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Insulation and metal roof sweating concerns for cathedral ceiling

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  • Insulation and metal roof sweating concerns for cathedral ceiling

    I am going to have cathedral rafters that are 2' deep on a 6.5/12 pitch over a great room. The rafters will get covered with a sheathing board and then 2x4 purlins on top of that. The metal will then be attached to the metal Purlins.

    For insulation I was going to pray closed cell foam to the underside of the sheathing. At least enough to make a vaper barrier. Then either open cell or fiberglass or batts to make up the R-value needed (R-49) in SE Indiana. If this is a bad idea please let me know.

    What about the metal roofing sweating.. Should I put tar paper on the sheathing board or a membrane under the metal to prevent it from routing out the sheathing board?

  • #2
    Tim, are you a builder or the homeowner? Is this an addition or new house build?

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    • #3
      Homeowner building some of this myself. New house build. I am thinking about Accuvent Catheral vent extensions so I can get Ventilation under the roof sheathing. I just don't want condensation to build up under the metal dripping on the OSB sheathing causing rot or mold issues. [FONT=Arial][COLOR=#b00000] [/COLOR][/FONT]

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      • #4
        Tim,

        The overall idea is a great one because it isolates your "roof" from the house. In that regard, the metal roofing (true standing seam presumably) won't be sweating because it's isolated. Sure, a bit hotter underneath because it's only 1.5" above something but not actually touching it.

        I have never built a vented roof like that but the idea is perfect. If I was going to do it, I would have a "sacrificial" roof under the vented metal roof. Would I trust just high temp Grace?...maybe.

        Check with some commercial roofing places and see what they have seen...and do double back here and share what you have learned.
        Portland Renovations, Inc.
        www.portlandrenovations.com

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