For many years — primarily during the cheap energy eras — the placement of insulation as it pertained to the myriad radiant panels was not given much thought.
If the panel did not perform to required abilities, the simple solution was to turn up the panel’s operating temperature. In many cases involving radiant floor heating systems on upper floors, this resulted in an increased back loss from the panel (downward flow of energy), which typically resulted in an underperforming floor on the area above the radiant floor and a significant back loss (actually a gain) to the area below the radiant floor.