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Insulating Radiant Under Wood

Maynard108 Member Posts: 3
So . . . I installed pex under my first floor maple floors (old and well conditioned) and now I need to insulate them. At the same time, I went through an energy audit and they (obviously) told me I need to insulate my floors. The contractors through the audit program are recommending either fiberglass insulation with a reflective foil on it (one piece) or rigid foam with a reflective foil on it (one piece) with spray foam sealing of the airspace between the joists. Are either of these options the right option? I am trying to find a balance between cost and efficiency, but I am hearing different things. Some say the foil should be right against the pex and some say an insulating cavity will increase the efficiency. Any advice is helpful on what the best thing to do at this point is. Thanks much.


  • Under Maple

    A few questions:

    1) It sounds as though you have an existing maple floor and stapled PEX tubing between the joist bays.  Is this correct?  If so, do you have a subfloor under the maple and what is it made out of and how thick is it?

    2) Did you use extruded aluminum plates attached to the subfloor that the PEX snaps into or is the tubing running wild, i.e. without plates (please tell me you didn't)?  Heating with plates is heating by conduction and requires no insulation air space.  Heating without plates is heating by convection and requires at least, a 2" air space.

    We typically use Radiant Engineering's Thinfin accelerator plates and tell the owners to use R-30 fiberglass insulation - no reflective foil or vapor barrier necessary.  With tubing without plates, I'd say to install insulation with reflective foil facing the tubing.

    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    a couple of things

    sealing the joist bay is an excellent detail and is strongly recommended. at the very least seal the ends of the joists and the outside rim joists.

    Fiberglass is a poor insulation and my normal stance is "anything but fiberglass". Especially if you're over unheated space. it's a bit translucent to radiant energy as well, so we do recommend a kraft paper or reflective layer in the assembly, preferably on the heated side. the system will work with thicker batts without a face, but it will be better with the face.

    I definitely concur with alan that plates are usually the right way to go, and I'll take it a step further and say before you seal this up to be sure that a room by room heat load has been done to be sure what you have will work. Now would be the time to add plates if you don't have them and need them, and/or rough in feeds for supplemental heat if needed.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • nature of heat

    convective= hot air rising. conductve=heat transfer by contact. radiant= waves which are unaffected by gravity which allows for overhead radiant heaters to effectively SHINE the heat below them.

    So, the pex conducts heat to the floor by contact & the floor & joists become more radiant as convective patterns begin to develop within the cavity.

    I'd cover the underside of the joist& cavity with foil facing up so to reflect the radiant . insulation below the foil i dont thiink would make any diff as which type where its inside & wind wont affect fiberglass which is cheaper than foam

    By leaving the cavity empty , the heat can better disperse onto floor above vs if the pex was foamed in, it would create a hot spot on the floor moreso. Bottom line is if the heat cant escape to the area below the floor structure, it'll have to go up as convective heat

    I'd love to test this wiith 4 schemes

    1-foil stapled to underside of joist

    2-foil attached to foam trips attached to the underside of joists

    3-foilfaced insulation [foilside up] attached to joists

    4- foil bubblewrap attached to joists
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    I'm not a fan of foil

    the stuff gets really dusty and loses a lot of effectiveness in upward facing applications.

    any opaque layer is sufficient in my mind. Foil is helpful at first, but I think it's ultimately a waste of money and time.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.

    & EEVEN a vacumn wont work= ask the sun
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666

    when it is clean. But few want to pull it apart every few months to clean it, I imagine.
  • Gordy
    Gordy Member Posts: 9,537

    Try this link if you are refering to a vacuum not being an insulator.

  • Gordy
    Gordy Member Posts: 9,537
    edited May 2011

     When clean Pook. As Rob pointed out. How many years do you think a reflective foil material would stay clean, and Give the same performance as when initially installed. It would be diminished returns every year it was in place,

    Radiant heat is a long term comfort system, and so should be the materials that support its performance. Bubble foil is a joke, and I'm with Rob on the fiberglass.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    About vacuum and insulation.

    There are the conventional ways heat can be moved around: conduction, convection, and radiation. Now there are ways to prevent moving each type of heat transfer.

    To stop conduction, you can impose a suitable insulator between the the surface that is the interface between the hot and cold surfaces. Typical insulators include suitable ceramics. Another commonly used insulator is air, provided the air is not free to move, so the air space is typically filled with a poor conductor of heat that takes up most of the air space, such as fiberglass. If the space were not filled with the fiberglass, the air would be free to move and convection would be set up and remove the heat. A vacuum would work too, if you have some way to keep it there.

    To stop convection, you must stop the movement of whatever medium would convect the heat from where it is desired to where it is not. Air with fiberglass, suitably fireproofed cellulose, etc can do this. Physical barriers, sufficietly close to each other so that temperature gradients can be maintained sufficiently small, can also work. Vacuum will work quite well if you can produce the vacuum and keep it there, should be extremely effective.

