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Insulation under hydronically heated slab

mcbane | Posted in Energy Efficiency and Durability on

I am building a garage in CA zone 16, US zone 4B.  Last year I buried temperature recorders at depths of 18″ and 42″.  The deeper probe hovered at 33 degrees all winter.  The shallower probe occasionally froze to 31 degrees.  Well water from 200 ft deep is typically 40 degrees.

The building will have an 8″ structural slab to support some concentrated loads.  It is important to me to be able to drill anchor holes anywhere in the slab so I want to put the hydronic tubing at the bottom of the slab.

My current plans call for 8″ aggregate base and 2″ EPS under the slab, with the tubing on top of rebar at the mid height of the slab.  If I move the hydronic tubing to the bottom of slab should I be adding more insulation under the slab?  Also, in theory, air entrained concrete has a high R value that would limit efficiency of a hydronic system but I have seen opinions ranging from “air entrained R value is same as normal concrete” to “air entrained R value is 10x normal concrete”.  I will pour the slab after the building is in place so conceivably I could order non-air entrained concrete.

BTW, slab perimeter insulation is R10 and it is difficult to increase that much.  Most I could do is add another R5 to inside of my frost wall.

If it is relevant, my heating objective for the structure is 60 degrees F minimum air temp, with no variation 24×7.  I want to keep plumbing and stored equipment from freezing and want to be able to work without my fingers freezing.  I will be on time of use electric billing so am hoping to use a time of day thermostat to electrically heat the slab at night and then use its substantial thermal capacity to keep temperature reasonable during the day.

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  1. rhl_ | | #1

    2” of EPS is not that much. I think Martins advice in one of the GBA articles is, how much insulation with floor heat? More.

    I did 3” in my basement, and 2” for the picture frame. This is in Zone 4A Marine. The tubes were supported on a wire mesh sitting on chairs.

    In terms of tube depth, you want the tubes to end up about 1.5” or so from the top of the concrete. John Seigenthalers book has a whole chapter on this. The further down you go, you need to be to add more heat (hotter input water) to get the same warming effect.

  2. Expert Member
    Dana Dorsett | | #2

    >"The building will have an 8″ structural slab to support some concentrated loads. "

    Air entrained or not, the thermal mass/R value thermal diffusivity and lag time of an 8" slab makes it nearly impossible to use as a heating radiator without major undershoots & overshoots.

    >"My current plans call for 8″ aggregate base and 2″ EPS under the slab, with the tubing on top of rebar at the mid height of the slab."

    That makes it even MORE impossible to control, since it increases the lag time to the max!

    If radiant floor is a "must", an above-the slab solution with at least some amount of thermal isolation from the slab would be needed. A minimalist Roth panel above the slab would be one solution to making it more responsive, but wouldn't allow you to game the TOU rate structures without using buffer tanks.

    R8.4 isn't much insulation to put between an 80F+ slab and near-freezing dirt with 40F subsoil. It should probably be something like twice that.

    To have any chance of making this work well using the slab as the radiator AND storage medium is going to require doing some math. (And it's not a "design by web forum" napkin-math kind of problem.)

  3. Expert Member
    Akos | | #3

    Assuming 1500sqft (makes the math easy) of floor with 8" thickness you have 1000 ft^3 of concrete. That has the capacity of around 32kBTU/degF, which is not that much. To make it work for say a 12h period, you would need a house heat loss of around 10kBTU.

    Asssuming 75F slab, 40F ground with 2" you would loose around 6200BTU to the ground. That is a fair bit compared to the house load above. You would probably want around 5" to 6".

    Overall, it is possible to make it work, but you need to do some real thermal simulations, much more than the quick calcs above. Probably no way to make it work without a highly insulated envelope.

    As Dana said, controlling this would not be easy, if your place has a lot of solar gain, it would cause a lot of overheating when the sun is out.

    Concrete is a pretty good conductor of heat, with any reasonable amount of insulation bellow (more than 2"), the location of the pipes makes very little difference for energy loss. Placing the pipes lower reduces striping, so you can go with a larger spacing between runs.

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