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We're about to have a wet room floor re-laid after the previous installers installed the gully and under floor heating incorrectly.

The base floor is wooden joists with cement board on top to provide a sturdy base (the previous install was with marine ply).

The next step - starting today - is to lay a latex screed layer on top of the cement board to provide a level surface. (There is a shower former in the shower area with correct fall gradients ashtray that will not be latexes)

Once the screed is down the tanking membrane will be instead on to according to the manufacturer's instructions. Then the underfloor heating mat (electric) will be installed on top.

All this sounds good so far.

The tiler who will lay the floor tiles, however, had asked for another latex screed layer to be laid on top of the underfloor heating. He claims that this is normal and protects the underfloor heating from damage from the tile laying process (in particular his sharp trowel).

This is what is being planned: enter image description here

In the previous installation there was no second latex screed layer, the tile adhesive was put directly on top of the heating mat which allowed heat transfer directly into the tiles.

I am concerned that a second latex layer might create an insulating layer between the heating mat and the tiles on top.

Or am i worrying about nothing?

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    Yes and no, the extra layer will create a small insulating layer, but this should not be significant provided the heat path is primarily up and not down. I'd be more concerned with the cement board. It is going to soak up a lot of heat. – Trevor_G Mar 1 '17 at 16:48
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    Heat rises, having a good base for the tile will help distribute the heat more evenly. On several jobs I bedded the cables in ~1/2" of mortar. These floors are really nice and the tiles will be in place longer. Not all cables / mats are listed to be encapsulated but if yours are I would want the extra layer. – Ed Beal Mar 1 '17 at 20:43

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