Insulation under radiant floor in bathroom
We are planning a little luxary in our bathroom of about 25 sq ft of electrically powered radiant floor (power is from solar). We want to be able to switch on the floor heat ,go into the shower, and when we come out the floor would be if not warm, at least tepid. So it only has 10 to 15 minutes to heat up.
We don’t want to leave it on for long periods — too wasteful.
We don’t want to have the floor heat on a timer so it comes on 30 minutes before we want to use it – not practical.
Thus I think that it is important to have a low thermal mass under the heating elements.
Thermally I think that best would be:
Tile, heating element, and then high compressive strength XPS, then floor boards.
But using thin set on top of XPS without something stronger would risk cracking the tiles.
In the UK I have seen a fiberglass board bonded to XPS, the fiberglass board provides strength and rigidity, the XPS the insulation, and it is all nicely low thermal mass. But I cannot find anything like that in the US.
Here installers recommend 1″ of mud. But that has a high thermal mass which will lead to slow heating.
Since we only use the bathroom for a few minutes a day having to heat that thermal mass (of the concrete / mud ) seems a bad idea. I only want to heat the thermal mass of the top tiles and our feet!
So are there any recomendations for an insulated board or something similar to go under a heated tiled floor, something that I can thinset the heating floor to?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part