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Community and Q&A

Cape Cod roof ventilation

TIMGABRIEL | Posted in General Questions on

I have a 2007 heckaman modular cape cod home.  Cold in winter hot in summer.  The roof rafters are 24” on center.  Inside the knee wall cavity, every third rafter opening by soffit had a vent chute while all others were jammed with fiberglass insulation.  Along the top portion (where top of stud and lower portion of cathedral section of roof) of knee wall it looked as though those cavities were filled fiberglass insulation.  Along the kneewall stud spaces (24 IOC) had r19 paper faced toward drywall and heavy plastic stapled to back of studs.  Is all this correct?  I have not investigated the ceiling portion of roof.  What fix if any would be recommended?  Thank you for any help.  Tim

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  1. Expert Member
    BILL WICHERS | | #1

    I’m hoping I’m understanding your situation from what you’ve said. It’s difficult without a few pictures. Sometimes a picture is worth WAY more than a thousand words :-)

    Do you have a ridge vent? If yes, and you probably should have one, there needs to be vent space above the insulation in EVERY rafter cavity. Code is 1”. The mighty joe lstiburek recommends 2” (I think he’s right too, 1” isn’t much, especially for shallow pitch roofs). Outside air needs to be able to come in through the soffit vents and travel along the underside of the roof sheathing (so there can’t be blockages) up to and out the ridge vent.

    It’s good that you have an air barrier on the attic side of the knee walls over the insulation. My house lacks this, and I’m in the process of adding one using rigid foam panels (which helps to eliminate thermal bridging caused by the studs). Make sure voids under the kneewall between the joists have been filled. Those voids are a common leakage path for inside air.

    There are several articles on here about air sealing and insulating attic kneewalls that can provide more detail for you.


  2. GBA Editor
    Martin Holladay | | #2

    If your house has fiberglass insulation between the rafters, every single rafter bay must, according to code, have an air gap between the top of the insulation and the underside of the roof sheathing, This air gap is usually established by the installation of a vent baffle (or chute).

    Every single ventilation gap needs to connect to a soffit vent at the bottom and a ridge vent at top.

    For information on the proper way to insulate a kneewall, see this article: “Two Ways to Insulate Attic Kneewalls.”

    If your kneewalls have polyethylene on the attic side of the kneewalls, that's a wrong-side vapor barrier. Instead of polyethylene, you want drywall, OSB, plywood, or rigid foam in this location.

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