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Zone 6 exterior wall construction

user-7034864 | Posted in Energy Efficiency and Durability on


I have a question about proper wall construction here in Helena, MT.  We get 11-12 inches per rain, and usually havd a period during the winter where outside temps don’t break 0° F for 3-4 weeks straight.  There is no code enforcement where I will be building.  90% of new homes in our area are built outside if a code enforcement zone.  That said, I want this house to last 100 years without any major redos.  So my first question is exterior walls…Here is my plan from inside out
1.  gypsum w/ latex paint
2.  5.5 of craft faced fiberglass
3.  OSB
4.  Draining Houswrap (obdyke hydro gap)
5.  1.5 in xps foam
6.  3/8 strapping for rain screen
7.  hardie panel (board and batten).

Does this look like a good long term solution?  Thanks for any comments and advice.


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  1. Trevor Lambert | | #1

    That doesn't seem like enough insulation to me for zone 6. Looks to be about R25. Please reconsider the XPS for environmental reasons. It's the least green option by a wide margin.

  2. Jon R | | #2

    Consider two tested air barriers (for example, the drywall and the sheathing), 2" of EPS and also something more moisture proof than OSB. Cellulose has some advantages over fiberglass.

    Follow the recommendations here regarding foam thickness (2" of EPS will work) and interior/exterior perms. Also here for why double air barriers and drainage gaps are a good idea.

  3. Expert Member
    Peter Yost | | #3

    You can never go wrong introducing a free-draining, ventilated space between your claddings and the rest of your wood-framed building assemblies. Technically at 12 inches precip annually, you don't "need" that space but all claddings like the "relief" that space gives.

    Since bulk water management is the top priority, next on your list should be air tightness. And then thermal so Trevor and Jon above are both on the money.


  4. GBA Editor
    Martin Holladay | | #4

    If you want to install continuous exterior rigid foam over a 2x6 wall in your climate zone (Zone 6), the minimum R-value for the rigid foam layer is R-11.25. You are planning to install R-7.5 of rigid foam, which isn't enough.

    For more information on this issue, see this article: "Calculating the Minimum Thickness of Rigid Foam Sheathing."

    I agree with Trevor that XPS should be avoided for environmental reasons. Instead, choose EPS or polyiso. For more information, see "Choosing Rigid Foam."

  5. user-7034864 | | #5

    Thanks For all Your Help

    Martin-11.5 would mean 3 inches of polysio...I don't think I could get my siding up. It would take 5.5 inch screws to get one inch into studs. How is that dealt with?

    Two questions about the exterior insulation minimums. My understanding is that those minimums exist to prevent condensation on the inside of the sheathing. 1. Why does the rainscreen not cool the sheathing back to the point that the R-value behind it is immaterial...and 2. Why do we not have the same problems with our cold roofs in this climate?

    Thanks again for all of your help and comments.

    1. Expert Member
      Zephyr7 | | #6

      R-11.5 worth of polyiso is a bit shy of 2". Even if you de-rate the polyiso for cold temperatures, you'd need only 2.3", so round that to 2.5". 3" would be closer to the amount you'd need if you use EPS.

      The reason for the minimum R values for exterior foam is to keep the interface between the inside face of the rigid foam and the batts in the stud cavities to a warm enough minimum temperature. The purpose is to keep this area, where the wood structural sheathing would generally be, warm enough that moist indoor air doesn't condense here and cause moisture problems within the wall. "cool the sheathing" isn't what you're trying to accomplish, you're trying to keep things inside the wall warm enough.

      Roofs DO have similar foam ratio requirements.


    2. Expert Member
      Akos | | #7

      Up here in the great white north (zone 5 and 6) there are plenty of 2x6 walls built with 1" of rigid on the outside. No issues with them.

      With the lower than ideal ratio Martin quoted, you need a decent warm side air barrier (air tight drywall) and a vapor retarder (6 mil or 4 mil poly, craft faced insulation or vapor barrier paint).

      I'm a big fan of felt faced roofing polyiso, generally it tends to be the cheapest here and can also be found recycled for cheap. Most is vapor permeable, so it would allow drying to the outside as well making the wall even more robust.

      1. user-7034864 | | #9

        And won't the obdyke hydrogap help, too?

        1. Josh Durston | | #10

          Does the hydrogap between the OSB and foam create a thermal bypass?

        2. Expert Member
          Akos | | #11

          Crinkly house wrap under foil faced foam helps with water diffusion, so it does speed up drying. If you go with that type of foam, it is a good idea.

          If you use something like felt faced polyiso, the house wrap should goes over the foam. Having a crinkly house wrap there especially with 3/8" rain screen doesn't do much. Since the foam is vapor permeable, you don't need anything extra underneath it to "help" with drying.

    3. GBA Editor
      Martin Holladay | | #8

      Q. "I don't think I could get my siding up. It would take 5.5 inch screws to get one inch into studs. How is that dealt with?"

      A. You install furring strips on the exterior side of the rigid foam. The siding is attached to the furring strips with conventional fasteners. Of course, you still need long screws to attach the furring strips.

      For more information, see these articles:

      "Fastening Furring Strips to a Foam-Sheathed Wall"

      "How to Install Rigid Foam Sheathing"

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