Ceramic floor tile and floor heat....

[mrcaptainbob]

The bathroom floor has hydronic floor heat. Four 1/2 pex between the floor joists the entire room length. The floor heat is used only in the winter time and is close to around 100*'s. Sounds like it would be too hot, but it's not. None cold weather we do not use the wood stove, thus, no floor heat. And it then goes as low as maybe 60*'s. Sounds cold, but it's not.
Presently, after rebuilding the floor with double criss-crossed 3/4 ply, leveled, screwed and glued to the floor joists, I covered it with luan and laid down wife's choice of stickey-back floor tile. It looked GREAT! But then, the change of temps was enough to mess it all up. The c of e was so great that the floor tile is all messed up.
The plan now is to lay down some cement backer board and lay ceramic tile on that. The difference of expansion between the two (so far what I have found...) is about 3:1. But it's so extremely little, that it's about .002" over ten feet.
Should I let the backer board float over the ply subfloor?
Any other suggestions?

[TinyTool]

It sounds like you’ve put a lot of effort into your flooring project, and dealing with temperature fluctuations can be tricky. To address the issues you're facing, make sure to securely attach the cement backer board to the plywood subfloor using thin-set mortar and appropriate screws. Don’t let the backer board float; it needs to be properly anchored to ensure a stable base for your ceramic tile. Using a flexible thin-set mortar for the tile will help accommodate any minor shifts due to temperature changes, and remember to leave expansion gaps around the edges and transitions to prevent issues.

Additionally, since you’re dealing with hydronic floor heat, it's a good idea to install a moisture barrier between the subfloor and the backer board to protect against any potential moisture problems. After installing the tiles, allow them to cure fully before using the floor to ensure a strong bond. Keeping the temperature relatively stable in the room, when possible, can also help reduce problems related to expansion and contraction.