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How do you create an unvented attic, i.e. bring the attic into the conditioned space?

tignarius | Posted in GBA Pro Help on

The 2009 IRC says, in part, that an unvented attic requires R15 worth of Class 1 vapor retarder insulation applied directly to the bottom side of the structural sheathing. It was my intention to use 2.5″ of polyisocyanurate against the sheathing, which in this case is 1×6 planks, and fill the rest of the 2×10 cavities with BIB cellulose. The project is in Climate Zone 4, there is 1/2″ drywall on the ceiling joists and loose fill fiberglass in the ceiling joist spaces. There are 10 recessed lights, a whole house fan we intend to remove and patch, a pull down stair and an air handler with 10″ flex duct. I was considering not air sealing the ceiling as the space will now be unintentionally conditioned. Is not air sealing still OK and does the sheathing type affect the impermeable insulation install?

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  1. kevin_in_denver | | #1

    Your'e probably OK to neglect the ceiling plane. Be sure to find and seal all the soffit and ridge vents.

  2. GBA Editor
    Martin Holladay | | #2

    Q. "Does the sheathing type affect the impermeable insulation install?"

    A. No. But make sure that each piece of polyiso is installed in an airtight manner, using caulk or canned foam at the perimeter.

  3. tignarius | | #3

    Thank you.

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