Subfloor for underfloor heating

[mattkelpie]

Hello. We're going to be installing large limestone tiles with wet underfloor heating in our kitchen/living area of the house. At the moment it's gutted and we have the opportunity to start from scratch as we're back to the joists. I'm looking to tile on to either 18mm routed cement boards or 20mm routed cement coated xps board. What would you recommend for a subfloor? Would 22mm chipboard and 12mm ply be about right, or could we get away with less? Already aware we'll be building the floor up quite a bit, but there will only be one threshold with a step, so not so bad. Cheers in advance!