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Vapor Permeability of Roof Membrane

4khz | Posted in General Questions on

Owner Builder; CZ 5 (lower NY State – not near the coast); roof mostly shaded by trees in summer.

I was originally going with corrugated roofing. However, this now won’t happen based on multiple factors… and about $20K less to go with asphalt shingles.

My 1st question: Am I good with the Adhero underlayment with a perm rating of 9 directly under the shingles? I read a building science.org article/study that doesn’t recommend a vapor permeable underlayment in a hot & humid climate because of too much moisture migrating into the sheathing. NY is not Florida, but this makes me concerned enough. Shingles are about .6 perm

2nd question: with the highly permeable underlayment, can moisture that gets into upper tier plywood sheathing dry to the interior  ?

My roof
Unvented Roof: 5 1/2″ mineral wool, 1/2″ plywood sheathing, mento 1000 vapor permeable (perm: 38?) air barrier; 5 1/2″ polyiso (3 layers staggered seams), 1/2″ CDX; Adhero (Perm: 9) self adhered membrane; black asphalt shingles

 

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Replies

  1. user-1072251 | | #1

    Assuming the polyiso is aluminum faced, it is a vapor barrier, so no, nothing above it will dry to the interior. (or the exterior asphalt) Seems like the lower tier with mineral wool, plywood sheathing and a permeable membrane could allow moisture to get to the polyiso, except that most moisture travels on air leaks which should be minimal with the Mento.
    I've given up on hot roofs, since venting is pretty basic and avoids the questions.
    Also, if you are thinking about corrugated metal, it will probably last 3X as long as asphalt, at which point your savings will have long disappeared. I made the same decision with one side of my barn roof fifteen years ago, and have regretted it since; just did the other side with standing seam, which will long outlive me.

    1. matt2021 | | #2

      Can I chime in with two quick questions regarding metal roofs? Given what you say about longevity:

      - Does what you say apply to standing seam metal roofs that are quite thin as well? (I might be looking at the Junior HF panels by Ideal Roofing, which are only gauge 29.)

      - Are metal roofs (especially when quite thin) prone to damage, if nothing else cosmetic, when needs to walk on them, say, for a repair or to check something?

      (Sorry, 4khz, for asking these questions here! I am not trying to highjack the discussion in response to your post.)

      1. Expert Member
        MALCOLM TAYLOR | | #3

        matt2021,

        Metal roofs mainly rely on their coatings for their longevity. They typically start with a galvalum substrate, and then have various grades of coating applied. If your roofing panels start deteriorating due to rust or corrosion, the difference between a 29 and a 24 or 26 gauge won't save them.

        One of the reasons I prefer to install metal roofing directly to the sheathing is it helps share the structural loads, both of workers and any impacts from debris, making the gauge of the panels less important.

        All that said, 29 gauge is pretty thin.

        1. matt2021 | | #4

          Malcolm, thank you very much for the detailed and very clear explanation. It all makes perfect sense. (I was attracted to those panels also because they seem pretty easy to install; I would not be doing it myself, yet the contractors I have encountered so far, while comfortable with asphalt shingles roofs, have not been open to installing a metal roof for me; and that's really too bad, as we are talking about a small -- 24'x15' -- low slope roof, covering what is currently a porch.)

          1. Expert Member
            MALCOLM TAYLOR | | #5

            Matt,

            I install my own metal roofs, and just did a (fairly simple) 1100 sf one for under 7K including labor.

            The materials aren't that expensive, and they go on quickly.

            There is something odd about metal roof pricing, which may be as simple as there not being many roofers who do the work, driving prices up?

      2. user-1072251 | | #9

        Re: standing on a metal roof, you can hang a ladder from the ridge and walk on that. If it’s a standing seam roof, you can rent staging brackets that clamp to the seams. Re: life of a metal roof: I’ve seen lots of old barns with rusty metal roofs - they’re usually dry inside; , I’ve never seen one that has rotted through.

    2. 4khz | | #6

      user-107xxx - the polyiso isn't foil faced. I believe it is GRF. I guess my question is whether the vapor permeable underlayment is a bad choice.

      1. Expert Member
        Akos | | #7

        My reading is that with asphalt shingles it doesn't matter. Installaing a permeable underlayment doesn't hurt but also doesn't help much since the overall stack up is very low perm anyways. For metal roofs with outsolation, I like to use a permeable underlayment since there is more drying capacity.

        This research was brought to you by people that don't make permeable underlayment but was done by Joe L. who is a very respected building scientist:

        https://roofingca.owenscorning.com/docs/whitepapers/DeckDefenseWhitePaper.pdf

        Another reference point, most SIP manufacturers require felt (permeable) under shingles on a roof.

        1. 4khz | | #10

          Akos - thanks for the info and link. That's the consensus I'm seeing... that it doesn't matter... except in hot humid climates (per BSC study) and very wet regions, as Malcolm pointed out in the RDH study.

      2. Expert Member
        MALCOLM TAYLOR | | #8

        4khz,

        When RDH looked at it they found there was some small amount of drying through asphalt shingles with permeable underlayment. In the context of the wet PNW climate where they were doing the research, they felt that was outweighed by the greater chance of bulk water intrusion from above, and recommended using an impermeable one. So probably fair to say the answer depends on the climate.

        1. 4khz | | #11

          Malcolm,

          Thanks for the RDH reference. A lot of good info/studies on that site. The only other reference I could find was the BSC Study which recommended a vapor impermeable membrane in Hot Humid climates.

          In response to your comment about roofing contractors & metal roofs: I was surprised by the difficulty with even getting a quote. I contacted 20 + roofers (some from references) and only one would quote the corrugated and the quote was much higher than expected. In speaking with several roofers, they all said that they were very busy and preferred not to install corrugated because they felt it was too risky and less profitable. Most preferred to install asphalt shingled day in and day out

          1. Expert Member
            Akos | | #12

            You can also try to find an installer that does metal tiles. Some of them are very simple install, can be nailed up with a roofing nailer. These tend to be much cheaper than panels and an easier install for someone used to shingles.

            The problem with metal roof install is that it is a skilled trade. It takes a fair bit of experience and knowledge to do a quality install. For a simple roof shape (ie gabled with no dormers) a metal roof install is actually much quicker than shingles.

            The issue is that lot of homes have many architectural details, even simple ones can quickly add a lot of time when working in metal. As an example, I was siding with snap lock a simple flat wall. It took about the same time to side the whole 35'x12' section as it was to properly trim around two vent openings.

            P.S. You don't want corrugated or any exposed fastener on a roof, these are best left for barns and siding. All those fasteners are potential leaks and some WILL leak down the road. A snap lock is more expensive but quicker install and maintenance free.

  2. 4khz | | #13

    AKOS - unfortunately, I ran out of time... needed to get a roof on with this small window of decent weather in hudson valley, NY. Corrugated was a based on aesthetics + cost.. at least originally. I didn't realize that corrugated would be such a high labor cost considering my simple gabled roof. I am comfortable with corrugated especially with the vapor open membrane and venting between the metal and the membrane. But I now have a asphalt shingled roof- which doesn't look too bad imo. see attachement

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