Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Sprayed Foam over PEX in heat distribution plates...good idea?

Andrew Hagen_2
Andrew Hagen_2 Member Posts: 236
Are these extruded plates? If so, Corbond or Icynene work great sprayed directly beneath the plates.


  • DennisU
    DennisU Member Posts: 6
    Sprayed Foam over PEX in heat distribution plates...good idea?

    I have a residence in San Francisco with radiant floor heat via 1/2" PEX tubing in aluminum heat distribution plates @ 8" OC in a typical wood floor framing assembly . My engineer recommended R-19 foil-faced insulation batts (foil facing up) in the floor assembly to insulate between the zones. Does anyone know if it would be a good alternate idea to spray closed-cell foam up against these plates in lieu of the batts. I am interested in the sound abatement quality the foam would provide under the sub-floor (albeit the R-value would drop to R-10 with 1.5" of foam). Also, I have become quite handy spraying foam with DIY kits and I'm not too excited about working with fiberglass overhead. Also, wouldn't the elimination of an air space below the PEX/plates avoid convective transfer down and all (or most) heat would go up radiantly through the plates? Thanks.
  • DennisU
    DennisU Member Posts: 6

    I used the Joist Trak Panels by Uponor Wirsbo. I think they are extruded...they seem very heavy duty. The foam kits I am using are closed-cell Polyurethane available from Foam-it Green in Chicago. I believe a similar spec to Corbond, but not Icynene. Sounds promising, thanks.
  • Bart_9
    Bart_9 Member Posts: 2

    I would maintain the 2" air space to heat the whole joist
    cavity, more even heat on floor system, foam would insulate piping and promote stripping.
    try using the 16" wide bubble foil w/staple tabs,
    keeping the air space - then spray foam if that is the way
    you want to go.
  • DennisU
    DennisU Member Posts: 6
    Foam sticking

    Good points.
    I would have to test to see if the foam would stick to inverted foil facing....it does not adhere well to plastic sheeting.
  • troy_8
    troy_8 Member Posts: 109
    Foam sprayed under plates and tube

    This is the best, provided you first install a scrim layer of paper or cardboard or something to keep the foam from enveloping the tube. The foam will work its way around the tube and plates and inhibit the heat transfer to the floor. You need to prevent the foam from touching the plates and tube. I was called in to advise why the job we installed didn't heat. I had suggested foam insulation and the homeowner followed through but the foam installer didn't protect the tubes. It was a disaster. Once the foam was dug out and redone it worked great. thank god it was only one room. Don't miss this step. You will be sorry. On the ends make sure to keep your tube loops back from the exterior wall plate, this area needs to be foamed to prevent heat traveling to the exterior.
  • Dave Holdorf_2
    Dave Holdorf_2 Member Posts: 30
    Air Gap


    With the Joist Trak plates, we do not want to have an air gap. We are looking for direct conduction into the subfloor. Older plates or thin stamped plates work with an air gap, but with extruded Joist Traks, no air gap.

    Also, I would do as Troy suggested, separate the foam from the tubes/plates. Even a thin layer of plastic film does great. Keep the spray foam from getting in between the PEX and plates therefore not letting the heat transfer occur.

    Do I know people that have not done it? Sure do and no problems, but once it is buried, alot harder to fix.

    Dave H.
  • Gary Hayden_2
    Gary Hayden_2 Member Posts: 61
    Great Discussion on Foam and Plates - Joist Space Heating

    I myself have been following this topic and thinking about it. I agree that the air space is an important factor. And the fact that Troy has already done a job both ways with the best result being the one that has an air space is also great information. I saw the foam was used on the Brownstone remodel on "This Old House" - and I just went out on the net and tried to find the cross section picture that I saw on TV to see if they used an airspace and/or reflector in combination with the foam. Could not find the picture. Their article says they just used foam. Do not know what type they are specifying - but since they do not have an air space - they could get away with a closed cell product. But if you have an air space I think you have to consider Iceyne as the best choice because of moisture concerns. Iceyne breathes as far as I know? Maybe an Iceyne expert will show up here?

