Self-leveling cement was poured over the wiring of the in-floor heat system to level the floor. Then Schluter-DITRA was layed down in preparation for the tile.
Usually under tile, two layers of 5/8″ plywood subfloor is necessary for the proper rigidity to prevent cracking. Yet this would have made the bathroom floor much higher than the hallway floor. The Schluter-DITRA grid system is only 3/16″ thick and replaces one of the layers of plywood.