Heat Mat installation question..

[Ajax123]

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.

 

[Richard Edwards]

Sound advice - as always from Ajax - You might be alright without DPM as the floor might have a DPM and if not I think if you used cementous boards as opposed to std insulation boards and taped and sealed the joints, then this is waterproof too. You will have to check the K value of the two board types with the manufacturers and compare the test results for the same thickness.

The only aspect my advice might differ from Ajax is - If you used Gypsum SLC with a higher insulation value, this might work against you, as the SLC is used to cover the cables and what you should be aiming for is max insulation under the cables and minimum around / on top of them to transfer max energy to the tile surface, hence I would go for Cement based high flex / latex.