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low temp radiant ceiling panels

user-3697136 | Posted in Green Building Techniques on

We are installing a air to water heat pump in a very well insulated renovation and are hoping to put radiant  panels on ceilings where floor and wall radiation are not desired.  

The design model from Idronics which recommends this application for 110-120 degree water has from the top down (see attachment):
-insulation between ceiling joists
– 1/2″ OSB over ceiling joists
-1/2′ pex tubing on 8″ OC edged by 3/4″ poly-iso
-6″ aluminum heat transfer plates around pex and over the 3/4′ poly-iso
– 1/2″ drywall ceiling

We have two recommendations not to use rediant ceiling panels for low temp hydrondic heating and would like some other opinions about their efficacy.
We are also wondering in the above profile if the poly-iso is necessary if the infill insulation between joists is R-60.

Thank you to all insights!
Phillip Mulligan
Chelsea VT

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  1. charlie_sullivan | | #1

    I haven't personally used them, but I've only heard good things about them. I would be skeptical of anyone saying that a specific temperature is needed. The heat output scales smoothly with the water temperature. You should be able to calculate what water temperature you need given the heat load of the space and the specs of the panel system. Are you having trouble finding data for that?

  2. user-3697136 | | #2

    no, I have just had comments that the heating won't be as effective because in fact a percentage of it is convention and won't readily reach the floor. There is a coeffiecent that takes in it's placement w/ a formula of q=.71(water temp-room temp) q=BTU/sq'/hr.
    I have 8'3" high ceilings, wondering if height plays a part of this. Floors get covered, walls are busy, I really want to go w/ the ceiling if it will make a comfortable room.
    thanks for your reply.

    1. Expert Member
      NICK KEENAN | | #5

      The ceiling will be less effective than the floor, in that at the same surface temperature about 70% as much heat is released. That is true. However, two caveats: first, that heat is not lost, it just isn't released. So there is no efficiency difference. Second, the relationship between the surface temperature and the water temperature depends upon the material between the surface and the piping. Ceilings tend to have less insulation than floors, because they are thinner and drywall is a better conductor than most things that floors are made out of. Ceilings tend to be more consistent in their insulation, people don't add throw rugs or furniture to ceilings.

  3. charlie_sullivan | | #3

    It's true that the convection from a floor means you get more heat for the same temperature from a ceiling compared to a floor. But that is easily offset by the fact that you don't have carpeting or furniture on the ceiling (usually). And in a well insulated building you don't need that much heat.

    8' 3" ceilings only matters in that you have more wall area for heat loss and perhaps window area. So the thing to do is to run a load calculation to find out how much heat you need to deliver and then use your formula to figure out what water temperature you need.

    When you do that load calculation, for the heat output from the ceiling, you can neglect heat loss through the ceiling. But when you size the boiler or calculation annual energy use, you should include it. If you want to be super precise you can include the fact that you'll be keeping the ceiling extra warm in that calculation.

  4. Expert Member
    NICK KEENAN | | #4

    That construction is exactly what John Seigenthaler recommends in this article:

  5. Expert Member
    NICK KEENAN | | #6

    I believe that the polyiso is there not necessarily to provide insulation, but because a 3/4" spacer is needed and at around fifteen bucks a sheet it's pretty much the cheapest thing available. It's also light, which is nice when you're working overhead.

    The way Seigenthaler cuts it into strips I don't think it would provide much insulation, there's no insulation where the pipe is hottest.

    1. Expert Member
      NICK KEENAN | | #8

      I imagine the polyiso is also pretty quiet, hydronic systems can be noisy when the pipes expand and contract.

  6. BirchwoodBill | | #7

    The water temperatures sound high. In zone 6 and have Warmboard under the hardwood floor and keep the water between 85 to 103f. Warmboard is AL PEX with aluminum covering on plywood.

  7. radiantfan | | #9

    I would like to see the thermal image from the attic side. Since all the materials are touching you are getting substantial amount of heat transfer via conduction. A radiant barrier above the coils would direct +95% of the energy downward. But radiant barriers only work in free space, once the materials touch the foil it ceases to function as intented.
    Try placing foil (or foil faced ridgid insulation) directly on OSB, then strap with 1/2 -3/4" lath and then install ceiling.
    Your concern for the expansion gap is 'bang on'. My coworker removed an entire staple up system because of all the creaking and squeaks as the tubing moved on the sub floor. Drove the family crazy during heating season.

    1. Expert Member
      DCcontrarian | | #10

      Is there any reason to believe that conduction through framing members would be any higher than in any other heat distribution system? Certainly it's a lot better than ductwork in unconditioned space.

      1. Deleted | | #11


      2. Expert Member
        Deleted | | #12


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