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Peel and Stick Options

John Nooncaster | Posted in General Questions on

Looking for product for covering unvented roof assembly.

Should I use Grace Ice & Water Shield or an alternative product?

Thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    John,
    Unless there is something unusual about your roof assembly, I'd stick with 30-pound asphalt felt.

    In most applications, the use of Ice & Water Shield should be restricted to just eaves and valleys.

  2. Chris Koehn | | #2

    Installing peel-n-stick over the entire roof has the potential for trapping migrating water vapour underneath the membrane. Moisture migration (due to vapour pressure) would have to be non-existent, which is difficult to achieve. The colder your climate the more this is true.

  3. John Brooks | | #3
  4. John Nooncaster | | #4

    http://www.buildingscience.com/documents/reports/rr-1006-ba-high-r-roofs-case-study-analysis

    "To meet durability goals in most applications, the airtightness must be provided by a continuous membrane—preferably adhered or sandwiched—on the exterior of the framing. In designs where the air tightness is provided between framing elements, spray foam has been found to be a practical solution to the challenge of providing this airtightness."

    Then add this--
    "However, all wood‐to‐wood joints in the framing must still be solved with sealants or other means."

    R38 of open cell in Zone 4

    I worry above the moisture drive from the asphalt shingles...

    What about taping the sheathing joints?

  5. GBA Editor
    Martin Holladay | | #5

    John,
    Air tightness and vapor tightness are two different matters. It you are installing R-38 of open-cell spray foam, you shouldn't have to worry about air tightness unless you have a very unskilled installer.

    As long as you're not installing interior poly -- and you shouldn't be -- there is no reason for you to need a vapor barrier on top of your roof sheathing. If your roof assembly experiences inward solar vapor drive through asphalt shingles -- a phenomenon about which researchers are still arguing -- the roof assembly should be able to dry to the interior.

    All of that said, if you really want a vapor-impermeable roofing underlayment, you don't have to use Ice & Water Shield, which is expensive. You can use Tri-Flex 30.

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