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Best insulation for 12″ gable exterior wall

Sknu | Posted in Energy Efficiency and Durability on

Building a new 1 story ranch home with ICF walls for both the walkout basement and main floor. Location is zone 6, western South Dakota, dry hot summers, cold dry winters, less than 20″ precipitation per year, Windy year around.  ICF walls extend up above the trusses bottom chords/finished ceiling height in the end sections of the main floor.  Center section of main floor has cathedral ceiling with scissors trusses and gable end framed walls siting on top of icf walls.  Gable end walls under scissors trusses are framed with 2×12 lumber to match the 11.75″ thick icf walls.  Gable end walls exterior is finished with LP smartside prepainted wood panel siding 4×8 sheets, tyvek house wrap, 7/16″ osb attached to the 2×12’s on 16″ centers. The inside covering of the gable end walls will eventually either be drywall or wood shiplap boards. 

How to insulate the gable end framed walls?  I have several ideas but do not want to trap moisture in the wall but need a durable air barrier.
1.  2-3″ of closed cell spray foam to provide air/vapor barrier on external osb, 8-9″ of mineral wool batts or cellulose loose fill, 1/2″ drywall taped on interior, cover with shiplap wood.
2.  Same as #1, except replace drywall with membrane or intello smart vapor barrier and cover with wood shiplap boards.
3.  11-12″ of mineral wool bats or loose fill cellulose, smart membrane, cover with shiplap wood.
4.  Other options?

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Replies

  1. Jon_R | | #1

    Follow the recommendations here (Table 3) and also code (exactly as written, without presumptions). Always air seal well - a double air barrier (interior and exterior) is beneficial. So is a smart retarder.

    1. Sknu | | #4

      Jon - thank you, this was helpful. I am studying it.

  2. GBA Editor
    Martin Holladay | | #2

    SK,
    First of all, can you tell us your name? (I'm Martin.)

    If you go the flash-and-batt route, remember to keep your foam-to-fluffy ratio in the safe zone. In Climate Zone 6, you want at least 36% of the total R-value of your wall to come from the spray foam layer. Two inches of closed-cell spray foam (R-13) and 9 inches of batts (R-33) won't work -- but 3 inches of closed-cell spray foam will work.

    Otherwise, any of your proposals will work. In all cases, pay attention to airtightness.

    1. Sknu | | #3

      Martin - thank you. My name is Scott. Will update my profile to reflect that.

      A follow up question, if I have the closed cell spray foam, is it necessary to have the drywall as an air barrier if we end up with just shiplap wood and is it necessary to have the smart membrane as a vapor retarder? Or does the closed cell spray foam cover both the air and vapor barrier?

      I also have thought given the gable wall areas is not that large, the cost is not that much greater to just go to 8-9" of spray foam and skip the bat insulation. Is this acceptable, or does the insulation need to extend fully to cover the 12" wall thickness from exterior to interior surface.

  3. GBA Editor
    Martin Holladay | | #5

    Scott,
    Q. "If I have the closed-cell spray foam, is it necessary to have the drywall as an air barrier if we end up with just shiplap wood and is it necessary to have the smart membrane as a vapor retarder?"

    A. If 100% of your insulation is in the form of closed-cell spray foam, you don't need to use drywall as an air barrier. Nor do you need a smart vapor retarder on the interior side.

    If you go with flash-and-batt, it's best to have an interior air barrier (for example, drywall), because the interior air barrier greatly reduces the chance of potential convective loops or hidden air leakage paths through the fiberglass batts. That said, many people who follow the flash-and-batt approach are casual about the interior air barrier -- because it is admittedly less crucial in a flash-and-batt job than in a job without any spray foam.

    Q. "The cost is not that much greater to just go to 8-9" of spray foam and skip the batt insulation. Is this acceptable, or does the insulation need to extend fully to cover the 12" wall thickness from exterior to interior surface?"

    A. For a full explanation of my view of this issue, see the following article: "Installing Closed-Cell Spray Foam Between Studs is a Waste."

    1. Sknu | | #7

      Martin - thank you, would you recommend skipping the spray foam all together and just using mineral wool/rock wool or loose fill cellulose with a smart vapor barrier and drywall? Then just putting the shiplap over the drywall?

  4. igrigos | | #6

    Have you considered an alternative to 2x12's for the gable end wall? You might be able to save money and improve thermal performance going with a double stud wall, or a narrower stud wall with enough exterior rigid to match the ICF thickness. 2x12's just seem overkill to me for framing purposes.

    1. Sknu | | #8

      Good point, but I already framed it up in 2x12's. This is my first time building a house, so there have been several things that I would do differently next time around.

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