There will be no DPM under the existing tiles. That puts your new tiles at risk especially if the new tile face is heated. Also if there is no insulation the heat mat will simply be trying to heat up the world cos there will also be no thermal insulation under the quarries. Best thing would of course be rip em up and do it properly but I would say minimum would be 1200g DPM over the existing tiles then some insulation board suitable for the UFH mat, then SLC followed by tiles. To get the highest thermal conductivity you might wat to consider SLC based on Gypsum and then adhesive based on the same.
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