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Wall assembly details?

Pete_VS | Posted in Energy Efficiency and Durability on

My wife and I are building our first house. I have a pretty good grasp on “normal” building practices but as I’m digging more into building science I’m realizing quickly I need to re-think a lot of things.
So currently the wall assembly we have drawn up is…
Half inch drywall
2×6 walls with either open cell foam or cellouse
Osb
1.5 inch layer of 2lb density eps.
Cladding will more than likely be vinyl siding and some stone veneer.
I would really like to have the windows on the same plane as the foam. So my question is… can I put the house wrap on the outside of the foam in order to make flashing the windows a bit easier? Any other comments on the assembly are welcomed!

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Replies

  1. ssnellings | | #1

    This article should help:
    https://www.greenbuildingadvisor.com/article/where-does-the-housewrap-go

    Either location works fine as long as you design your details correctly.

    My only comment on your assembly is that you haven't included information that allows evaluation of your air-sealing strategy. Are you taping the seams of the OSB?

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Peter.

    The answer to your question is yes, you can install housewrap outside of the foam and install the windows to the plane of the foam insulation.

    I agree with Sam. Now is the time to make a very detailed plan for air sealing, which should be continuous around all six (or more) sides of the the building.

    Also, it would be helpful for people wanting to reply to your post to know where this house is located.

    I suggest you take a look at these articles:

    The Four Control Layers of a Wall
    Where does the Housewrap Go?
    Innie Windows or Outie Windows?

  3. Pete_VS | | #3

    Thanks Sam and Brian. Sorry for not including where I lived. Western Michigan Zone 6a. For the air sealing I was thinking of taping the rigid foam since that would be more or less flush with the face of the foundation and then I could tape or liquid apply some sealant at that transition. I was going to have the rim joist cavity sprayed with closed cell foam and have that tied into the basement wall and floor vapor barrier. Does that make sense? Thanks so much for the advice!

    1. GBA Editor
      Brian Pontolilo | | #4

      Hi Pete.

      In general, yes. You can spray foam the rim joist area, but closed cell spray foam is economically and environmentally costly and you will have air sealed this area from the exterior. Though redundant air barriers are a good thing, in your case it would be more cost-effective and environmentally-friendly to use another insulator.

      Mineral wool batts are easy to work with and fit into areas like the rim joist, but fiberglass is an option. You may also use scraps of the rigid foam you are installing, which is a nice way to minimize waste.

      Keep in mind that the R-value of the rim should match the R-value of the framed walls above.

  4. Expert Member
    Dana Dorsett | | #5

    >"1.5 inch layer of 2lb density eps."

    HUH?

    Why the heavy-duty Type IX EPS? Even Type-VIII goods (1.25lbs nominal density) is more than fine for wall insulation, and fairly close to the performance of Type IX goods.

    >" Sorry for not including where I lived. Western Michigan Zone 6a."

    In Zone 6 it takes a minimum of R11.25 on the exterior of a 2x6/R20 wall to meet the IRC prescriptive for dew point control at the sheathing. If you're keeping it at 1.5" of EPS you'd have just a bit more than half that, which would mean air sealing at the wallboard layer becomes critical, and it would need something much more vapor-tight than standard latex paint as the interior side vapor retarder. See Table 702.7.1:

    https://codes.iccsafe.org/content/IRC2018/chapter-7-wall-covering

    With 1.5lbs or higher EPS you'd be good to go at at 3", but at the bare minimum if only 2.5", where it would be labeled only R10.5, but would be over R11.25 at outdoor temperatures that mattered.

    With foil faced polyiso you'd meet spec at 2" (labeled R12-R13), and a foil facer next to the air behind vinyl siding would add another ~R1 of seasonal performance, enough to compensate for any temperature derating of the foam. Foil facers are also easier to seal reliably with foil tapes than sealing bare EPS foam.

  5. Expert Member
    AKOS TOTH | | #6

    Dana's reply is spot on just keep in mind there is nothing technically wrong with your wall assembly provided you have at least a class II interior vapor retarder.
    You can read more about it here:

    https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-026-they-all-laughed

    The R11.25 exterior insulation is not a must, just if you have that much you can skip the interior retarder.

    2x6 with R5 exterior insulation is code around me (zone 5 and zone 6) built with interior poly. Works just fine and quite durable.

    P.S. Going with unfaced EPS allows for a bit of drying towards the exterior, this along with crinkly housewrap under the foam makes the assembly even more robust.

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