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Community and Q&A

Cape Cod attic insulation

1954Chevy | Posted in Energy Efficiency and Durability on

Hello, my first question. I have a 1960 cape cod style house and I’m getting ready to put a new roof on with all new sheathing. I also wanted to improve my insulation to make my upstairs more comfortable and reduce or eliminate ice dams. I’ve had three insulation companies out for quotes and they all want to insulate the roof and create an unvented attic. I only have 2″x6″ rafters. Is this normally something people do with these old cape Cods? My budget for everything is only $20K so I can’t afford to do alot.

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  1. GBA Editor
    Martin Holladay | | #1

    You now have an excellent opportunity to solve a fundamental problem of the Cape Cod style. (That fundamental problem is an awkward shape with a thermal barrier that is often unclear.) If you need new roof sheathing, now is the best opportunity to solve your problems -- by installing one or more layers of thick rigid foam on the exterior side of your roof sheathing, or by installing a layer of nailbase.

    Empty your bank account or borrow the money; you won't regret it. If you don't do it now, you never will.

    Here are links to two articles to guide you:

    Insulating a Cape Cod House

    How to Install Rigid Foam On Top of Roof Sheathing

  2. Dana1 | | #2

    R20-ish fluff in the rafters and 4" of polyiso or 5" of EPS above the roof deck will get you to current code-min performance on a U-factor basis for climate zones 4 & higher, and would have very decent dew-point margins in zones 4-6. In zone 7 you'd need a bit more foam on the exterior, or a class-II vapor retarder on the interior. If you use reclaimed foam from commercial building re-roofing/demolition the material cost can be well-bounded, but depending on the complexity of the roof lines there can be a significant labor cost, if say, you have a bunch of gabled dormers creating compound-angle valleys and hipped roof rather than flat rectangular planes.

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