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Opinions on this Wall Assembly

WpYMkdD4Ze | Posted in Green Building Techniques on

I’m evaluating different wall systems for an upcoming residence, and I’ve come up with something that I haven’t seen before. I would like some critique.

The wall would be composed of 2×6″ studs @ 24″ O.C. with OSB sheathing on the inside to act as a vapor-permeable air barrier. It would be filled with fiberglass or cellulose. It would be strapped with 2X3 studs horizontally inside of the OSB to provide room for extra insulation and a wiring chase. The exterior could be covered with an inch of XPS to provide a little more insulation and a drainage plane behind furred wood siding.

I like the idea of not having to worry about sealing up electrical boxes, etc. This wall should be easy to make fairly tight. I’m in a cold dry climate and don’t know if I should be concerned at all about the impermeability of the XPS on the outside should some moisture find its way into the wall.

Is this worth giving any more thought?

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  1. Riversong | | #1

    What climate zone are you in?

    OSB is not very vapor permeable and qualifies as a class II vapor retarder, which is what code requires on the warm side - but don't expect it to be able to dry much to the interior.

    Depending on your seismic and wind codes, you may not be able to use interior sheathing for shear bracing since it doesn't tie the floor and ceiling assemblies to the wall assemblies, and you'll have to treat it like Air-Tight Drywall with caulk or gaskets at every framing junction if it's to be the air barrier.

    Also most electrical boxes are 3¼" deep and won't fit in a 1½" cavity. One inch of exterior XPS may not be enough for your zone ?????

    With relatively impermeable XPS on one side and relatively impermeable OSB on the other, your wall system won't dry well in either direction. I'd suggest not trying to re-invent the wheel.

  2. Seth Hassinger | | #2

    Thanks for the reply Robert. I am in zone 5, NE Oregon.
    What would be a good vapor permeable structural sheathing to use in place of the OSB, and how about eliminating the foam (using house wrap instead) and making the interior wall thicker (and vertical (double wall)?
    I don't know about the seismic or wind requirements here. We do get some wind.
    I am not aware of any good easy ways to seal the outlets well, which is what motivates me to explore this system.

  3. wjrobinson | | #3

    Seth, Bensonwood is making your wall now. It has received lots of media coverage, Google it also here's a link.

    Some Passive House structures are doing something like you are looking to do also. Read up on Passive House

  4. Riversong | | #4


    Sawn lumber makes the best sheathing for breatheability, but plywood is much better than OSB (and more durable). And good old #15 felt outperforms any plastic housewrap.

    Wall options include, a basic cross-hatch wall, interior foam board, double wall, truss wall or exterior truss extension. Cellulose is orders of magnitude better than fiberglass.

    There are several air-tight outlet options on the market, including:

  5. Seth Hassinger | | #5

    Thanks for the link AJ. I'll read up. Looks like the Bensonwood wall would have the same inability to dry in/out that Robert mentioned about my wall. Is the zip wall vapor permeable?

  6. Riversong | | #6

    Zip Wall is highly impermeable, less than 1 perm wet or dry.

  7. Seth Hassinger | | #7

    Assuming the wall is assembled as discussed, would it be feasable to (hang) horizontal 2x4s (edgewise) on the interior from the sheathed 2x6 wall?

  8. Riversong | | #8

    Never seen it done. It would be near impossible to keep them from rolling downward under the load of the drywall. Perhaps if they were screwed with Timberloks or equivalent.

    You'd be far better off with a double wall, staggered studs.

  9. Seth Hassinger | | #9

    Yes, I guess the only real advantage of the horizontal studs would be not having to drill through them to run wiring/plumbing. Any reason the interior wall needs to share plates with the outside wall? Could they be joined only at window and door openings with plywood? The walls could be separated by any distance to deepen the wall, if desired.

  10. Riversong | | #10

    Yup, that's how it's done. And no drilling for wiring if it's a single cavity. It' s a near-perfect wall system with negligible thermal bridging.

  11. cornercanyon | | #11

    You may find this an interesting video -

    it was posted by John Brooks in another GreenBuildingAdvisor article showing how one group builds very tight walls in Europe. -

    Staggering the studs in double stud walls may not be worth the effort from an energy savings stand point. See this from the 100K House

    Good luck.

  12. homedesign | | #12

    Comments about the Euro-Construction Video

    (I am not promoting this wall.... just commenting)
    The house is "tuned" for a cold climate
    There is a continuous Airtightness skin(racking board) on the interior
    Airstop tape is used on the interior
    the tape is mostly on the conditioned side of assembly and not subjected to temperature extremes

    The assembly has a service core ...
    horizontal furring (looks almost square in cross section)
    No insulation inboard of the "racking board" (allows drying)
    The horizontal cavities in the service core have less "stack"(and convection) than vertical cavities

    The outside skin for the cold climate house is more vapor AND AIR open than the inside skin.

    ("Zip System" would probably not be a good choice for the outside skin) for cold climate

  13. john9987 | | #13

    Hi I’m John Meusch,

    I’m not an expert in this area, but I build a small 700 sqft home and we have some condensation issues that we need to correct, likely with exterior rigid foam. I was considering how I would do this again and I had this same idea about putting the osb on the interior wall and then placing the drywall over the OSB. Since osb is relatively vapor impermeable it could be useful in a cold climate. I’m zone 4. With a little creativity, I think this could be a more elegant solution than the standard wall. In my case we have ribbed metal siding. If there was a breathable WRB attached after the studs to the exterior with a rain screen arrangement, the steel would be a great barrier, rain screen would allow lots of drying potential, and the interior osb would make a nice substrate to air seal and stop hot air from getting to a cold surface in the wall. I wouldn’t compare this to reinventing the wheel, more like an electric car. It doesn’t use gas, you have to find a place to charge it and depending on your location that can be challenging… etc. Would like to know what Martin thinks about that?

  14. Expert Member
    MALCOLM TAYLOR | | #14


    I don't see much p0int in sheathing the interior with OSB unless you are also going to strap the walls. Otherwise the OSB doesn't do much the drywall covering it won't.

    An interior service cavity may make sense for some situations. The air-sealing layer behind it can be OSB, some other sheet good, poly or a variable-perm membrane. Before deciding to include one it's worth looking at what it will really contain. On some walls that might be one electrical outlet - making building the cavity a bit like using a framing hammer to swat a fly. My preference w0uld be to use them selectively where there are a lot of services, and in other areas use conventional construction techniques to achieve vapour and air sealing.

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