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insulating under tubes

dmoni Member Posts: 1
I have a question about methods to insulate under pex tubes fastened up between joists with heat transfer plates.
I’ve read around on the internet at radiant company and insulation sites and have seen several different methods discussed. I understand a need to insulate but I can’t seem to find anything definitive (other than from people trying to sell it) on the benefits or need for the foil. In my case the foil faced bubble pack would add about $350 to my project assuming an unheated basement below and a desire for all heat to be utilized by rooms above only. More than willing to spend the money if it’s the right thing to do.
1. Foil faced radiant barrier 1 -2 inches below the tubes, followed by rest of cavity filled with fiber glass.
2. 1-2 inch gap and fill rest of cavity with fiber glass, in my case R19 craft faced would work good
3. foam bourd under, 1 inch down , either with or without foil face
This has probably been answered on here before I would like your option because I think this is the most credible site on any heating subject.
Thanks in advance for your help , I used to read this site on regular basis almost 8 years ago ( in my old job when boss was not looking ) but I am just know getting back to it , got great help then and see many of the same old pros still helping
Hope you are all doing well.


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    Dan, reflective surfaces do have a benefit, however, in order for them to be beneficial, the aluminum should be 99.5 % pure, which is hard to find in these days of constant recycling, should be as smooth as a mirror, which is impossible to do with BFB, and can not have any dust on the reflective surface.

    The last item is really the deal killer. I don't know where it comes from, but dust gets into these joist bays and settles out on the upward facing reflective surface and destroys any spectral reflectivity of the surface. At that point all reflection ceases.

    You MUST maintain a slight air gap beneath the tubes if you are doing bare naked staple up, because if you don't you will lose the convective energy transfer from the tubes, and won't get the heat out of it that you'd expected.

    When you use heat transmission plates, the 2" air gap is not a critical dimension, but it is generally recommended that you don't want ANY air gaps that would invite convective currents from occurring when you are trying to maintain conductivity as your primary means of upward heat flow. Ideally, a small gap between the insulation and the plates would be better than no gap, but unless you go rigid insulation with standoffs, is hard to do.

    Un-faced fiberglas insulation has been proven to work well over the years. Just make certain you encapsulate the FG insulation to avoid free particles from falling down into the conditioned space below the insulation, if need be.

    I suspect that some day in the future, that loose fiber glass particles will be put int the same classification as asbestos insulation finds itself in. Avoid friables (loose small breathable particles) at all costs.

    And save the aluminum for more imprtant things, like beer cans :smiley:

    And to anyone else reading this thread, those plates are not "reflectors". They are energy transfer plates. Quit listening to the internet peddlars of things hydronic and radiant. Most of them don't know what they are talking about...

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Rich_49
    Rich_49 Member Posts: 2,692
    Are these heat transfer plates you have tight against the sub floor or are they not in contact with sub floor ?

    If they are not in contact leave whichever air space you desire ( less is better ) , skip the bubble foil and just use whatever insulation you are comfortable with , backing is good for reasons mentioned by Mark . Resistance is the key here and the higher the R value the less downward losses you will suffer .

    In contact with subfloor . NO AIR SPACE . All else applies
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • 4Johnpipe
    4Johnpipe Member Posts: 477
    Heat is blind cant see the shiny foil. Just use a quality insulation as much as you can afford to fit "properly" into the beam space.
    Considerate People, Considerate Service, Consider It Done!
    email: [email protected]
  • tyronesheat
    tyronesheat Member Posts: 33
    Mark, so let me get this right. You are saying to use encapsulate the FG insulation below the staple up. If so which one r15, r19?
  • Rich_49
    Rich_49 Member Posts: 2,692
    Minimum of R19
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833