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Community and Q&A

Wall Assembly to Avoid Spray Foam

Mhomedream | Posted in General Questions on

We are in the planning stages of our home in Northern Idaho(extreme mountain climate).  We need to avoid spray foam, and are very sensitive to mold so must avoid an assembly that will create condenstation/moisture build-up.  Considering the Huber R-12, with rock wool on the interior and a rain screen.  Overwhelmed by dew point and all the extra considerations in this climate.  Do I need exterior insulation as well?  Siding will be either Hardi-Plank or cedar.  Is there an optimal real world solution?  We paid A LOT for a professionally designed wall assembly, but then couldn’t find a contractor willing to actually build it.

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  1. Expert Member
    Akos | | #1

    One option to consider for a standard, easily buildable, decent R value wall is 2x8 24"OC without any exterior insulation.

    This wall is only slightly more expensive to build than a standard 2x6 wall, with high density batts you end up with an R24 assembly and avoids all the issues of dealing with exterior rigid insulation.

    In zone 6 an R20 assembly already gives you most of your energy savings, going much above that is only worth it if you are looking for certification such as passivhaus.

    For extra durability, go for plywood sheathing and install your siding over a rain screen.

    1. Mhomedream | | #4

      Thank you for your feedback. This is the first I've heard of 2 by 8 assembly. We do not care about passivhaus, only decent insulation and no mold. Can you explain why this eliminates the need for exterior insulation? Would you be willing to spell out in detail(with brands, if possible) your recommended assembly. This is what I'd been considering: Montana timber or Hardi Plank>Ventilated rain screen>Huber Zip System R-12>2 by 6 studs>rock wool bats(or if we can’t find them regular formaldehyde free insulation(is there a specific R value we need for the interior?))>paperless drywall.

      1. Expert Member
        Akos | | #7

        In almost all cases moldy walls are caused by improper air sealing or bad flashing details (windows and doors). A well detailed "standard" wall works well enough. The key to these walls working well is the interior permeability should be much lower than the exterior permeablity to prevent moisture from the house getting into the wall and allowing the small bit that does get in to dry to the exterior.

        A wall with a lot of exterior rigid insulation tends to be a more robust assembly in colder climates since it limits the amount of condensation and moisture buildup in the sheathing. This does come with cost and complexity though.

        A 2x8 wall is nothing much different than a 2x4 or 2x6 wall. Pretty much the same wall that packs more insulation. The reason for the 24"OC is to reduce the thermal bridiging form the studs, plus it is cheaper to build as you need fewer studs.

        In your case, the assembly would look like this:

        -drywall (paper/paperless/fiberglass, doesn't matter)
        -smart vapor retarder (faced batts, Membrain, Intello plus)
        -R30 batts (high density fiberglass or mineral wool)
        -Plywood sheathing, seams sealed with a high quality tape
        -rain screen strapping (strips of 3/8 ply or 1x4)

        Everything except for the tape and the variable perm membranes should be stock at your local lumber yard making it a very easy assembly to build.

      2. Expert Member
        Zephyr7 | | #8

        Note that if you go up to 24" stud spacing, using 5/8" drywall on the interior instead of 1/2" drywall will make a noticeable improvement in the solidity of the wall. 1/2" drywall gets a little too "bendy" for my taste with 24" stud spacing. I'm a fan of using 5/8" drywall everywhere to get a more solid wall, but with the wider stud spacing there is a much more noticeable improvement.


    2. Wooba Goobaa | | #9

      Arrow describes his 2x8 wall assembly ...

  2. Forrest Stanley | | #2

    I agree with Akos but, be aware that you will end up with an R value of about 24 with mineral wool but considerably less if you choose (in stock) fiberglass (probably about 18) - maybe 22 with cellulose.

    1. Expert Member
      Zephyr7 | | #5

      You can get high density fiberglass with the same R value as mineral wool. While it is usually special order, it's also becoming easier to find as people want more R value in their walls. High density fiberglass is a good middle-cost option between "regular" fiberglass batts and the more expensive mineral. Having installed all three of those kinds of batts myself, it is my preference to work with mineral wool though -- it's easier to install well, and I like the product better.


  3. Dennis_Vab | | #3

    Have you looked into the cost of zip r12? It would cost less in material to use wall sheathing with taped seems, and exterior insulation. Labor would be more as you are now detailing all the windows.

  4. Expert Member
    Zephyr7 | | #6

    You don't NEED exterior insulation, but exterior continous insulation is a big help. You don't have to use Zip-R to get exterior rigid foam though -- you can use regular plywood or OSB sheathing, then put a layer of rigid foam up over that. Sometimes the cost is less to do it the "old fashioned way" with a seperate rigid foam installation step, but not always, so you'll want to check with your contractor first.

    Mineral wool is probably a little safer than fiberglass in terms of mold growth potential, but not by much. What would make a much bigger difference in terms of limiting the potential for mold grown would be to put up a smart vapor retarder (MemBrain, Intello, etc.), and detail the drywall airtight too. This will help to limit how much moisture can get into your wall assembly, and the drier that wall is, the less chance you'll have any mold grown in it.

    There is no need for spray foam in a wall, and it doesn't really gain you anything in a wall. I consider spray foam as a niche product, mostly used for unvented roof assembly and irregular masonry walls (like cut stone foundations). In a wall, spray foam really doesn't have any benefits over other forms of insulation, and it costs a lot more to install.


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