    I found a detail in the ASHRAE 90.1 User's guide that shows that it is not allowed to have the batt insulation installed with an air space under the sub-floor. Maybe a modification would be to install some plastic tubes with holes (like irrigation PE) that you use to provide recirculation air to the cavity that is conditioned? (I am thinking out loud here.)

    I am working with an architect here locally where we are ceiling a crawl space as part of the project. I have decided to seal the crawl space and condition it - dehumidify. All the insulation that is currently installed under the sub-floor I plan to reuse after we add the radiant with an airspace as there is no paperback or foil back - it breathes. The other option I see is the use of Icyene because it breathes and a perforated reflector material to maintain the air space - Maybe heavy aluminum foil would work with small holes punched in it. I am always impressed the way those Chic Fella sandwiches stay so hot! Most of the Insulators that we have interviewed to seal the crawlspace walls are using closed cell.
  • Marc_22
    Marc_22 Member Posts: 3

    spray foam is the best way to go. Icynene is the best product available. All polyurethanes 2lb foams use a hell of a lot of HCFC's as a blowing agent. Major green house gas....very irresponsible look up HCFC's and global warming..the 2lb guys claim it is good because last blowing agent ate the ozone this one lesser of evils but still a major green house gas... nothing better than water as blowing agent ICYNENE
  • that it is not allowed to have the batt insulation installed

    with an air space under the sub-floor? huh?

    if there's radiant as a staple-up installation, 2ish" of air space between it and the insulation is called for
  • troy - very good

    point and example

  • Andrew Hagen_2
    Andrew Hagen_2 Member Posts: 236

    Let me guess, these were sheet metal "plates"? Foam cannot get inside or behind tightly installed extruded aluminum heat transfer plates. People use the same name for a lot of different things.
  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,479
    I have a job

    3 years old that is extruded plates with pex. It has no air space and has been foamed. Works great. No problems. WW

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • hmmm, now that makes me wonder

    what the 'science' is between the two opposite symptoms
  • hot_rod
    hot_rod Member Posts: 21,872
    I don't see the value in an air space

    with heavy gauge direct contact transfer plates. I have 4 or 5 jobs with 3 years running with foam sprayed against the floor and plates. Spray foam makes a great sound insulation between the levels also.

    If the tube is "air hammered" into tight plates the foam should not be able to get in between the tube and transfer channel.

    Maybe a strip of painters masking tape over the tube if you are concerned about this detail?

    Yes are a real hassle to find with closed cell foam over tubes.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • troy_8
    troy_8 Member Posts: 109
    Plates and tube foamed

    Let me be clear- I don't want an air space. We only use extruded plates. But- try foaming plates and then dig an inspection hole. I will tell you that foam does get around any exposed tube(not snapped in plates) and will sneak under any small spaces above plates. I've seen it. Like Hot rod suggested, you may be able to just tape over the exposed tube. My point is don't just spray directly at the plates and tube, especially at exposed tube. It has been proven that extruded plates respond quicker when you minimize the air space. The dead air will have to be heated up when you want all the heat to transfer directly to the floor. IMHO
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    totally correct

    You want the least amount of downward heat travel for good response time. Bury the plates. I'm thinking maybe a BX clip or two at the returns to keep the PEX from lifting if the foam sneaks under it (not on the apex). I see no point of tape or plastic doodads covering the plates/tube, unless it was a fact that the foam turns aggressive and attacks the PEX.
  • aside from cementitious insulation,

    i thought all foams, including icynene, require a 15 minute fire barrier over them, like drywall, bec they all produce toxic fumes when burned.

    Not intended for exterior use. Must be covered by an approved fire barrier. Not to be installed within 2" (50 mm) of heat emitting devices.
This discussion has been closed.