Thank you all
no. directly on the tiles.
I specifically asked the installer and the builder 5 times... they said it's not a problem.
It is supposed to go on the tiles for us too. Was the substrate reinforced in any way? The screed layer installer also says no problem for us, but I am a bit uneasy. Do you know how compressive the insulation is for you? What material is the bathtub made of?