that greenboard is designed to be capable tile backer, but I dunno if that qualifies in such a damp location /forums/images/graemlins/dunno.gif. Whatta y'all think?