Hi, I know about the Wedi and kerdi board but was looking for a lower cost alternative to creating a thermal break between a concrete slab an a slate floor I have a total of about 2 inchs to play with 1/2" of that would be the slate and morter.

Can you use XPS or poly iso with Backer board or ply and backer board or ply only? may be alternate 2 pcs of 1/2" ply on top of xps or polyiso (screw down)?

Anny help would be appreciated, thanks!