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Wrap-and-Strap Wall System

mdb_az | Posted in General Questions on

I’m in early stages of planning a new home build for my family; 4BR/3BA sub 2000 sq ft house in CZ-5. I intend to self-perform most of the carpentry within reason.

We intend to build a pretty good house with R-30+ walls, R60 roof, R20 FPSF, triple-pane windows, <1.0ACH, heating and cooling from minisplits etc. Our early designs are farmhouse modern-ish with simple geometries.

I am a GC and in my professional capacity have built a number of high-performance houses for clients using a few different systems (all in CZ6):
-thick exterior rigid insulation (both foam and mineral wool)
-Klingenberg style walls with 2 x 4 frames and TJIs outboard with blown-in cellulose
-load-bearing TJI stud walls

…all of which perform great but are often overwrought in their complexity.

For our project I’m considering using the method outlined in this article from the Maine Indoor Air Quality Council:

A Practical, Easily-Built Wall System: R-30 and Beyond

…which is essentially a wrap and strap system with a load-bearing 2 x 6  wall and 2 x 4 service cavity on edge. I would follow more or less their details with the following assembly:
-3/4″ wood siding
-3/4″ vertical battens
-Solitex Mento or equivalent WRB
-1/2″ CDX sheathing
-2×6 @ 24″ OC w/dense-cellulose
-Intello or equivalent “smart” VB
-2 x 4 @ 16″ or 24″OC strapping on edge fastened w/countersunk 4″ screws Or 5″ pneumatically driven nails w/3/5″ cellulose or mineral wool batts
-1/2″ GWB (if using 16″ OC strapping) or 5/8″ GWB (if using 24″ OC strapping)

What I like about this wall (in theory):
-the primary structure is typical construction which makes it easy to build and potentially sub out to a framing crew if bandwidth or schedule necessitates
-the service cavity keeps air and vapor barriers continuous and allows MEP work to proceed once the insulation is in place and makes the house more comfortable to work in
-less thinking than a double-stud wall (the other approach I would use)
-fewest trips around the exterior of the house

The potential downsides:
-cold sheathing: I know this is a bugbear of the double stud wall crowd but the sheathing isn’t in the ideal, warm location in the assembly; however I will use CDX not OSB and a vented rain screen and a moisture-buffering insulation
-interior strapping; I know pole barn folks routinely install 2 x 4s on edge as roof purlins and it may be easy enough to do so; however I slightly worry about wonky material creating poor results

Anything I’m not considering? Anyone else built in this fashion before? Any well-founded advice is much appreciated 🙂

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Armin,

    What you are suggesting is called a Mooney Wall. It was developed by Tim Mooney and Mike Smith (both good guys!) https://www.builditsolar.com/Projects/Conservation/MooneyWall/MooneyWall.htm

    They work well. One minor issue is that the interior strapping means the standard framing at corners and at the intersection of interior/exterior walls will need to be modified.

    It might be worth considering whether the 1 1/2" service space deep enough for what you want to put in it? An interior 2"x4" wall will use almost the same amount of lumber but allow you to run drains, deeper electrical boxes and insulate with standard batts. It would however take up more space.

  2. mdb_az | | #2

    Thanks Malcolm! Nice to know the heritage of the design. To be clear: I would be putting the 2 x 4s on edge so the cavity space would be 3.5". I'm used to designing with thick walls and accommodating for this. Your point about the corners is well taken. Perhaps building California corners with one corner stud being a 2 x 6 on the flat would alleviate the problem?

    1. Expert Member
      MALCOLM TAYLOR | | #3

      Armin,

      Sorry I missed that. I think your plan will work well, and has the advantage of burying the vapour barrier that is missing in the usual Mooney wall insulated with cellulose. My only reservation is whether it still doesn't make more sense to run the studs vertically, rather than as strapping.

      1. mdb_az | | #4

        thanks again, Malcolm. When you say run the studs vertically wouldn't that align it with the other framing members and provide less of a thermal break than running them as strapping?

        1. Expert Member
          MALCOLM TAYLOR | | #5

          Armin,

          I'd either offset them, or use a different spacing. Running them vertically removes the problem of providing a strong enough moment connection to support the 2"x4"s on edge, and leaves you with a conventional wall everyone recognizes, that you know how to frame, insulate and board. I'm not sure Mooney walls make sense when you go beyond 1 1/2" thickness.

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