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Community and Q&A

insulating a slab with hydro radiant heat

JimT | Posted in Energy Efficiency and Durability on

A builder has suggested using Perma Columns for a post frame home. It will then have radiant heated floors. The interior slab will not have a footer. 

One person wants to insulate the underside of the slab with ridged foam board. 
My concern is that the interior slab will raise and fall with the seasons. We live in SE Ohio.
Should the underside of the floor be insulated or would doing the Frost protected shallow foundation insulation method be better in this situation?

Not certain if insulating the underside to help with the efficacy of the radiant heat could cause frost heave issues. 

Thank you

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  1. Expert Member
    Michael Maines | | #1

    You absolutely need something to keep the slab from moving with frost. The IRC has guidance: Note that the perimeter doesn't necessary need a concrete wall; it's the insulation specifications that are important.

  2. JimT | | #2

    That is what I thought. The way the contractor wants to do it definitely has me wondering on the best way to do things. Originally, I envisioned a monolithic slab with footers. He seems to think that frost line is only 18 inches in Ohio which is definitely not correct.
    Insulating the underside of the slab leads me to believe that frost heave will be more likely since the soil underneath will not get warmed much from the radiant heat. This makes a deep footer even more important.

    1. this_page_left_blank | | #3

      You don't need to heat the ground from above. If you put the proper amount of insulation under a slab, it will keep that ground from freezing. No need to even have a building on it, let alone be actively heated. The heat comes from underground.

      The columns complicate things a bit, because you do need to insulate at the slab edge as well, either extending horizontally around the perimeter, or going down into the ground. Those columns are going to be a pretty big heat sink, so you'd have to insulate them as well.

      What is the reason for choosing post frame construction? I think it's generally regarded as presenting more problems than it solves when it comes to making a conditioned space. I remember asking about how to convert a pole barn to a workshop several years ago, and was told that it would be cheaper to just start a building from scratch.

      1. Expert Member
        MALCOLM TAYLOR | | #4


        There was a blog following the construction of a small high performance post frame house on GBA several years ago, but I can't find it searching. It's not something I'd do, but if you are going that route it was done very well.

    2. Expert Member
      Michael Maines | | #5

      This is a good overview of how FPSFs work. The frost depth in Columbus, OH is 32". That's the line we have to design to; frost actually penetrates deeper than that, it just doesn't usually cause problems below that point. Some years the frost may not even extend to 32" but other years it will.

  3. JimT | | #6

    Thank you everyone. I appreciate the input. I definitely was not feeling comfortable with the post construction and the slab not having a footer below frost line. I do like the idea of FPSF insulation even if the footer is deeper than "shallow".
    I have some concern that a well-insulated slab will encourage frost heaving. I understand why insulation would be important due to the radiant floor heating. Not sure if just having an insulated slab there would keep the soil temp from below high enough

    1. gusfhb | | #7

      Put another way, things don't get cold, they lose heat.
      With foam insulation above, and then heated slab above that, where is the soil going to lose heat to?
      Being frozen is not the soil's natural state

    2. Expert Member
      Michael Maines | | #8

      Jim, did you read the builders' guide I attached above? It sounds like you're still missing the concept.

      The ground wants to be warm; it only freezes due to cold air. So you have cold air from above trying to get into the ground (technically the ground is losing heat to the air but I find this an easier way to visualize). Fighting against that is the earth's natural heat, and heat from the building above--whatever gets through the sub-slab insulation. The prescriptive requirements have been around for 40 years and I have never heard of a failure--though I'm sure some exist--but it's a time-tested system if built properly. I've designed and/or built many frost protected slabs in CZ6A and none have moved as far as I know.

      1. JimT | | #9

        I understand it thanks. 2nd law of thermodynamics. What threw me was a contractor stating that a footer is not needed. I know this is not correct.

      2. pietrasm5 | | #10

        I'm looking to build an unheated garage with apartment above. I was thinking of building a FPSF using a small ICF stem wall and floating slab within the interior. Do you have a preferred design? Will be located in the Portland, ME area.

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