said by cdru:Fair enough, I'm no expert. said by IIIBradIII:
Well the first layer of ply should be fine, but I'd then add CBU on top of that, ensuring that the seams are overlapped.
He's already using Ditra, so there is no advantage to using a CBU.
I'd go existing 1/2", another 1/2", heating wire, ditra, tiles with the appropriate thinsets between all the layers of the sandwich. You'd be up ~1/4" at the threshold of the bathroom, but it's not uncommon to have some type of a transition there anyways so I don't think it will be a stumbling point or look wrong.