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Vapor permeability and SRI for unvented roof?

jpbrown00 | Posted in Energy Efficiency and Durability on

Building a new home, Climate Zone 5. Roof will be Uvented. Underside of rafters will be insulated with closed cell spray foam R-49 which will serve as A/V barrier also. Roofing is asphalt shingles. Most of roof is 12:12 cathedral with 5:12 dormers, with some minor attic area that will contain A/C ductwork. Overhangs are 16″.

1.Since my roof sheathing cannot dry to the interior I am seeking a vapor permeable self-adhered product to use at the eaves where I would normally use Ice & Water Shield, 6 ft up the slope. (The field of the roof is a typical vapor permeable underlayment). I am having trouble locating such a product…suggestions?

2. Would you recommend a higher Solar Reflectance for the shingles to reduce surface temperature of roofing & sheathing? (The best I can get for the aesthetic I want is SRI 28 so not terribly high)

Thank you

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  1. jpbrown00 | | #1

    Sorry...typo -it should say Roof will be UNVENTED.

  2. Stockwell | | #2

    I have previously seen answers here that state it is ok to use an impermeable product on both sides of your roof sheathing as long as the sheathing is dry when you do it.

  3. jpbrown00 | | #3

    Thanks and I should add that the roof of this house is being built in a factory (custom modular) and will arrive on site shingled. So I am confident that the sheathing will be dry. I guess I am concerned about inward solar moisture drive and potential leaks. So I am looking for a product that is waterproof (to larger water molecules) but vapor permeable. Has anyone used Slopeshield SA?

  4. Expert Member
    Michael Maines | | #4

    Julie, I have not used Slopeshield, but have used Pro Clima Solitex Mento 1000 and it is good for your situation. Siga Majcoat, or Siga Majvest SA are also good options. But like Kevin said, with an impermeable roof surface and impermeable insulation below, there is no reason to use a permeable roofing underlayment. If anything I would err on the side of the highest water resistance, and possibly consider a self-healing membrane if you are concerned about leaks.

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