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Polyiso insulation under radiant floor heating slab?

acoops | Posted in Energy Efficiency and Durability on

Hi there,

I’m looking to install a radiant foor heating system in my house. I recently purchased a large amount of used (good condition) Polyiso Insulation namely RMAX Thermasheet Type 3 and some of Firestone’s equivalent.

Looking into it further I’m now wondering whether I shouldn’t have gone with EPS instead of Polyiso. (Sounds like Polyiso has issues with moisture wicking and decreasing R values in cold temperatures).

Seeing that I’ve already bought it, I’m wondering if in my particular situation Polyiso will work – The 4″ of Polyiso will be installed onto an existing slab and sandwiched top and bottom with 6mil poly vapour barrier. I will not be putting any holes through the top layer as the pex tubing will be fastened to rebar mesh with zip ties. The rebar mesh will be raised up with 1/2″ wide foot plastic spacers to avoid putting holes in the vapour barrier.

The house is located in Interior British Columbia. I have attached a cross-section diagram of my plan. Any advice, experience or opinions are much appreciated!

Thanks for your time!
Adam

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Acoop,

    Out of curiosity, are you planning to also rebuild the door and window openings?

    1. acoops | | #3

      Hi Steve,

      Yep, completely changing the layout, new doors and windows are going in. House was quite run down so stripped it back to start fresh!

  2. Expert Member
    Dana Dorsett | | #2

    >"Sounds like Polyiso has issues with moisture wicking and decreasing R values in cold temperatures"

    The temperature derating is not an issue in your stackup, even if you have permafrost for subsoil.

    With 6 mil poly between the existing slab and polyiso there is no wicking issues, but it would be nice to avoid low permeance layers between the polyiso and indoors, otherwise it creates a moisture trap. If dry when it's installed and the installation is PERFECT it would be fine, but perfect is only in the mind of the designer- in real world construction there is good/better/best, but no "perfect".

    A 4" concrete slab has a substantial carbon footprint, and it's not cheap. If it's only being installed to be a radiator it may be greener & cheaper to install a floating 3/4"-1" subfloor (double-layer of half-inch OSB or CDX glued & screwed together) and an above-the-subfloor radiant solution,. that would also have a much greater vapor permeance than 6 mil polyethylene.

    How much heat per square foot does this radiant floor need to provide?

    1. acoops | | #4

      Hi Dana,

      Thanks for the reply!

      Good to hear that the temperature derating won't be an issue, luckily we down have to worry about permafrost here! I might try to keep the Polyiso away from any sewer pipes/ vents and use EPS or XPS around those areas of the house just to be safe.

      Hadn't thought of installing that kind of floor/ radiant heat set up but I'll definitely look into it! Was leaning towards the finished concrete slab option for durability as half the building is commercial. We have ordered a geothermal unit that's capable of producing 60,000BTU's - that will eventually be powered by our solar set up.

      1. Expert Member
        Dana Dorsett | | #5

        >"We have ordered a geothermal unit that's capable of producing 60,000BTU's - that will eventually be powered by our solar set up."

        That's a LOT of ground source heat pump, and oversized for the loads of most normal sized houses! How big is the house? How was the heat pump size selected?

        If you have a heating history on the house from prior winters it's possible to infer the design heat load by correlating wintertime fuel use with heating degree-day data from a nearby weather station. The details on that methodology live here:

        https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

        1. acoops | | #9

          For sure, it's definitely on the large side for our current space of 1800 square feet, looking at adding a second story in the future - wanted a geothermal unit that could cover us for that eventuality. Unfortunately we don't have any heating history on the house, just bought it and prior to that it had been abandoned for over a decade.

          Some friends close by with a similar size house have the same size geothermal unit, that's what we went off for purchasing ours.

  3. Expert Member
    Akos | | #6

    Basements floors will eventually get wet, no matter how careful you are, it will happen.

    I would be worried about poyiso espcially if there is no drying path. Once saturated, it will also loose a fair bit of R value and takes forever to dry. I would only install polyiso on the floor over some drainage mat and leave a path for drying to the inside.

    I would check the heat load for the space. For most basements, a full coverage is not needed, it would actually make the space less comfortable as the surface would never get warm enough. You'll never get the warm toes feeling.

    You can save a fair bit in install and materials by only heating high traffic areas and colder section (under windows /exterior doors/walkout areas).

    1. Expert Member
      Dana Dorsett | | #7

      >"Basements floors will eventually get wet, no matter how careful you are, it will happen."
      ----------
      >"For most basements, a full coverage is not needed, it would actually make the space less comfortable as the surface would never get warm enough. You'll never get the warm toes feeling."

      The diagram shows a slab-on-grade foundation with stemwalls, NOT a basement. That has considerably lower risk than basement slabs, but it still calls for a drying path for the foam board.

      1. Expert Member
        Akos | | #8

        Funny how the mind works. When you are in the land of basements, you assume concrete slab means basement.

    2. acoops | | #10

      Thanks for the reply!

      Is there a particular type or brand of drainage mat you'd recommend? I might end up using the Polyiso in the walls and roof instead. Would drainage mat be necessary with EPS or XPS foam boards?

      For sure, have designed the pex zone layout to favour the high traffic areas and colder sections you mentioned.

      1. Expert Member
        Akos | | #11

        The drainage mat was based on my incorrect assumption that it is a basement. With slab on grade, water is less of an issue.

        Polyiso is great for walls and roofs, but doesn't work well for floors. If you are going to pour a new slab on-top, I would go with another insulation type.

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