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Under the slab insulation

Post in 'DIY and General non-hearth advice' started by Fredman, Jan 1, 2008.

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  1. Fredman

    Fredman New Member

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    Dec 22, 2007
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    U.P. Michigan
    I was wondering how much and what kind of insulation one might put under a concrete floor if they are going to do radiant heat? Do the footings have to be insulated and if so how far down?

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  2. Reggie Dunlap

    Reggie Dunlap Feeling the Heat

    Joined:
    Dec 13, 2005
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    Loc:
    Northern Vermont
    I use 2 inches of blue board under the slab and at least one inch as a thermal barrier between the slab and the frostwall. Footing depth should be whatever is considered normal in your climate. Here in Vermont we usually go five feet below grade.

    Hope this helps,

    Reggie
  3. jklingel

    jklingel Feeling the Heat

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    Loc:
    Fairbanks
    See the "under-slab insulation thickness " thread, several below this. Lots of opinions there, but MY bottom line is: LOTS, unless your earth is going to ever be warmer than what you want your floor to be. Heat goes to cold. Period. End of story. If your earth is cooler than your floor, heat will travel out of your floor. It is a law of thermodynamics (energy travels from a state of higher entropy to a state of lower; paraphrased, I don't recall the exact wording, but look it up) and won't change because someone says his place is toasty with no insulation. Fine. It would be toasty w/ less heat put to it if you insulated (assuming ground is cooler than desired floor temp). 1" is better than nothing, 2" is standard in cold places, and I am going to use 4". BTW, extruded poly board is the stuff, not expanded, and I have not read of anyone on this forum that thinks much of the bubble wrap. And insulating around the footer wall, deep, is imperative if you live in a coldish place. Good luck. j
  4. amkazen

    amkazen New Member

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    Loc:
    Albuquerque, NM
    If I remember correctly we used the rigid styrofoam insulation that is about 2" thick under our slab/radiant floor. We bought it at Home Depot. We followed the radiant pipe manufacturer's installation instructions and the State of NM code guidelines. We installed the piping & insulation ourselves when we started building in the Spring of 2001.
  5. Fredman

    Fredman New Member

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    Dec 22, 2007
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    Loc:
    U.P. Michigan
    Our frost line is 42 inches. So you are saying to go with 1 inch 4 feet down with 4 inches of insulation and I will be fine. If I am to do that, then am I better to pour a foundation with the insulated forms? I was looking at a pole barn. Front 2/3 at 50 degrees and the back 1/3 at 65 degrees.
  6. amkazen

    amkazen New Member

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    What does your local Home Depot / Lowes/ local building material supply store recommend?

    What do any of your local builders recommend?

    What about the radiant heat supply store recommend?
  7. Fredman

    Fredman New Member

    Joined:
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    I talked to a local contractor the other day and he doesn't recommend foam board. Because he has seen it break up when installing the pex tubes and pouring the concrete over it. He recommends "Barrier" blankets(not foil bubble wrap). They offer more give and you can wrap them around the foundation.
  8. amkazen

    amkazen New Member

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    Loc:
    Albuquerque, NM
    Ok, good, you have some info now to think about. As a devil's advocate, I would say his is one opinion, and I would also want to know how he is aware that the boards crack. I might agree with him that the board would crack when pouring concrete on the end of a board that is hanging over the empty footing (nothing to support the board). However, if the board is supported I cannot see how the concrete would crack. We had almost 1,300 sq. ft. of rigid board laying on the dirt and 4x4 metal grid with the radiant tubes (we used Kitec pipe) tied to the grid. There was probably 10 guys running and walking all over the board and grid pouring the concrete and it never cracked from what we could see. Also, when we were installing it and walking on it we never saw it crack. I did see it crack where the board hung over the footing and I stepped on the board, and also when we poured concrete in that same area. In fact, the Kitec pip in one area was not tied for 5 feet and the concrete pushed the Kitec down into the footer. We got it back out but oops...lesson learned...make sure the Kitec is tied every 6".

    The bubble wrap may be good stuff, or better, or just as good, but just make an informed decision that you feel comfortable with. Call the manufacturer of the pipe you are using. I have found manufacturer's tech support is usually pretty nice and ready to help.

    I will find my pics of our radiant installation and post them later tonight.
  9. jklingel

    jklingel Feeling the Heat

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    Fairbanks
    If you are conscientious about preparing your gravel under your foam, it will not crack because it will be on a flat surface and therefore be in compression. Spend a little time w/ a rake , a 2x4 screed board, and some fines (sand, even) and get your gravel FLAT. I would bet that a rare contractor takes the time to do that, from what I have seen. Up here, 1" blue foam is almost the same price as 2". Go heavy and far w/ insulation, bury it, and forget about the cost. My 2. j
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