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Community and Q&A

Attic Floor / Rafter Insulation and Roof Venting

lou94stang | Posted in Green Building Techniques on

I am building a new construction 2 story + walk up attic.  I am In NY Zone 4, if I finish the attic during building phase I would have to install fire sprinklers and a separate water meter 🤦‍♂️ plus my taxes would be around a extra 5k a year.

My approved plans call for R38 Batts in attic floor joist R21 in the walls and R21 in my unfinished basement ceiling.  I can not install any insulation in the rafters now because they will consider it finished area forcing me to sprinkler the house.

Later on if I decide to add R38 Batts in my 2×12 Rafters and also add a vent from my central air to condition the space . Do I still need to vent the roof being that the attic floor joist are now insulated ?
Spray foam is out of the budget I got a quote recently for 41k so that’s out of the question. I  read the Cool hand Luke and other article by Joe Lstiburek already but still confused.

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  1. Expert Member


    Your roof will need venting both before you change where the insulation is and after.

    During the first stage, when the attic is not conditioned, you will need to vent the attic, and include an air-barrier at the attic floor.

    When you move the boundary between the inside and out to the roof line, you will need to vent each rafter bay at the eaves and peak. You will also need to provide an interior air-barrier under the rafters. Having insulation on the attic floor doesn't affect this one way or another.

    1. lou94stang | | #2


      When you say air barrier for attic floor, would paper faced batts with paper to the interior suffice or maybe some 6mil plastic taped to ceiling of 2nd story. Or is there another product I should be looking into.

      1. Expert Member
        MALCOLM TAYLOR | | #3


        The best option may depend on what you are using for an air-barrier elsewhere. What is your air-barrier on the walls? Paper faced batts aren't sufficient. The most common approaches are to use either poly or a variable-perm membrane in the underside of the attic floor framing, or detail the ceiling drywall to be airtight. Sealing penetrations is important. Your biggest challenge will probably be the access stair.

        1. Deleted | | #4


        2. lou94stang | | #5

          Using Zip System for my walls and roof. My access stair case will have a fire rated door at top of landing into the attic space so should be able to get a good seal there. With the zip and paper back R38 should that be sufficient? Or am I better of with poly and taping off all penetrations through the poly?

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