On top of my subfloor I have radiant foil barrier, then 5/8" (od) pex tubing and aluminum heat plates. I have filled in the balance of the floor (7x10 foyer) w/1x3s screwed to the subfloor. Now I have to attach cement board over the sleepers to lay down my porcelain tile. I have just recently learned that in the voids where the pex tubing runs, I will have to fill in w/thinset. Am I understanding this correctly? If so, it's o.k. to fill these voids 5/8" deep w/thinset?? This method worked great when I laid wood over the sleepers, but tile prep is presenting a problem. thanks for guidance