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Vapor barrier under polyiso on V-joint pine boards (and more)

user-228058 | Posted in Energy Efficiency and Durability on

I am moving forward with converting my timber frame barn into a home. North Connecticut zone 5.
I am planning on doing a build up roof of sandwiched foam polyiso and a layer of eps on top followed by a vent channel of 2x4s fastened down with headlock screws or such, then plywood and roofing. My question is , as I would be installing the layers of foam over the v joint pine what should I use for a air barrier? I imagine I could do a variable vapor product like intello. However as the rigid is impermeable to begin with could I simply go with regular plastic? If I stop the air flow ,it seems that the foam would be quite unlikely to take on moisture even during summer with ac. Am I missing something science wise?

Next I want to do some areas of the insulation on the walls as outboard Larsen type trusses , however in some areas( where it would be asthetically challenging) It makes more sense to do a combination of infill and outfill if you will so I would have 3.5 inches between the timber posts and 5.5″ outside the timbers . I will be insulating the walls with dense pack cellulose. ( I will need to Sheetrock then pack the inside and outside cavities, separately ) I am a bit hesitant with the putting interior vapor barrier fully inside as it would start and stop by taping to the timbers. So Can I just put the vapor barrier in between the stud walls , (3.5 inside air barrier 5.5 outside )being that it will be a perm variable product like intello, will the somewhat more outboard then ideal work for the location of the vapor retarder ?? Planning on doing a sheathingless wall with a layer of the proclima mento followed by rain screen and pine novelty siding. Any other ideas?
Thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Scott,
    Your plan for insulating the roof requires a high-quality interior air barrier (not a vapor barrier). What product to use depends on your budget; choices range from Grace Ice & Water Shield on the exterior side of your existing board roof sheathing, to synthetic roofing underlayment (carefully taped), to one of the European air barrier membranes sold by Small Planet Workshop of 475 High Performance Building Products, to taped Zip sheathing.

    If you are very conscientious about your taping, you could consider creating an air barrier by taping at least two layers of your rigid foam. I wouldn't do this (because there have been reports of air leakage through foam seams when this approach is used, and because there have been reports of foam shrinkage).

    Polyethylene would work, in theory, as an air barrier except for two important issues: its use on a roof is dangerous for the installers (because it is slippery), and it is so fragile that it is almost impossible to install without flaws.

    For more information on this issue, see How to Install Rigid Foam On Top of Roof Sheathing.

    You don't need an interior vapor barrier for your walls; all you need is one or more air barriers. Codes usually require an interior vapor retarder -- a less stringent layer than a vapor barrier -- but this requirement can be met with the use of vapor retarder paint. For more information on this issue, see these two articles:

    Vapor Retarders and Vapor Barriers

    Do I Need a Vapor Retarder?

  2. user-228058 | | #2

    Thank you Martin.
    For the walls my concern was that the air barrier would act as a vapor retarder ,to far out in the assembly. Meaning it would be better to have more insulation out board. What could I use that would act as a air barrier but fully vapor open? Like a woven type of house wrap?

  3. GBA Editor
    Martin Holladay | | #3

    Scott,
    If you want to use Intello membrane as an air barrier, you can safely place the membrane as you suggest. (Intello is not a vapor barrier.)

    But it still sounds as if you are thinking too much about vapor diffusion, which doesn't matter much. If you focus on air sealing, you'll usually be fine. You can let go of your worries about vapor diffusion.

  4. Matt_Michaud | | #4

    So if Scott is going sheathingless (so taped OSB and plywood are out), smart and vapor-open membranes have been mentioned (Intello and Mento), and PE is fragile, what new between-walls air barrier option does he have - accepting that he needn't worry about vapor diffusion? Scrim reinforced PE?

  5. GBA Editor
    Martin Holladay | | #5

    Matthew,
    Scott's plan to use Intello is fine, as I already mentioned. Of course, the air barrier doesn't have to be in the center of the wall; it can be on the interior side of the wall (usually taped drywall) or the exterior side of the wall (in Scott's case, carefully installed Pro Clima Mento), or both.

    Many builders with thick double-stud walls use taped OSB between the two stud walls (in the center of the wall assembly) as an air barrier and vapor retarder -- but if Scott wants to avoid the use of OSB, he certainly can.

  6. user-228058 | | #6

    Thanks guys, I think I have a clearer picture of the walls. Maybe I am dense but if i am layering the foam on top of vjoint (roof)I still am missing what a good choice for a air barrier is? This barrier in my mind would be stapled to the top of roof deck and taped prior to the layers of rigid. Planning how to work on it is a issue, as you mentioned Martin, my plan would be to install the barrier top down.. i.e. Roll out a 4 ft wide or so roll ( parallel to the ridge) stapling well , then moving down installing the next rol( slip it under the above)l and reaching up taping the seam, then repeat. Then installing the foam from the bottom up. So there would be no standing or staging on the air barrier if you will. So being this is simply a air barrier could I use synthetic tar paper, house wrap, or such? Seems this step would be eliminated with taped sheathing or such.
    Thanks again, while I may seem nutty, planning these details prior to stripping and hanging out up on the roof seem prudent. :)

  7. GBA Editor
    Martin Holladay | | #7

    Scott,
    I think you should re-read my first response. I listed several options. The more robust air barriers cost more than the thinner, more fragile air barriers -- so your choice will depend on your budget and your goals.

  8. user-228058 | | #8

    Oops... yes you did.

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