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Open-Cell Foam Directly on Sheet-Plate Radiant Underfloor Systems- Pros/Cons?

SteambackSteamback Posts: 26Member
edited October 2017 in Radiant Heating
My insulation contractor says that the only way he will use open cell spray foam to insulate my underfloor radiant install is if I use bubble-foil stapled-up between the joists and about 1" or more away from the radiant, making an air space between the radiant and the foil. Then he'll spray foam on the foil between the joist bays.

I would much rather skip the foil install and just apply spray foam to the radiant directly. I can't understand WHY? I would want to create an airspace in the joist bay between the radiant and the insulation, if I'm willing to commit to 3+ or more inches of foam making sure the floor to be heated is colder than below the radiant (ie, heat goes up, right? especially with insulation making sure so...)

My Main Question: Will I regret applying foam directly to my radiant without leaving an airspace in the joist bay between the radiant and the insulation by using the bubble foil sheets???

Background - I installed an underfloor radiant system using staple-up sheet plates which cover the PEX piping. The plates are spaced about a 1/4" apart and are 14.5" wide, meaning they cover the entire exposed underside of the sub-floor. Essentially, we used plates everywhere we could, except for the bends in the PEX at the ends of each bay loop. So, almost the entire house floor is covered with sheet plates.

The plates were ideally for 5/8" pipe but I used 1/2" PEX. I did my best to install the plates with as much contact with the PEX as possible. But, it was inevitable that contact between the plates and the PEX was good but not great. There are slight spaces between the PEX and the plate channels at the ends of each plate. So...

I'm a little worried expanding foam might get in those cracks and isolate the PEX from the plates and floor...but am considering using 2 foams. One would hopefully be a low-expanding type and could be applied to the spaces between plates only. Then, another higher expanding foam would be applied to finish the install.

Is this nuts, or OK practice? Thanks in advance for any thoughts...


  • hot rodhot rod Posts: 8,146Member
    edited October 2017
    I'd worry about the pex moving in the plates if the fit is loose? If it is heating pex with an EVOH outer barrier that stuff will really squeak.

    Did you ask the tube manufacturer about spraying against the tube?

    True that foam will find those gaps and reduce some contact, but it is already air space if the tube is loose. Any foam getting in there will adhere the pex and limit or eliminate tube creep and potential noise, at least :)

    Any way to run the system before you insulate to observe the tube behavior?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • Paul PolletsPaul Pollets Posts: 3,087Member
    Bad idea to use plates that are larger that the tubing. Emissivity will be reduced and noise is more than possible, as HR said. It's far easier to use R19 or R21 batts of fiberglass under the plates so accessibility is never in question. Hope you've considered a mixing valve and control as the heat source.
  • Jamie HallJamie Hall Posts: 8,233Member
    Spray foams as a group do not like to be heated. Even to the relatively low temperatures of radiant. They will degrade dismayingly rapidly. So -- don't do it. Your contractor's idea will work.

    Remember that the purpose of insulation is to provide a non-conductive (relatively) material which prevents much air movement. The 1 inch space between the plates and the insulation is just that.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • hot rodhot rod Posts: 8,146Member
    Always best to check with the manufacturer of the product. Around here foam is sprayed right against the roof sheathing and even right against metal roofing. My shop was spray foamed on metal over 20 years ago, I have not seen any breakdown, I do poke holes through it occasionally to mount things on the roof.

    No harm in that layer as your contractor suggested, that spray foam chemical reaction does generate some heat, make sure any bubble type wrap can handle that.

    Here is some info from the polyurethane site.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
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