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Cathedral-type roof assembly

JTuck | Posted in General Questions on

I’m building a new construction 2700 sqft bungalow in NC zone 4 but just a couple miles from zone 3.  The house has a hip roof with a dormer on all 4 sides. Pretty much the entire second floor is an unvented cathedral type ceiling/roof assembly. My plan is to spray a couple inches of closed cell to the underside of the deck with batts under that to fill the space between the foam and drywall.

Then on top I plan on placing roof underlayment and light-colored metal roof. Do I need an air gap between the metal and underlayment? If so, it will not ventilate at the top with the hip roof and all the dormers. Is there any risk of condensation under the metal? I’ve heard it mentioned a couple of times but nothing definitive.

Thanks everyone!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    J. Tuck,
    You are planning an unvented roof assembly using the flash-and-batt method. For more information on the approach you are planning, see "Flash-and-Batt Insulation."

    In Climate Zone 4, the code mandates that the minimum R-value of the closed-cell spray foam that you plan to install is R-15 for roof assemblies. "A couple of inches" of spray foam isn't enough. Install 2.5 inches or 3 inches and you'll be safe.

    You don't need an air space between the metal roofing and the roofing underlayment unless it is required by the metal roofing manufacturer.

  2. Expert Member
    Peter Yost | | #2

    HI JTuck -

    can any roof assembly benefit (greater drying potential and free-draining space between the cladding and the rest of the roof) from this vent space? Yes.

    Is it required by code? No. Do you "need" the vent space? No, but we consider ventilated rainscreens best practice for wall assemblies where there is more than 20 inches of precipitation a year, why not roofs?

    Peter

  3. JTuck | | #3

    You guys are awesome. Thank you!

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