    To stop radiation is a different story. Stuff that is transparent to infra-red radiation allows radient heat to pass freely. The space between the sun and the earth is an example of a nearly perfect vacuum, and the distance is around 93 million miles. Yet the energy of the sun has little difficulty getting here. That heat is neither conducted or convected here in any noticeable amount. But it gets here. About the only practical way to stop it is to put a suitable reflecting surface in the way, such as a white cloud, or a parasol. On earth, mirrors work well on a small scale. The reason a thermos bottle works is that the vacuum in the walls prevent conduction and convection of the heat, and the silvered sides prevent the radiation. If the vacuum insulated panels actually work, they must have both the vacuum to prevent conduction and convection, and they must be silvered to prevent the radiation. In the typical thermos bottle (the glass ones), the silvering is on the inside to keep it from getting dirty and tarnishing.
  • Gordy
    Gordy Member Posts: 9,537
    radiant, vacuum, sun, and distance.

    We as radiant heating proponents often use the Suns ability to radiantly transfer its heat long distances.

       Using the Earth, and the Moon as two celestial bodies that are similar in distance from the Sun. One planet has a large reflective surface, and an atmosphere. While the other has no atmosphere, and its surface while some what reflective is not like snow, and water on earth.

    The Sun's outer layer is 11000* F

    Mean Surface Temperature

                              F      R      C      K

              Earth          59    519     15    288

              Moon           -9    451    -23    250


         Minimum Surface Temperature

                              F      R      C      K

              Earth        -128    332    -89    184

              Moon         -233    227   -147    126


         Maximum Surface Temperature

                              F      R      C      K

              Earth         136    596     58    331

              Moon          212    672    100    373


    As you can see the temperature extremes are not that far apart considering the levels of protection relativley speaking between the Earth, and the Moon.

    With delta Ts of 105* F on the low end. 76* F on the high end, and 68* on the mean end of temperatures.

    The size of the heat source, and the distance from it plays the larger role.

    My point is the vacuum of space has little to do with the ability to tranfer radiant energy the distance it travels. Its the lack of objects, and particulates to block the transfer.
  • meplumber
    meplumber Member Posts: 678
    Just a thought.

    In most jurisdictions, foam has to be covered by drywall for fire protection, even in a basement.  Check your area
  • Maynard108
    Maynard108 Member Posts: 3
    Thanks for the replies

    Sorry - I had some email trouble.

    Yes - it is a wild array of pex under Maple floors. I don't know how thick the subfloor is or what it is made of. 1900-1920's New England colonial house, if that gives you any ideas.

    I have had the room by room heat load done and it should work pretty well (been heating nicely in most spots with no insulation, but not very efficient . . .).

    So, it looks like we can get by with the current plan, but not recommended. Can someone give me a per sq foot ball park on plating? It seems like it will be prohibitively expensive. Also, is it too late? Are they just plates, or are there channels that the tubing goes in and if it wasn't installed with the plates in mind, the tubing won't fit the channels?

    Thanks again for all your help.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    How Would One

    Design the proper water temps needed or know if the radiant would even work if you did not know the R-Value you are trying to heat thru?

    If you are not using plates you MUST leave a 2" air gap between the bottom of the tubing and insulation. This allows you to create a convection oven in the bays. Without it you won't heat the space. If this is your main heat source then plates are a must. Without them you give up, lower water temps, even floor surface temp and maybe more importantly response time.

    For insulation you want to make sure what ever product you use has a minimum value of R-11 although would rather see R-19. Don't know how you would do a proper radiant heat loss without knowing the R-Value of the insulation. I wouldn't put my trust in your heat loss. I guess your winging it on the design based on not knowing the floor r-value or the insulation value below.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • scott markle_2
    scott markle_2 Member Posts: 611
    ultra fin

    For joist bay heating (high temp radiant) this is the type of plate to consider.


    From what I remember It's recommended that this product is used with pex/Al/pex, because it's stiff and doesn't move so much with expansion, but if you do a good job with hangers I think you could make it work with regular pex, just confirm with the manufacturer that the pex your using (if it's not pexALpex ) will make good contact with the plates.

    R-19 would probably do the trick, but as Chris points out how much goes up and how much goes down depends on the R-value of the floor as much as what's bellow it, maybe a bit less so in a high temp joist bay heating arrangement where the dynamic of rising hot air comes into play. I'm sure ulta fin has some guidelines that you could use for determining appropriate levels of insulation
  • Maynard108
    Maynard108 Member Posts: 3

    Thanks - this looks like a better product for me, since it seems that you can put it in post hanging. I will have to go take a look and see what space on either side is needed for the fins and whether we have that space.
This discussion has been closed.