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Moisture problems with double-stud walls

Larry Hanneman | Posted in Green Building Techniques on

I have read the article on double-studded walls very carefully. Moisture elimination is one of the issues that I am trying to resolve in my mind before attempting a low-energy home.

I am considering using the six-inch wall with staggered 2X4s since it reduces the thermal transfer by approximately 20% in my calculations. The design that I am considering is as follows:
•Cement board siding
•One-half inch vented rain screen
•ZIP System R3 as the exterior sheathing to replace OSB, Tyvek and blue-board insulation
•Two inch layer of high-density foam sprayed on the interior of the ZIP System R3
•Three inch fiberglass batting
•One half inch plaster board sheeting on the interior face

I am trying to achieve an R30 wall with zero air leakage due to:
•the moisture barrier and seam taping of the ZIP System R3 product
•the two-inch layer of high-density spray foam

Based on your extensive experience, do you see any moisture problems in such a construction?

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Replies

  1. Eric Habegger | | #1

    Speaking as a non-professional, I'm sure the professionals will weigh in here, I think you are moving in the right direction using the Huber zip system. That is provided you live in a climate that is appropriate to the level of wall insulation you are attempting. If you really are attempting an R-30 wall though a double staggered 2x4 then it seems excessively complicated for the 6" thickness. Why not just use 2x6 advanced framing techniques on 24" centers. For the small loss in insulation and or thermal bypassing then use the 1.5" R6 zip sheathing? I'm sure Dana could weigh in numerically if they come out even or if one is slightly better than the other.

    You mentioned that you are trying to achieve zero air leakage. That is pretty much impossible, though a worthy goal. I wouldn't worry about it myself and just the Huber recommended tape to get to 95 percent of your air tightness goal. Also (others will know more about this) depending on the high density foam you chose that is between the studs it may limit drying from the inside, which is probably what this system requires.

  2. Eric Habegger | | #2

    Just read that the polyiso part of the zip faces inward so I'm a little confused myself on how that would affect the drying. I'll defer to the expertise of others on spf between the studs affecting drying.

  3. GBA Editor
    Martin Holladay | | #3

    Larry,
    First of all, it's hard to give advice unless we know your climate zone.

    It looks like you want to design a wall with foam insulation on the exterior side of the wall assembly, and fluffy insulation on the interior side of the assembly. This approach is common; variations include 2x6 walls with exterior rigid foam and flash-and-batt walls. Your suggested assembly is one more variation, although it is rather complicated.

    All of these assemblies share one characteristic: the need to balance the R-value of the foam layer with the R-value of the fluffy insulation layer. For more information on this approach, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  4. Larry Hanneman | | #4

    I am in climate zone 4 - Cookeville, TN

  5. GBA Editor
    Martin Holladay | | #5

    Larry,
    My guess is that it would be a lot less expensive to build a 2x6 wall insulated with dense-packed cellulose, with taped OSB or plywood wall sheathing as your air barrier, and a layer of rigid foam on the exterior side of your sheathing to address thermal bridging.

  6. Eric Habegger | | #6

    Martin, I've been thinking about Larry's stack up plan and the more I think about it the more I like his zip sheathing idea. If I was having a house built from scratch (I'm not) and not doing the labor myself it really is a good idea to use a zip system, especially with the R6 sheathing in Tennessee. It will save tons of labor on windows and doors over the method of rigid foam because the OSB part is on the outside of the sheathing you don't have to create a buildout buck for them.

    It also saves labor on putting up separate sheathing, foam board, and Tyvek. So I'm sure any slight cost of the zip panels over the combined cost of sheathing, foam boards, and Tyvek, if there is an increase in cost, will more than be made up in reduced labor costs. Also it will be much easier to get the trades on board for this without additional training over what you're suggesting. That's a big deal in many parts of the country.

    Lose the HDspf. and lose the staggered 2x4 in a 6" wall and go with 2x6s using advanced framing And yes, cellulose is much better than the fluffy stuff, but at least he will have the option this way of using 6" thick 24" wide FG batts even though I wouldn't. The trades will love him for this type of construction and if they implement it well will be much cheaper in labor. Putting up with poor, or nonexistent, execution of a superior new building technique is always a problem, especially in some parts of the country.

  7. Eric Habegger | | #7

    "Putting up with poor, or nonexistent, execution of a superior new building technique is always a problem, especially in some parts of the country." I guess I better make clear that I wasn't putting down Tennessee. When working on my house here in California I was not able to find any contractors in my rural location that could or would do cellulose dense packing. I ended up having to do it myself. I hope that isn't Larry's situation.

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