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Spray Foam

2001 Member Posts: 1
Hello,  I have a system that is suspended under the sub-floor with staples.  Then the tubing was covered in a 2' strip of plastic so the foam didn't touch.  That was just a precaution wasn't sure if the foam chemicals would react, so that's just a barrier.  Then the spray foam was installed.  These are scissor trusses.  Two zones and the floor is heating great.  Just wondering about efficiency and if the system will function as it should.  Heating with a A.O. Smith Vertex LP.  I've had a person look at it and they said you have to have a 2" air gap and the aluminium track.  Is that correct?  Thanks for your help.


  • STEVEusaPA
    STEVEusaPA Member Posts: 5,189
    Yes and no...

    You have a staple up radiant system. This is deemed by most people here as not the best way to do radiant, but it will work.

    Aluminum plates would have been a better job, giving you better contact and better heat transfer. It just means that you're going to have to run hotter water through the system.

    If it was designed properly, and its working the way it is, then your ok.
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,189
    edited April 2012
    Yes and no...

    Sorry, double post.
  • kcopp
    kcopp Member Posts: 3,784
    you only need....

    the 2" space if you don't use plates.... If you use plates DON"T leave an air space. A lot of people confuse that.
  • bugaboo
    bugaboo Member Posts: 14
    Air gap

    I would like to learn more about the why, if possible.

    Why should there not be an air gap if plates are installed?

    Does it matter what kind of foam is used? 1/2 lb vs 2 lb? The 1/2 lb is a little soft, and about R4 per inch. The 2 lb is relatively solid, and about R7 per inch.
  • SWEI
    SWEI Member Posts: 7,356
    air is an insulator

    So the heat in the pipes has to transition from conduction to radiation to conduction to radiation.

    If you use plates, and fully insulate behind them, there is only a single transition from conduction to radiation.
  • bugaboo
    bugaboo Member Posts: 14
    conduction vs. radiation

    Okay, so I understand that radiation is much more efficient than convection.

    I understand that warming up the air in an additional area takes extra energy., which could slow the response.

    Where I am not following completely is what the real downside is. Under a wood floor, with plates installed, there are areas where there is direct contact, and there are areas where there is no contact. With spray foam you will have no heat transfer other than conduction in the wood. Now, we obviously have some conduction, because the heat makes it from the plates to the living area, but I ask how much lateral conduction there will be in the wood flooring.

    If we leave an air gap then we have direct conduction up where the plates are, plus convection between the plates. In my mind this means we will have a more even floor temperature.

    I believe that if the insulation beneath is the same the average floor temp should be the same, for the same BTU input.

    Again, the response will be slower, as the air must warn up, and then transfer the heat up into the wood above, but I think that's the only drawback.

    Does this sound about right, ohave I got something completely wrong? Does anyone know if there is any significant difference between the heat that would have to conduct laterally through the wood between the plates and the heat that would come up through air convection?
  • Gordy
    Gordy Member Posts: 9,514
    Working great

    I have to ask if the system is working great, was this at your design temp or below? Last year was a mild winter for most of the country.

    If spray foam was applied, and encompassed the tubing between the floor, and the tube you may have issues, at colder outside temps.

    The job of a plate is to give direct transfer of heat from the tube to the flooring. Suspended plateless needs the air gap to reduce stripping , and use the air convection to spread the heat more evenly over the floor. The plate would have done this, and allowed a lower water temp to be used.
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