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Correcting a thermal bridge

Nate Weiner | Posted in Energy Efficiency and Durability on

I’m building a house in zone 6a, Central NH. My plan is to wrap the house with 2 layers of eps totaling 4.5 inches with the joints staggered. The framing is 2×4 and no interior vapor barrier will be used. I was planning to add 3.5″ of fiberglass to the cavities.I’m using DuPont drainwrap over OSB with the joints and stud lines taped with zip tape. The foam will be furred vertically with 3/4″ boards to ventilate behind the siding.
The roof will receive the same 4.5″ of foam and is 5/8″ Advantech with Tamko Moisture Guard between the foam and the Advantech. The roof will be re-sheathed with plywood and papered with metal roofing over. The rafter bays will be filled with fiberglass batts with no vapor barrier.
The foam I’m using has a permeance of 4.95.
My question regards the overhangs which are to be installed directly to the first rafter through sheathing and wrapped on both sides with foam but will contact the upper layer of roof sheathing. I think I’m creating a thermal bridge and may be in danger of moisture problems at the top of the wall.
An architect friend has suggested flashing the interior of the house with spray foam assuring me that there will be no dew point issues. I’m not too concerned about the wall as I believe the Drainwrap (36-50 perms depending on test method) and ventilation behind the siding will allow the wall to dry outward if it is necessary but I’m concerned about the roof as the Tamko product isn’t vapor permeable (.001 perms).
Naturally the house will have an HRV installed.
Sorry for the lengthy post and thanks in advance.

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Replies

  1. TJ Elder | | #1

    Nate,

    When you say the foam has a "permeance of 4.95", is that at the intended 4.5" thickness? I would guess that's for one inch, and it's a fraction of that for several inches. Basically the walls need to dry toward the interior, same as the roof.

    As for thermal bridging at the roof/wall transition, it's not really feasible to have overhangs without some degree of thermal bridging. You may note a lot of Passivhaus projects do not have overhangs at all. If you could post some drawings, your audience here could more easily evaluate whether thermal bridging is problematic with your design.

  2. John Klingel | | #2

    I'd be curious to see a drawing too; still learning. On this "The rafter bays will be filled with fiberglass batts with no vapor barrier", if I am not mistaken, fg is the worst batt insulation there is, esp if exposed in rafters. Apparently it is not dense enough to prevent inevitable air currents from robbing btu's. Roxul instead? Wool? Cotton? Cellulose?

  3. GBA Editor
    Martin Holladay | | #3

    Nate,
    If your concern is that the roof underlayment isn't vapor permeable, you can just use asphalt felt (asphalt felt is a smart vapor retarder, with a permeance of 5 perms when dry and 60 perms when wet) or one of the new vapor-permeable synthetic roofing underlayments (which behave like asphalt felt but cost four or five times as much).

    Here is more information on roofing underlayments: Synthetic Roofing Underlayments

  4. Nate Weiner | | #4

    Martin,
    The Tamko product is already installed. I'd hate to have to go backwards at this point.

    I'm considering building the overhangs as suggested here...
    http://www.buildingscience.com/documents/insights/bsi-056-leiningen-versus-the-ants-redux/

  5. Nate Weiner | | #5

    @ TJ Elder After looking at the literature you're right about the permeance of the foam. Drawings are not my forte. We've mocked up a bit of what we planned on doing at one corner of the mechanical room.
    The photos I have are too large to upload. I'll take one of the proper size and attach it.

    edit...the attached photo shows the curb style o/hangs I had intended on building. The piece attached to the building is a 2x10. It will be surrounded by foam on three sides, but as I said the top will contact the second layer of sheathing creating a bridge.

    @John...I feel the same about fiberglass in typical installations, but with an airtight drywall approach I feel it will perform just fine. Should there ever be a need to go back in to this assembly I don't want to deal with anything loose. I may reconsider. It isn't set in stone.

  6. Clement Edgar III | | #6

    I am kind of confused about the foam on the roof. If the attic is vented and cold, then foam on the roof is useless. If the attic is sealed and warm, then won't there be moisture problems in the foam as warm wet air condenses in the foam when it gets cold. If you put a vapor barrier between the foam and the fiberglass, then there will be a moisture trap. I am sure there is a way to make this work, but I think you really need to think about where the moisture is going in each layer of the envelope.

  7. Nate Weiner | | #7

    Clement,
    The attic is warm.
    I've thought this out for a long time. I'm seeking advice on thermal bridging from the o/hangs, not the basic design.
    It was suggested I add a layer of spray foam on the interior to solve the thermal bridging of the o/hangs.
    I'm looking for information to support or refute that idea.
    In other words, if I have an impermeable layer outside the sheathing with foam on top will it cause an issue if I add foam below that sheathing?

  8. TJ Elder | | #8

    Nate,

    My impression is that thermal bridging will not likely be a problem, with only a convoluted path for heat through the narrow dimension of these rafters. I would recommend against adding impermeable foam at the interior, so you retain the ability to dry the assembly toward the interior.

  9. Aj Builder, Upstate NY Zone 6a | | #9

    Head scratcher.

    Why frame a home in the dead of winter? There is no excuse that will fly.

    The building is complicated and the plans are being second guessed now midstream and in weather that is just wrong. Why? Whatever the reason, not valid.

    Submit all the details for your build here and maybe someone can see what to recommend.

    Better yet, someone right now is responsible for plans you have now. Get with them and get your solution as you should have done before the first order of lumber.

    OK... we all have done what you have done... but... really, it is no fun shooting from the hip at this stage of a build. Try not to repeat such!

    Good luck,,, I for one can not understand your build without being there and having your plans and designer and insulation contractor with me for a long lunch meeting.

    Nate, around my parts a vented space below the roofing is the only trusted way to build over ridged foam applied as you are on your roof. And I have witnessed some amazing rot when not done though they had a very permeable T&G ceiling broken by false beams.

    As another recent thread noted, your build is trapping moisture in it as you build that will have to get out.

  10. Nate Weiner | | #10

    Thanks TJ

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