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Your experiences regarding hydronic floor heat in concrete slabs

jollygreenshortguy | Posted in General Questions on

Typically when we have a concrete slab floor and hydronic floor heat, the piping is placed inside the slab itself.
Is there any compelling advantage to placing the piping on top of the slab and pouring something like a self-leveling mix to encase the piping?
If you have first hand experience with that I’d be interested to hear your thoughts regarding the pros and cons of each approach. Thanks.

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  1. Expert Member
    DCcontrarian | | #1

    With concrete you need a minimum thickness for strength, the problem with putting tubing too close to the surface is the concrete above it is thin and liable to crack. What I've done where I had limited thickness under a tile floor is embed tubing at the top of a mortar bed, and then cover the whole thing with 1/4" concrete backer board that is bonded to the mortar bed with thinset. Then tile over the backer board. I really liked the feel of that floor.

    As a general note, I don't advocate using concrete for a floor material because you want radiant heat -- it's not a good material for that. In the early days of radiant floor heat it was promoted for that purpose based on some mistaken beliefs. It doesn't make for a good floor, and it doesn't make for a good radiant surface. You want your radiator to have as little heat capacity as possible so that it responds as quickly as possible to temperature changes in the room.

  2. Expert Member
    Akos | | #2

    Around me heated slabs are done pretty much always with insulated pex panels. These put the tubes bellow the slab so there are no issue with holding the pipes in place and keeping it away from the surface plus cracking. The cost delta between the pex panels and plain EPS is not that much, you can also always add an extra layer of EPS bellow them if you want higher R value.

    +1 On DC's comment. Radiant slabs are slow to react which is the opposite of what you want. I only have a 1.5" concrete overpour and it still takes about 4h to react to a setpoint change. Not something you want in a place with much winter time solar heat gain.

  3. buildzilla | | #3

    is there a strategy that uses something like warmboard on top of the slab?

    something like dri-core and warm-board had a baby...

    1. Expert Member
      DCcontrarian | | #4

      I'm not a big fan of dri-core, it addresses a need that shouldn't exist.

      Don't see why you couldn't put WarmBoard over the slab, or on sleepers.

    2. Expert Member
      Akos | | #5

      What you'll find is with decent insulation and air sealing the heat loss in a basement is so low that anything floor heat makes no sense. Maybe with something like a walkout with a lot of windows changes the math but I think even there are still easier/cheaper ways of getting heat there.

      I know there is the common perception out there that floor heat equals comfort, which is true in older leaky homes.

      In anything near 1ACH with a bit above code levels of insulation (especially sub slab insulation in a basement) the temperatures are so even that no surface ever feels cold. Because the loads are also so small, adding floor heat also might only bring up the surface temps by a couple of degrees which is barely noticeable.

      1. Expert Member
        DCcontrarian | | #6

        I agree. Before going too far down this path I hope JGSG does a room-by-room Manual J and figures out what the floor temperature will be for this basement. I suspect it will be disappointingly low.

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