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Siegenthaler-style radiant ceiling below unvented roof

NICK KEENAN | Posted in General Questions on

I’m considering a radiant ceiling of the type that John Siegenthaler recommends, as in this article:

Basically he covers the ceiling with a layer of OSB, then covers that with a layer of foil-faced polyiso with aluminum PEX transfer plates glued on with contact cement. 

I’m trying to think through whether this is going to cause problems below an unvented roof. The assembly is going to be essentially vapor-tight, between the faced polyiso and the aluminum plates. I’m worried that with an unvented roof it’s going to need to dry to the interior, and the assembly is going to block that. Is my worry unfounded?

The roof has 4″ of external polyiso insulation and 12″ of batt insulation. Climate zone is 4A, Washington, DC.

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  1. Jon_R | | #1

    IRC R806.5:

    "2. No interior Class I vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly or on the ceiling side of the unvented enclosed roof framing assembly"

    Same for walls in 4A.

    Wallboard instead of foam + reasonably sized gaps between plates should provide sufficient inward drying. Make sure the interior side is a smart Class II or Class III (paint may not be enough).

    1. Expert Member
      NICK KEENAN | | #2

      You're confirming what I was thinking. The foam isn't really there for insulation, it's to provide support and spacing. (And with sheet goods now pushing $100 a sheet, the fact that polyiso has stayed around $18 is a consideration too). I was thinking maybe homasote instead. Or even strips of 1x3.

      What I'm having a little more trouble with is what are "reasonable sized gaps between plates." The aluminum is completely impervious.

  2. Expert Member
    Akos | | #3

    Putting fluffy insulation between two impermeable layers is asking for trouble.

    I would see if you can leave enough area around the perimeter covered with drywall only to allow for some moisture movement or go for permeable foam and don't cover the whole surface with aluminum.

  3. Expert Member
    NICK KEENAN | | #4

    So it seems to me I want my air sealing layer above the radiant tubing. Siegenthaler uses OSB for that but I'd like to maximize permeability. What's a high-perm air sealer? Drywall? Anything lighter?

  4. Expert Member
    NICK KEENAN | | #5

    My latest thinking is to put a layer of Tyvek between the insulation batt and the radiant tubing, to act as an air barrier but allow moisture to flow down. Along the ceiling joists I'll put 7" strips of 1x3 on 8" centers and the aluminum transfer plates will be screwed to them. That will leave gaps between the transfer plates where moisture can flow and out through the drywall.

  5. Expert Member
    NICK KEENAN | | #6

    We ended up just using open cell foam for the insulation.

  6. Scott_Upstate | | #7

    Hi Nick, Any chance we can connect re: Siegenthaler-style ceiling radiant ceiling? I'm hoping to install one using his method in a residence that's currently being designed and I'm also Zone 5-ish -- Upstate NY.

    1. Deleted | | #8


  7. Scott_Upstate | | #9

    Ah -- you replied to my other thread. Thank you. I'll be in touch soon.

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