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How would you design non-load-bearing exterior walls?

BlueSolar | Posted in Energy Efficiency and Durability on

Hi all – I’m looking at a design like the Blue Sky steel-bolted moment frame. Basically, the steel corner columns and beams provide the structural integrity. The walls don’t need to be load-bearing.

Since the walls don’t need to do any load-bearing work, the only reason to have studs would be to support siding and other attachments like drywall inside, mounting TVs or cabinets, etc.

I’m intrigued by the ThermaSteel wall system. They’re steel-framed with solid EPS foam. They eliminated thermal bridging by turning the steel studs sideways, which is brilliant. But they’re standard size studs, which is a somewhat arbitrary decision given that they’re not being used the way they were intended, and these walls are designed to be load-bearing.

https://www.thermasteelinc.com/content/revpanel-system

Since I don’t need the walls to be load-bearing, it seems like there’s an opportunity to optimize. ThermaSteel walls have roughly twice the amount of steel as a normal steel-framed wall because there are now two side-turned studs facing each other, where normally there would be one stud spanning the depth of the wall.

And since the studs are turned sideways, the full width of the stud faces the installer (of siding, drywall, etc.) So we’ve got a 3½ inch wide stud surface, instead of 1½ or 1⅝ inches. That’s sort of nice and luxurious, but it also seems unnecessary and possibly wasteful.

Since the studs in this context are only there to be able to attach siding, for example fiber cement panels, would it make sense to just design or choose a stud size and shape that serves that purpose? I’m thinking of something of normal width, maybe a little wider, like 1¾ inches. Minimally, it could in theory just be a narrow steel strap / sheet to be able to screw into, not a stud per se, but it might help to have a stud-like shape for more strength and rigidity. For example, a ThermaSteel stud is 3½ inches face/web with 1½ inch deep flanges. A non-load-bearing stud like I’m imagining might be 1¾ inches face/web and some TBD flange depth, maybe an inch or so, for bracing strength.

Would this make sense? How would you approach high R-value exterior walls that aren’t load-bearing? It seems like they need something like a stud in order to install the siding, and it also seems like we could use a lot less steel than load-bearing walls require, or ThermaSteel uses. I’ve looked at Hardie and Nichiha docs, and it seems like they just need a certain gauge steel at certain spacing – it doesn’t seem to matter if it’s a traditional stud shape or size, since I think the key issue is pull-out strength or screw retention.

Thoughts?

Thanks.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    BlueSolar,

    You have run up against the same problem that confronts all post and beam or concrete structures. The necessity for the exterior walls to have a certain insulative value dictates the depth of the studs - and that usually means they are large enough to have been load-bearing.

    The most common ways around this are SIPs in residential construction, and propriety insulated panels in the commercial context. You can use light gauge, non-loadbearing steel studs, and add exterior insulation to cover them and the steel structure. You can also just use wood infill, which is probably the easiest.

    I'm pretty sure I remember a discussion where some of us tried to dissuade you from using purely steel framing. Most residential construction that wants the benefits of the large spans and openings you get with steel, uses those elements selectively as part of a mostly wood hybrid build. However if you are set on it, the options above are what I'd suggest.

  2. Expert Member
    NICK KEENAN | | #2

    I can't find the link, but on this very site I was just reading about a guy who made non-structural exterior walls using I-joists. You get a thick wall, something beefy for drywall on the inside and siding on the outside, and a small framing fraction.

    1. Expert Member
      NICK KEENAN | | #4

      They are called "Larsen Trusses" and here is the link:
      https://www.greenbuildingadvisor.com/article/all-about-larsen-trusses

      1. Expert Member
        MALCOLM TAYLOR | | #7

        DC,

        Lucas Durand, who used to be a frequent poster on GBA, used them on his house:
        http://ourhouseuponmoosehill.blogspot.com/p/details.html

  3. Expert Member
    BILL WICHERS | | #3

    Are you trying to minimize the overall footprint of your walls to gain square footage? I think you'll find that many fancy wall designs will cost more than just changing the foundation outline a bit (assuming you haven't begun construction yet).

    As Malcolm points out, and I seem to recall also participating in a discussion along these lines some time back, steel is usually used selectively in residential structures in places where it's particular properties are needed. This usually means in basements to split joist spans, where a girder basically acts like a super header, and in other areas where long, clear spans are needed. You don't usually use steel in a residential structure to try to keep loads off of walls. It's not a big deal to build a load bearing wall.

    I suppose the big question here is "what are you trying to achieve with this design?". I personally would want 2x6 walls for rigidity myself, 2x4 walls aren't as rigid as I'd like. I would absolutely not want to make a wall thinner than a 2x4's 3.5" since the wall would get to flexible. There are other concerns with a wall than just insulation, even if it's not load bearing.

    Bill

    1. BlueSolar | | #5

      Hi Bill, do you have a visual on the ThermaSteel wall? It's a 2 × 6 wall. Well, 5½ inches. The studs are turned sideways so there's no bridging. I envision keeping to the same concept, but perhaps with much smaller studs since they don't need to be load-bearing.

      Rigidity is an interesting question. I'll need an engineer to model it.

      1. Expert Member
        BILL WICHERS | | #6

        I looked at their site, looks like the have several variations on their basic design. It looks a lot like a SIP but with metal studs instead of wood sheathing bonded to insulation. I agree it should avoid thermal bridging with the metal studs in the way they are, but I'm not sure there is much advantage to this product on a typical site built home. Some of the info on their site ("arrives with openings precut", etc.) seems to imply that their product can be used in prefab building assemblies.

        Usually prefab products offer a cost savings where there is a large volume that can be made very similarly. This means manufactured homes and larger projects with many similarities that offer reuse of similar elements (we call this "step and repeat" sometimes in the design world).

        If you're building a one-off custom home, you may find a conventional wall assembly can be built with the same or better thermal performance to the ThermaSteel panel but for less cost. I'd recommend checking into that before going to far along with their product (they might even say the same thing).

        You may be able to get some rigidity/deflection under load info from the ThermaSteel people. What you don't want is a wall that flexes when you lean on it. 2x4 walls, to me at least, just don't seem solid enough to make me happy. They are perfectly fine structurally in nearly all cases, I just want them to feel more solid. It's similar to the "this IS safe" and more conservative "this LOOKS safe" deflection tables you find sometimes.

        Bill

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