So thinking of putting some electric u/floor heat in the downstairs loo which will have procelain tiles.

Do people recommend I remove the old floor boards and install some other board onto the joists ?

I need to stick to about 40mm total makeup including a 10mm tile.

(Thinking of using thermosphere - can you integrate it into heatmiser ?)

Edited Saturday at 17:363 days by Spinny

I would keep the boards if they are solid and level, and put a tile backer board over them rather than taking them up. That saves height and gives the tiles a stable base to work from. With 40mm total and a 10mm tile, you have about 30mm left for the backer, mat and adhesive, so a thin backer board makes more sense than a thick one. Screw it down well, stagger the joints, and check the floor does not bounce before you tile. For the Heatmiser question, I would check it with the mat maker, as some mats need their own sensor and controller setup.

Thanks, stevenlee - wow you are far away - do you live in CA ?

However I didn't make it clear - the 40mm is from top of joist - so it would include 21mm floorboards leaving around 19mm...

Top of Joist

Floorboard 21mm

backer board ?

Flexible Tile adhesive to stick down matt 2.5mm

Matt with electric cable 5.5mm

Flexible Tile adhesive 3mm

Porcelain Tile 10mm

Total 42mm + backerboard... so too high

I don't have any insulation between the joists at the moment.

With 40mm from the top of the joist to the top of the tile, keeping the 21mm boards doesn't really leave room once you add adhesive, mat and tile. I'd lift the boards and go back with 18mm exterior grade tongue and groove plywood, glued and screwed to the joists at about 150mm centres, with noggins under any edges that land between joists. That gives you a few mm back and a much stiffer base than old boards, which porcelain needs.

A cement backer on top would be ideal but you're out of height. Instead, prime the ply and set the cable in a flexible self levelling compound rather than adhesive. Roughly: ply 18, cable in leveller about 6, flexible adhesive 3, tile 10, so around 37mm. It also gives a flatter result than sticking the mat down and combing adhesive over the cables.

While the floor is up, put insulation between the joists, either mineral wool on netting or PIR cut tight and foamed at the edges. Otherwise a good chunk of the heat goes down into the void and the floor takes ages to warm.

On Heatmiser, they do electric floor versions of their stats, so check the mat's wattage against the stat's rating and fit the floor probe between two cable runs, not on top of one. It'll need an RCD protected supply, and bathroom circuits are usually notifiable, so get a sparky to connect and test it.

Before you close it up I'd get some insulation between the joists, even just PIR cut tight or mineral wool on netting. With a suspended floor and nothing under it a good chunk of the mat's output just heats the void, so it runs longer and the tiles still feel lukewarm. If you can spare a few mm, an insulated tile backer board on the ply instead of plain backer also makes it warm up a lot quicker.

On Heatmiser, they do electric floor stats with a floor probe (neoStat-e I think), so you could run the Thermosphere mat off that instead of its own stat. Just check the stat's amp rating covers the mat wattage, a small loo mat is usually well under.