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I live in a large apartment house in Central Europe and I plan to add electric floor heating in my little bathroom for convenience. I own the appartment. The building has central heating units in the other rooms, but it's nice to have the ability to preheat the bathroom (and mostly the floor) before taking a shower.

The composition of the floor is 15cm hard rebarred concrete panel, 2cm of some sort of soft/brittle cement something, and then finally 10mm ceramic tiling on glue. There's another apartment's bathroom under mine.

We'll be re-tiling the whole bathroom, so my plan is to harvest the 2cm cement layer and use the vertical space to install the heating cable and then some self-levelling concrete.

My only concern is thermal insulation. Will the efficiency of the heating be lowered much by the absence of insulation under the mat? I'd hope not, given it lies on thick concrete that's not cold on the bottom, and there's only like 2--2.5cm material on top (a bit of concrete and then the tiles) but I better check.

Tech details: The bathroom is 175x175cm, but there's a tub, so the floor is only 175x105cm. I plan to use a 300W cable, so it's about 16W/m2.

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    So is the thickness 2cm or 3cm? 10mm for the tiles and 2cm some cement stuff or not? Without insulation under your heating panels you will be heating that concrete slab, which if the below neighbors keep their bathroom at a lower temperature will increase your costs..
    – Solar Mike
    Feb 23, 2023 at 10:29
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    This is a tremendous amount of work and expense which should only be undertaken after the most careful and professional analysis. Do you have to get permission from a building supervisory board? Feb 23, 2023 at 10:49
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    @JimStewart Supervisory what? (Sorry, I'm not in the US, in case that's a US thingy.) Yes, I know that adding a heating like that without proper paperwork is very borderline here, but I certainly won't be the first one to do it as a DYI, with an electrician coming later to wire it. As for the work, sorry, but this is not a tremendous amount of work, certainly not for a 2m^2 room :)
    – yo'
    Feb 23, 2023 at 12:45
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    I presume that this is an apartment which you own and have the rights to make such a significant change. In the US, "apartment" usually refers to a rental, and renters don't have the rights to make major changes such as this. Your use of metric indicates you're not likely in the US, but it's always good to confirm these things.
    – FreeMan
    Feb 23, 2023 at 13:47
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    Also in much of the US, an apartment commonly used to refer to something you rent from the building owner. If the unit's owned separately it'd be called a condominium. Feb 23, 2023 at 16:09

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Of course some of the heat generated by your cable will be conducted down into the slab but since it's located much closer to the upper surface, most of the heat will go where you want it to go.

I am not clear how this might be insulated in any significant way due to the fact that you don't have access to the underside of the slab and you don't have much space on your side to add any insulation.

For your purposes, however, it's probably not needed since what you want is a warm surface for the relatively short period of time you are using it each day.

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    Also good news: The thermal mass of the concrete will be approximately the average of the air temp of the apartment below and the OPs apartment. Heat applied will only have to raise the temp of the floor a bit for it to feel warmer, however, I'd expect a fair bit to go into heating the mass below, not just the mass above. It may not be a particularly power efficient endeavor, and I've seen a lot of complaints lately about the cost of electricity in Europe (making assumptions here...) skyrocketing with no sign of abatement.
    – FreeMan
    Feb 23, 2023 at 14:36
  • @FreeMan if we talk about turning on a 300W heating for 60 minutes every day (that's more than it will be), it's about 100kWh a year. My current price is €0.30/kWh, so it's €30. If proper insulation costs me extra €150 and I save 30%, the investment repays in 10+ years. That's really not interesting on such a small scale.
    – yo'
    Feb 23, 2023 at 17:15
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    Just because the wiring mat would be closer to the bathroom floor does not mean (to me) that most of the heat would go in the direction the OP wants. Concrete conducts heat. There would need to be insulation under the electric coils to prevent the mass of concrete from conducting away most of the heat input. The OP must find out how the good quality European designers retrofit electric heating mats on a concrete floor. Feb 25, 2023 at 7:30
  • @JimStewart I believe you are mistaken here. If you were to cut a cross section of the slab and visualize the temperature you would see a parabolic curve dropping away from the heat source both above and below it. Since the top is closer to the source the temperature will be higher. Yes the heat does flow both ways but we're interested in temperature here. Concrete does not conduct heat all that well (about 1.0 W/(m K)) so the slope of the temperature curve will be fairly high. That will make the TOP heat up much faster than the bottom. Remember the problem here, making the floor feel warm
    – jwh20
    Feb 25, 2023 at 11:57
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    A couple near me got one of these heated bathroom floors retrofitted with I believe a Schluter mat that holds and positions the wires. There is a built in insulation layer as part of the tray that holds the wires. If the thinset that the wires are embedded in would be in direct physical contact with a concrete slab below my physical intuition is that the tile floor above would not heat up as expected. In the installation near me the bathroom is on the 2nd floor of a wood frame house. Feb 25, 2023 at 22:29
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just have insulation directly under your heated mat , over concrete so that the heat is spread out evenly. regardless of heat loss , just more comfortable not to be able to feel hot and cold spots where the wire is and isn't

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If you wish to keep the current level of the floor you are limited to 3cm thickness total, including tiles. This is too little to install insulation.
You could add some minimal foam pads and make the floor free-floating but it probably wont be worth the effort.
You want a level surface, with just enough space for some glue and the tiles, rest should be filled with concrete.
Make sure the cable is installed directly under the tiles, in the glue layer. From experience - thinner tiles work better, 0,6mm tiles heat up within 30m, the thicker 1cm ones take over hour. If there will be pause in the works between laying the cable and putting down the tiles, ask the cable to be covered with tile glue 1-2mm to keep it safe.
Do NOT put insulation directly under the cable. It needs the floor thermal mass to work properly, or will burn out.
By 300W cable I assume you mean 300W/M2, not 300W for total unit? The cables are rated per square meter, and 150/300 are normal values. So 300W/m2 cable you will need 450-600W of power - make sure you have that much available on bathroom circuit.
As far as I know, you don't need any permits in europe to install heating mats, but you will need electrician to connect it to get the warranty. For 175x105 cm you will need the 1,5m2 standard heating mat. Some of the floor will not be covered. If you plan on full wall mirror, you can also go with 2m2 mat and use the remaining heating cable behind mirror to keep it steam free. Just spread the cable wider. Really helps.

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