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Community and Q&A

Vapor Permeable Membrane and Insulated Nailbase on Roof

user-7135166 | Posted in GBA Pro Help on
We have a Cape-Cod-style late-’30’s brick house (unvented, multi-gable roof) with a finished second floor and the only roof insulation is the original between the rafters, now thin and ineffective.  Climate zone 4 (Maryland).  The house feels very chilly in the winter.  It’s time to get the asphalt shingle roof replaced, so we’d like to have someone remove the existing shingles, install 4″ insulated nailbase (this is the thickest that will fit) over the existing plank roof deck, then new shingles over that along with new fascia trim and all the other detailing that has to be done when you raise the roof 4″.  After we recover from this project :), we may or may not get dense-pack cellulose installed between the 6″ rafters, behind the plaster and in the unfinished area behind the knee walls.  I have two questions:
1.  PERMS/VAPOR BARRIERS:  I have basic drawings from an architect and an estimate from a contractor – the architect and contractor have done a similar job on another house, so we are using a similar scope.  On that job, the contractor installed Solitex Mento 3000 over the sheathing, taped the seams and edges with VANA tape, then Zip-R panels over that (with seams taped), then the underlayment and asphalt roof (I am having him install an insulated nailbase instead of Zip-R, which is not approved for roofs).  MY QUESTION:  I understand why he might want to use the Mento 3000 vapor-open air barrier on the roof deck – because vapor goes from warm to cold and we don’t want condensation to form on the inside of the roof deck but instead to dry to the outside, but the asphalt shingles are vapor impermeable, so isn’t installing a vapor-permeable membrane on the roof deck creating a risk that we’ll trap vapor between the roof deck and the shingles (i.e. in the polyiso of the nailbase)?

2.  My stepson, who’s a contractor, keeps telling me that the estimate from the contractor we have been talking to is very high, but that contractor is the only contractor I can find who’s willing do the job – the commercial roofers who work with insulated nailbase on flat roofs don’t want a job as small as ours (and may not be able to do the finish work that goes with changing the roof plane on a sloped roof); the residential roofers generally have no idea what I’m talking about; and the job is too small for the remodeling contractors we’ve spoken to.  My husband doesn’t want to go forward without another estimate.  We are in the Washington DC/Maryland/Virginia area.  ANY RECOMMENDATIONS?

Thank you!

Rebecca

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Hi Rebecca,

    I will give your post a bump.

  2. user-7135166 | | #2

    Thanks -

  3. Jon R | | #3

    > asphalt shingles are vapor impermeable

    They are more like .6 perms, which is low but not impermeable. This can make a difference (see Fig 20 here).

    > vapor-permeable membrane on the roof deck creating a risk that we’ll trap vapor

    Vapor-permeable allows things to dry. It's impermeable that traps moisture. That being said, I don't have a definitive answer for your case.

    Air seal well, preferably on the interior side or both sides. Test to confirm.

    1. user-7135166 | | #5

      Jon R. - Thanks - gotcha on the shingles.

  4. GBA Editor
    Kiley Jacques | | #4

    I think you might find this article of interest:
    Choices for Roof Underlayment Continue to Expand.
    “A paper published by Owens Corning in 2011 discussed this issue as it related to a roof clad with asphalt shingles. The authors, including Joseph Lstiburek of Building Science Corp., found that making synthetic underlayments vapor permeable ‘provides no advantage to the building performance.’ The reason is that overlapping asphalt shingles create their own vapor barrier and prevent moisture infiltration from the outside as well as vapor escape from the interior.”

  5. user-7135166 | | #6

    Thank you Kiley. Someone else pointed out to me that there's unlikely to be condensation of any vapor that reaches the underside of the shingles (through the polyiso) from the inside of the house until it meets the "top deck" of the two "roof decks" - at which point it will rot the top OSB (and that this will be visible) well before the polyiso gets wet. I think I just need to make sure to air seal everything.

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