I have removed the old floors from a bathroom, around 5 square meter, and there was a celotex (50mm). It was sitting on a concrete slab. I repeated the same model, but I added a membering first then 50mm celotex then 12mm Hardibacker. I fixed the hardibacker with screws to the main concrete slab through the celotex. Any potential problem with that? Any advice before tiling?