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Vapor permeable membrane underneath insultaed nailbase?

user-7135166 | Posted in General Questions 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 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?    

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  1. b_coplin | | #1

    The insulation on top of the roof deck keeps it warm enough that vapor cannot adsorb into the wood. (It dries to the inside only). Long before the insulation takes on moisture the nailbase OSB will turn to mush (It gets wet from rainwater, not interior seasonal moisture drives). Any drying to the exterior for the nailbase deck is through the shingles. There is very limited drying through the shingles.

    I assume the contractor likes the Mento membrane, and is more comfortable using it than a peel n stick. Perhaps it is less expensive. In any case, you need an air barrier below the nailbase, as they carry some risk of air leakage induced rot. Mento will provide an air barrier.

    You might be able to get a lower bid using reclaimed foam and the method Akos describes in this thread:

    All this said, I would have reservations about a contractor that has installed a product in violation of the manufacturer's instructions.

  2. user-7135166 | | #2

    Thank you, Bryan! Understand now that there will be no condensation on the exterior of the roof deck planks because the insulation will keep the planks warm. Thanks for referring me to the tongue-and-groove thread - very helpful, and I would never have found it otherwise using the search terms I was using.

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