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Outsulation-to-insulation ratio?

Stephen Edge | Posted in General Questions on

I have 2 inches of foil-faced polyiso over Typar on the exterior of my north wall. It is strapped with vinyl over the strapping.

How much interior insulation can I have on this wall, and do I need an air barrier? Reason I ask about the air barrier is because some of the gable wall space is behind the kneewall where I really don’t feel like hanging sheetrock or paper unless I really have to…

Also, can I use XPS on the interior or will this create a double vapor barrier? I’m in zone 6.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Stephen,
    In your climate zone, your 2 inches of polyiso will allow you to install up to 5.5 inches of fluffy insulation between your studs without any worries about moisture accumulation in your sheathing or wall cavity. As long as you have 2x6 or 2x4 studs, you're fine. For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    I don't recommend that you add any XPS on the interior side of your wall. If your wall has polyiso on the exterior, you want to allow the wall to dry to the interior; that means no interior rigid foam.

    I don't quite understand your question about insulating the kneewall. However, if you are using fluffy insulation (like fiberglass batts) to insulate your kneewall, you definitely need an air barrier on both sides of the wall.

    That leads up to your question, "Do I need an air barrier?" Every wall needs at least one, and sometimes two, air barriers. For more information on air barriers and insulating walls with fiberglass batts, see:

    Questions and Answers About Air Barriers

    Installing Fiberglass Right

  2. Stephen Edge | | #2

    Thanks Martin, My only open question is about that pesky little triangle on the gable wall behind the knee wall. Since I have an airbarrier on the exterior in the form of typar and 2in foil faced polyiso, can I insulate the interior of that wall with open faced roxul 23r batts. This area would be very difficult to sheetrock. Also, is there an rvalue ratio I should follow with exterior foam. Meaning a limit to the interior rvalue based on the rvalue of the exterior. I've got plenty of space there to fur out an 11in wall

  3. GBA Editor
    Martin Holladay | | #3

    Stephen,
    Q. "My only open question is about that pesky little triangle on the gable wall behind the kneewall. Since I have an air barrier on the exterior in the form of Typar and 2-in foil-faced polyiso, can I insulate the interior of that wall with open-faced Roxul R-23 batts?"

    A. Yes, but the wall won't perform as well as it might if you skip the interior air barrier (drywall, ThermoPly, or MemBrain).

    Q. "Is there an R-value ratio I should follow with exterior foam?"

    A. Yes. See the article I linked to: Calculating the Minimum Thickness of Rigid Foam Sheathing. The article tells you that you can have up to 5.5 inches of fluffy insulation in your climate zone if you have 2 inches of exterior polyiso. If you increase your fluffy insulation beyond 5.5 inches, you are making the wall sheathing colder and asking for trouble.

  4. Stephen Edge | | #4

    Got it. Thanks for the feedback.

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