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Community and Q&A

Insulation under radiant floor in bathroom

user-669103 | Posted in Green Building Techniques on

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?


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  1. user-659915 | | #1

    Put naturally-warm cork on the bathroom floor, forget the electric radiant, save your PV for more important stuff.

  2. user-669103 | | #2

    We are actually using cork on the entire rest of that floor, and have other cork foors too. However, the cork manufacturers specifically advise not to use it in the bathroom, and we like the look of tile.

    The area that we are looking at is so small and the time period so short that we would use only about 0.1KWh per day that it is used, which is about 20% of what the a cable TV box uses in a day every day.

  3. gusfhb | | #3

    Schluter on top of the xps.

    radiant is not an on off kinda thing.

    I have have built two curbless hydronic radiant heated showers and the shower itself is the only place you notice the heat, and the water heats it up more than the radiant does. When you step out you are on a towel/bathmat and when you are 3 feet away put down a rug.

    If you are over concrete insulate just to insulate as you will not notice a 1 inch step at the door, and that will do most of the work. Maybe an electric toe kick heater would be better.

  4. GBA Editor
    Martin Holladay | | #4

    I'm with James and Keith. I think you won't notice this luxury -- after all, you step on a bathmat, not directly on the tile. The bathmat is all you need.

    If you have invested in an expensive PV array, it's painful to think that some of that electricity is being used for electric resistance heating elements under floor tile. That's not a good use of electricity.

    I agree that the cable TV box is also a problem. We all need to lobby the cable companies (and regulatory authorities in Washington) to do a better job on set-top boxes -- the electricity required for these hogs could be cut in half easily if anyone bothered to focus on the engineering issue for about 30 minutes.

  5. user-669103 | | #5

    Keith: You said "If you are over concrete insulate just to insulate as you will not notice a 1 inch step at the door, and that will do most of the work. "

    I couldn't work out what you mean by this, please elaborate.

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