Will a tile underlayment work if I do not screw it to the subfloor?
I am making a tile hearth extension to meet code distance for a fireplace insert. For r-factor I will use 1/2" micore 300, then 1/4" Wonderboard, then thinset and matching tiles. My concern and question is whether my underlayment will be stable enough if I do not screw down the Micore and Wonderboard? The reason I do not want to screw it down is because the subfloor has radiant heating tubes with water. The tubes are under an inch of a gypsum/sand/cement mixture.
