The sand-lime bricks serve as impact sound insulation. Then panels are placed on top of them, into which the underfloor heating is installed – I have posted this here before, just scroll back a bit. If tiling is planned, Fermacell is placed on top. Parquet, to my knowledge, is laid directly on it. It is somewhat slower than an underfloor heating with tile screed – but since underfloor heating is slow anyway, we didn't care.