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Exterior wall construction dilemma and concerns about moisture in wall cavity

green654 | Posted in Energy Efficiency and Durability on

We are in the process of a new build. The Framing is up. We’ve attached Advantech Zip sheething to the frame with Tyvec drain wrap. The builder wants to put Hardie Board on top of drain screen and call it a day. However, I’m concerned about thermal bridging and the potential for moisture to develop in the walls
without exterior insulation (we live in zone 5). The builder said at this point, there is only room for ½ inch exterior foam
insulation and no space for a rain screen/air gap between the foam
exterior insulation and Hardie board. What’s our best option?? Keep it as it (Advantech zip plus drain screen plus Hardie) OR add the ½ inch exterior foam on top of drain screen sandwiched between drain screen and Hardie. Avoiding mold and moisture is the top priority.

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  1. Mark Nagel | | #1

    While I am only in a very early design phase of my build I have a similar sort of situation forming. I won't go into the details, but I am going to toss this out there in case it not only might help you but also that it might help me (answer a question that brought me to this potential "solution").

    Hoping that an actual knowledgeable person comes along...

    Install strips of 1/2" insulation vertically for alignment with the framing and as needed for supporting the siding (deflection).

    This will provide for a thermal break AND a rainscreen.

    1. Expert Member
      Malcolm Taylor | | #3


      Unfortunately intermittent insulation in the cavity won't provide an appreciable thermal break. You are better off sticking with plywood or 1"x strips.

      1. Mark Nagel | | #4


        I'm not quite sure of what you're stating as being "in cavity."

        I have a building vocabulary that is on a "3rd grader's" level- my understanding of "cavity" was something lying within.

        I'm trying to convey the strips as being on the outside of the external sheathing, kind of like standoffs. I suppose that the external sheathing is still acting as a temperature sink and can conduct to/from the framing. Was thinking that there had to be some sort of reduction in conductance from the siding directly to the framing (via sheathing that's between the two).

        1. Expert Member
          Malcolm Taylor | | #6


          Sorry Mark - by cavity I mean the rain-screen gap that the strips would create. Because they are surrounded by air that is usually at the outside temperature, and bypassed by the continuous sheathing, the strips won't do much to reduce the thermal bridging at the studs underneath.

        2. Jon R | | #11

          You are right that 3" wide strips of 1/2" foam over the studs/sheathing will provide some total-wall R value benefit over using wood furring strips. I'm with Malcolm - not much. THERM software would give a better answer.

  2. Expert Member
    Malcolm Taylor | | #2


    Until you start adding foam the exterior, your present wall assembly is very similar to the standard one used on most houses. What are the specific concerns you have about it that you think will lead to moisture problems?

    I'm also curious as to what your builder says is limiting the depth of your wall?

    1. green654 | | #5

      He said the way the foundation was poured, that is the limiting factor in how deep of a space we have to work with on the walls. And the concerns I have is a that we will not have a thick layer of exterior foam
      To control how cold the wood sheathing gets in the winter. I’m concerned that there will be condensation and moisture that accumulates on sheeting and will lead to mold within the wall structure without exterior foam insulation.

      1. Expert Member
        Malcolm Taylor | | #7


        If the framing stays in the same relation to the foundation below there is no limitations to how much insulation you can add outside the sheathing. Houses with exterior sheathing and rain-screen cavities almost always cantilever those layers over the concrete below.

        Absent issues with poor air-sealing, walls built as you describe don't usually see moisture or mold problems. They do perform better with a real rain-screen gap and a thick layer of exterior foam, but not having any is better that an inadequately thin layer.

  3. GBA Editor
    Martin Holladay | | #8

    It always amazes me to discover that walls can be framed before the owner has finalized insulation plans with the builder -- and that the owner and builder can have completely different ideas about wall construction details, even after framing has started...

    Planning ahead is essential!

    1. green654 | | #9

      True’! Hopefully people can learn from me so they don’t find themselves frantic in the same situation!

      1. Kirk Ellis | | #14

        Where are you located that building permits were even issued without all these details spelled out. I am in California and just finished reading the insulation sections of our "Title 24 Energy Code" document. 363 pages that are pretty specific about every compliance detail being required on the plans before a local building department is supposed to issue a permit.

  4. Jon R | | #10

    IMO, safest is to keep the 1/2" rain screen, use Intello on the interior side and fill with cellulose. This provides buffering, a good perm ratio and the ability to dry inward.

    You could flash-n-batt with closed cell spay foam.

    1. green654 | | #12

      Thank you! If we did flash and batt, would we need Intello?

      1. Jon R | | #13

        Flash-n-batt wouldn't need it and you could use just paint as the vapor retarder. See code for details.

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