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Community and Q&A

Heated bath floor – system and flooring?

C L | Posted in Green Products and Materials on

Is there any metric by which to measure the efficiency of electrically heated bath floors? Is there any system that if preferable to others? What about the Schluter Ditra system? If you have one, would you do it again? If so, what would you change about how it is installed?

In regards to finish flooring over the heated floor in the non shower part of the bath, cork looks like an interesting product. Although cork is sometimes discussed as an insulator, the mfgs of solid cork flooring propose it as an ideal finish floor for a heated floor; supposedly it heats fast. Does this sound reasonable, or is this just marketing hype?

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    C.L.,
    Q. "Is there any metric by which to measure the efficiency of electrically heated bath floors?"

    A. Yes. The metric is called "efficiency." It describes the conversion of electrical energy into heat energy. All electric-resistance floors have the same efficiency, namely 100%.

    Q. "What about the Schluter Ditra system?"

    A. The Schluter Ditra system can only be used for tile or stone, as far as my research shows -- not for cork. If you want more information on this point, contact Schluter Systems at 800-472-4588.

    If you want to use cork flooring above electric-resistance heating mats, first you have to find a manufacturer of electric-resistance heating mats that allows their system to be used with cork flooring. I've never heard of anyone installing cork flooring above this type of heating mat, and I'm skeptical as to whether it's a good idea.

  2. Steve Knapp CZ 3A Georgia | | #2

    C.L.,

    You also want to confirm that the cork is suitable for wet areas. A lot of the products I've researched are not recommended for baths because they will swell and buckle.

  3. Jon R | | #3

    A reasonable definition of efficiency would involve useful work - so I'd exclude any heat lost to the underside of the floor. This efficiency will be less than 100% and cork will lower it (as compared to something more thermally conductive like tile).

    When it comes to bare feet, cork will be substantially more comfortable than tile - perhaps to the point where you don't need radiant floor heat (overhead radiant might be enough).

  4. C L | | #4

    Thanks for the responses. Per the above, any electric product is likely to have the same efficiency (or lack thereof) and the finish floor thermal conductivity will impact the efficiency of the system.

    For a heated floor, the cork mfg recommends the Schluter Ditra system, and then top it with Ardex. This provides a waterproof membrane (Ditra) and a cementitious "subfloor" (Ardex). Then install their cork tiles with contact adhesive onto the Ardex. This also allows for a future finish floor replacement without destroying the heated floor - you scrape the cork tiles off the Ardex.

    In regards to cork being unsuitable for wet areas due to swelling and buckling, wouldn't that only apply to engineered cork products on a backing that would swell? The mfg does warn their engineered floating product is not suitable for wet area installation. They have no such warning on their solid cork product. Solid cork tiles have air pockets; would that preclude or reduce swelling?

  5. Steve Knapp CZ 3A Georgia | | #5

    CL,

    I assumed you were using one of the engineered plank click and lock systems. If you are using a glue-down tile and the manufacturer warranties bath installs, I wouldn't worry about suitability problems. One thing you might want to consider is whether the backer plus tile will create for an awkward elevation change between the bath and whatever space it adjoins.

  6. User avatar
    Stephen Sheehy | | #6

    We have Ditra heat under our bathroom tile floor. It works very well. We generally turn it on thirty minutes or so before a shower. I suspect tiling your entire floor, with heat underneath, won't be more costly than doing part with tile and part with something else. You wouldn't need to manage the transition between the two floor types.

  7. C L | | #7

    Thanks Stephen. Do you have the heated floor under all bathroom tile, including the shower, or all except the shower? I will use tile on the floor inside the shower. Not sure if the heated floor should be continued into the shower or not - someone pointed out the hot water heats the shower floor...

    The interest in cork is based more on wanting a resilient floor - tile is hard if someone falls. The secondary issue is whether epoxy grout would eliminate the problem with keeping a tile floor looking new (ie no discoloring of the grout).

  8. Malcolm Taylor | | #8

    CL

    Epoxy grout is a lot more resilient and stays new-looking much longer than regular grout, no matter how diligent you are at keeping it up with sealer. It is a devil to work with, so expect to pay a large premium to the installer.

  9. C L | | #9

    There is also a type of underfloor heating that installs under the subfloor, between the joists. This has the advantage of a) being able to install later and b) heated floor is not impacted if finish floor is revised.
    I think this would be less efficient as the heat would need to transmit through the subfloor. The subfloor material probably has an impact - ie Advantek vs OSB vs plywood. Is there any simple way to calculate the efficiency hit of going through that additional layer?

  10. User avatar GBA Editor
    Martin Holladay | | #10

    C.L.,
    In your latest post, it sounds like you are describing Pex tubing, installed as part of a hydronic heating system. It is the most common way of creating a so-called "radiant" floor.

    Of course, a hydronic heating system can be used to heat your bathroom floor, as long as you have a source of hot water (usually, a boiler or water heater) and you don't mind the expense of purchasing the necessary equipment.

