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Community and Q&A

Best 2×6 wall?

alcoprop | Posted in General Questions on

I asked a question a couple days ago about a huge remodel we are doing and I wanted to make another post to rephrase my question. This remodel is located in climate zone 5a and is down to the studs inside and out. We have 2×6 studs 16″o.c. I have zip going up as my WRB and paying great attention to air sealing. What is the best way to achieve the highest Rvalue? I was thinking 1.5″ of mineral wool exterior, 2″ closed cell and 3.5″ of mineral wool interior. for around R35. I know you guys say that spray foam is very expensive and very rarely recommend, but I have a good buddy that owns a spray foam company. He is doing our crawlspace already and is giving me a great deal while he is there to do the exterior walls. The cost is $1,500 to do the exterior walls in 2″ of closed cell. Please call me out if this is still a dumb wall design for this climate zone and please let me know what would be the best.

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  1. seabornman | | #1

    It depends how you are doing the exterior. Strapping and siding? If so, my vote is for 3" of exterior insulation, which meets code by itself (using the U-value table). Then you can add anything you want to the interior cavity.

    1. alcoprop | | #2

      Yes 1x4 furring strips and siding

  2. Expert Member
    BILL WICHERS | | #3

    +1 3” of exterior insulation. I’d use polyiso in two staggered layers myself. I’d use mineral wool batts inside. The spray foam doesn’t really gain you much here.

    If you really want spray foam, have your buddy flash the stud cavity perimeters to air seal any gaps, but don’t do any amount of fill. You’ll actually get a better fit with batts this way, and you get the perfect air sealing which is the ONLY advantage the spray foam has over other types of insulation here.


    1. alcoprop | | #4

      I already have 1.5" comfortboard 80 on site. I am sure I can probably return it but what about doing 1.5" of polyiso and then the comfortboard? Or just return the comfortboard and get the polyiso? Thanks

  3. GBA Editor
    Brian Pontolilo | | #5

    He Shaun,

    Keep in mind that if you use foil-faced polyiso, which is most common, and interior closed-cell spray foam, your sheathing will not be able to dry when it gets wet. Whichever arrangement of insulation you choose, make sure the wall can dry in one direction or another. I've written two articles about walls recently, which you may find helpful: Walls that Work and The Four Control Layers of a Wall.

    1. alcoprop | | #6

      After everyones comments I believe I will be returning the Comfortboard 80 and using polyiso. What I have available in my area is Dow SuperTuff R and I have 2" and 1" available. I will bypass the spray foam and do 5.5" of mineral wool on the interior.

      1. Expert Member
        Akos | | #7

        Going with the 2" polyiso + mineral wool batts with a rain screen setup should get you close to R33.

        If you are using vinyl siding, you don't need a rain screen, going with 1.5" would be less work as you can nail the siding directly on. The drop in assembly R value is not that much (R30) but would be much cheaper to install.

      2. Expert Member
        NICK KEENAN | | #8

        Polyiso loses some insulating ability as the temperature drops. Having another layer of insulation of another type on the outside keeps it warm and it performs better.

        I would tape the polyiso to form an air barrier.

        1. alcoprop | | #9

          So should I do polyiso and then the comfortboard to keep it warm?

          1. MattJF | | #10

            No the performance drop is generally not meaningful for enough the year to include another material just for that reason.


            Check craigslist for any vendors with reclaimed or 2nd's rigid foam. This would be the best and greenest option. If it is reclaimed, go inspect it in person.

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