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Vapor open or vapor closed?

pakrat1 | Posted in GBA Pro Help on

Houston tx
chainsaw retrofit 3/12 pitch rancher, longest rafters abt 15′
the stackups
roof: WRB, 4-5″ foil faced polyiso 2 staggered layers taped ,1×4 sleeper vent channel,tech shield foil down,underlayment, OC duration cool sh

what I have not been able to find is: is there any problem using a vapor closed peal and stick, would some sort of drain plane [drain wrap, etc ] under the ISO allow any wind blowen rain enters the ridge vent and gets under the foam, would it be cheap insurance to protect the foam from a leak. I have not seen any comments on this topic after reading just about every article on the web
Not keen on using vapor open peal and sticks unless some one knowlegable on the subject says my plan is flawed

same general question with the wall which has same basic config as the roof, just sub the SHs and underlay with hardie lap siding

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Replies

  1. pakrat1 | | #1

    what i ment to say was would a drain wrap under the iso allow any wind blowen rain that enters the ridge vent and gets under the foam to trickle out from under the foil faced iso. the foam will be taped but you know what DR Joe says, leaks are inevitable

  2. Jon_R | | #2

    A generic, theoretical answer: both drain wraps and permeable wraps typically promote drying of trapped moisture. Whether this drying is significant depends on how much moisture gets in and how susceptible the materials are to moisture.

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Michael.

    I may be be confused by the description of your roof assembly, because, if you have insulation above the roof sheathing, you shouldn't have a ridge vent. Exterior insulation on a roof would require the roof to be unvented. Have you read this: How to Build an Insulated Cathedral Ceiling?

    Also, these roofs are designed to dry to the interior. So a vapor-closed roof underlayment is fine. Just make sure you follow the insualtion ratios described in the article above and keep the inside of the assembly vapor-open.

    That said, in theory, yes, the water that got behind the insulation should have some drainage ability if you use a drainable underlayement. That's why many builders do exactly this on walls. Is it necessary? I don't think there is consensus on that. This article might be helpful: Wrinkled Housewrap Behind Exterior Rigid Foam.

    1. pakrat1 | | #4

      thanks brian

      detail left out
      above foam will be 1 x 4s eve to ridge [ vent channel ] with tech shield foil facing down [taking advantage of foils very low emissivity ] then normal "cool" shingle install

      1. Expert Member
        Dana Dorsett | | #5

        >"above foam will be 1 x 4s eve to ridge [ vent channel ] with tech shield foil facing down [taking advantage of foils very low emissivity ] then normal "cool" shingle install"

        That's not much of a vent channel for the foil-clad nailer deck to dry into, but since it's not the structural roof deck it may not matter (from a code compliance perspective.) Foil clad roof decking has perforations in the foil to allow the decking to dry toward the interior. Code-min deck venting for steep roofs is 1", and a 1x is only 0.75". The cool shingle roofing lowers the convective drive for moving moisture out too. Going with 2x furring would be better, especially if the roof pitch is <4:12.

        1. pakrat1 | | #6

          Thanks Dana

          Trying to avoid the fat fascia syndrome, any suggestions on 1" solution, ripped 1 to 1.125 plywood [ would the plywood hold nails better, would love to drop down to 3" width ] or is 1" dimensional lumber even available?

          Thanks again
          Mike

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