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Community and Q&A

Insulation for radiant heat

user-7330770 | Posted in General Questions on

I’m currently installing radiant heat in the 3 floors of my home. I’m curious to know what you guys think the vesb way to do it is
Basement-1/2 pex in slab (will be fully finished)

1st floor–in mud job on top of subfloor

2nd floor–staple up in between joists

I’m going to be spray foaming all the exterior walls . Should I also spray inbetween the floor joists on every floor or will fiberglass Batts be fineĀ 

Or do i not insulate at all since every Floor will have radiant?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name? (I'm Martin.)

    Of the three floors you describe, the most important place to include a generous amount of insulation is under your basement slab. In that location, you need a continuous horizontal layer of rigid foam; the thickness depends on your climate zone. For more information, see this article: "Determining Sub-Slab Rigid Foam Thickness."

  2. user-7330770 | | #2

    Good morning my name is Nick and ty for the reply, I have installed a continuous layer of 2" XPS in the basement . What do you recommend if any to insulate the two other Floors

  3. GBA Editor
    Martin Holladay | | #3

    To make sure that you're heating the space where you want the heat to go, it makes sense to insulate between the joists with fiberglass batts or mineral wool batts. See illustration below.

    1. user-7330770 | | #4

      Does the same concept apply for the 1stfloor where the tubing will in a mud job ON TOP of the subfloor?

      1. Expert Member
        Dana Dorsett | | #8


        For zone isolation there needs to be insulation between floors, whether staple up, or mud. It doesn't take a lot though, and the higher performance the building envelope, the less zone isolation matters. R13s snugged up to the subfloor is more than enough for most code-min houses when both levels are fully conditioned space.

        Is the past tense, ''...I have installed a continuous layer of 2" XPS in the basement.." and future tense, "...going to be spray foaming all the exterior walls..." both true?

        The reason for asking is, XPS is the least-green commonly used insulation material out there due to it's high polymer/inch but most importantly it's HFC blowing agents. That is followed closely by HFC-blown closed cell spray polyurethane, though there are less damaging HFO blown variants (for more money per R). Open cell foam uses less than half the polymer per R, and is blown with fairly benign H2O (yup, water!). When installed between studs the higher R/inch of closed cell foam is also largely wasted- more performance can be had with a half inch of continuous polyiso (blown with low impact hydrocarbons.)

        Also 2" XPS is really at best R9 over the lifecycle of the material. Most heated slabs would need/want more than that.

  4. Expert Member
    Akos | | #5


    The empty floor assembly with finished ceiling is about R3 (if the tubes are bellow the R value is slightly less). Depending on the temperature of your slab on the main floor and the heat load in the basement, it might be too much. I generally find that uninsulated ceiling with floor heat above in a reasonably insulated basement needs very little heat.

    You can probably save yourself a pile of money by not putting in full floor heat in the basement (just spot areas like washrooms) and leaving the ceiling uninsulated.

    1. user-7330770 | | #6

      So you're saying not to insulate the basement ceiling/first floor . If I understand correctly you want the heat to radiate up to the first floor

      1. GBA Editor
        Martin Holladay | | #7

        I think Akos was making the point that you are planning to install way more tubing than you need -- unless your house has no insulation and single-glazed windows.

        It's much better to invest your money in airtightness, extra insulation, and good windows -- rather than spending a fortune on an expensive heating system.

        Akos suggested that you skip the tubing that you plan to install in your basement slab.

      2. Expert Member
        Akos | | #9


        Pretty much what Martin says. This is one of those cases where figuring out what your loads are can save significant money and make the house more comfortable. Putting in more tube only increases your labour and part costs with out any benefit (actually it makes things a bit worse as the floors will never be "warm")

        For example, if you do a complete heated slab in the basement (provided it is reasonably sealed/insulated), the operating temperature on it would be so low that it would never feel warm, defeats the purpose of heated floors. If you want the warm toes feel, than it is better a smaller surface radiating, which wold be hotter. So in a basement, you probably only want to heat bathrooms, hallways and any other high traffic area.

        Also important to check heat loads, if you decide to skip the insulation in the basement ceiling, you don't get too much radiated heat from the first floor and overheat the basement.

        Also with 3 stories heated with only radiant heat, you will get a fair bit of stratification (the top floor will be hot). I would probably go with TRVs or individual thermostat for the bedrooms as there will be big difference in heat with the doors open/closed.

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