    Concerning your "efficiency" question: As long as your home's thermal envelope has adequate insulation, an electric-resistance heating pad or Pex tubing installed as part of a radiant-floor heating system aren't less "efficient" if there is a thick subfloor or inappropriate flooring. The heat remains indoors, so it isn't "lost." The problem is that a floor assembly with a thick subfloor or inappropriate flooring is less responsive, and takes longer to heat up, than a floor assembly with well-chosen materials. Moreover, the heat may end up in a different room than intended (the room below the floor assembly).

  11. User avatar
    Stephen Sheehy | | #11

    CL: Our shower is open to the rest of the room, with the floor in the shower area sloped to a linear drain next to the wall. There's no shower door or partition. The whole floor is heated. When I shower, I usually don't bother with turning on the floor heat, but my wife likes it.

    We put four inches of reclaimed xps under the slab.

  12. Sean Cotter | | #12

    You might find answers about varying systems here:

  13. C L | | #13

    Thanks all

    The under joist system to which I was referring is not hydronic, but rather simple electric resistance heat. There are a few such products; here is an example:
    There is also this low voltage system that claims to be more efficient. As far as I can tell it just uses less power because it delivers less heat:

    I guess I did not ask my efficiency question properly. I'm trying to understand if there is a way to calculate the delivery of the heat to the proper location - ie the bath floor surface. I understand a less effective system (same efficiency) may heat the area around the resistance heater better than heating the floor surface. I'm just trying to understand if there is a way to calculate the effectiveness.

  14. User avatar
    Dana Dorsett | | #14

    Most vendors publish a nomograph or chart of the heat rate per square foot it can deliver through different R-values of subfloor + flooring materials. If this system is not temperature controlled but has a watts per square foot spec or watts per length spec, the R-value of the subfloor + floor don't matter as much as the ratio of R of the floor materials and the insulation below. If it R1 of floor materials to R20 of insulation about 95% of the heat will be coming through the floor. If it's R2 flooring to R11 something like 85% will be coming though the floor. To convert watts to BTU/hr multiply by 3.412.

    If they don't publish a spec, there's no way to know how to estimate it's effectiveness. Didn't see any kind of spec for the low voltage stuff.

    The Suntouch product inked to is 120V x 0.6A= 72 watts for a 6' long section, or 12 watts (41 BTU/hr) per running foot. If it's between joists 16" (1.33 feet) on center it can deliver 41/1.33= 39 BTU/hr per square foot of floor going into the system, but only part if it is going up- some is going down through the insulation If it's only delivering 85% of it up, the rest going through the insulation & joists. It's about 33 BTU/hr per square foot coming through the floor, and the surface temp of the floor will be about 16F warmer than the room temp. In a 75F bathroom that would be a bit north of 90F, which is warmer than most people like on bare feet, but not super uncomfortable.

    At 50 BTU/hr per square foot out of a radiant floor the surface will be it'll be about 25F warmer than the room temp. On a tile or stone floor in a 75F room that can be pushing the limits for bare foot comfort.

    The more square feet of floor you can use for radiant, the lower the surface temps can be. Most people find 75-80F fine.

    Don't put radiant floor under a toilet- it's possible to melt out the wax seal to the drain. It's fine to put radiant heat under a bathtub or shower floor. It's usually best to avoid putting it under cabinets to avoid the potential for damaging any heat sensitive products stored there.

    Getting to the right heating solution starts by calculating the heat load of the room. How much heat does it need?

  15. C L | | #15

    Dana - thank you so much!!! Very helpful

  16. User avatar
    Peter Engle | | #16

    Just one comment on Dana's post - You do want to run the radiant at least under the toekick if you have standard vanities, or about 6" past the front if you have furniture style vanities (open bottoms with furniture feet). Otherwise, your toes are touching cold tile when you brush your teeth. Voice of experience, here.

  17. C L | | #17

    Based on Dana's information, putting the heated floor UNDER the subfloor, between the joists, may be the best solution because the heating function is not destroyed if you replace the flooring and it allows the heated floor to be installed after construction. Although it will have more to go through and thus be slower to heat the top surface of the finish floor, Dana's explanation makes sense. The heat has to go somewhere, so if I put enough insulation under the heat element, presumably a greater percentage of heat will go up to the finish floor.

    The challenge will be to figure out the R value of the subfloor and finish floor, so I can calculate the R value of insulation needed for under the heat mat.

    And I'm still not sure if I should put a heated floor in the shower...

    Dana - to answer your question on how much heat the bath needs, the answer is probably none. HVAC loads were calculated and equipment was sized without taking a heated bath floor into account. The heated floor is merely a luxury...

  18. Jon R | | #18

    A radiant floor will often produce more heating of the air than you want. If so, you can put radiant heat only under the areas that bare feet will touch.

    > 41/1.33= 39

    I'd use 31 and the system is thermostatically controlled (ie, can produce lower surface temperatures as desired).

  19. User avatar
    Stephen Sheehy | | #19

    We have a wall hung toilet and a wall hung vanity, so we avoided any worries about where to put the heat wires. We find 75° F is more than enough to warm the floor.

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