We took 21 feet off of our back deck and built a sun room. It is closed in with storm doors & storm windows only. Fully insulated with R-13 on walls & ceiling and R-19 on the floor. We removed the decking boards and insulated between the joists. I had already closed in underneath with vinyl with 1/4 inch slope to prevent rain drips on patio below. The new sub-flooring is 7/8" pressure treated plywood. I plan on putting ceramic tiles (12" square) over the floor, but was going to use Luaun as a base over the pressure treated to smooth out any bumps and then thin set the ceramic tiles on that. My concern is because it is technically a 3 season room, is the Luaun appropriate or should I use something more like hardie-board?