Hi,
As is attached, the image shows a crosssection of the layers in the underfloor heating system I am planning. I have got a company that is giving me a good deal on routing ply for use as an low profile under floor heating system. The system brings the floor up by 30mm; 6mm cement fiber board, 18mm structural ply and 6mm xps insulation. it will be in a living room and kitchen area and is a wet system. I want to know, If anyone sees any immediate issues with it as shown in the diagram, my doubt I have is if the xps will be rigid enough to support such a system but the reason I was thinking this, is to bring it up to 30mm as it brings the floor exactly level with the rest of the house and a little more insulation never hurt, though it is sandwiching the subfloor because a vapor control layer, so maybe it would hurt if water got in... Please let me know if you would do a system like this any differently, I already have the ply, pipes, spreader plates as surplus from a pervious job, so I am planning to work them in no matter what.