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Feedback on Double Stud Walls

Sasha Harpe | Posted in General Questions on

I’m considering using a double stud wall for a new home. in all the literature both the inside and outside stud spacing’s are insulated and often the walls are connected at each floor with the outside wall being the load bearing wall.

I’m considering designing the building such that the inside 2X4 wall is load bearing and un-insulated. The vapour barrier would be installed on the exterior of this wall and a second wall would be built hanging from the rafters so to speak with no connection to the interior wall except at window and door openings. The space can be filled with blown in Cellulose insulation and because the vapour barrier is on the exterior it of the inside wall it should be relatively continuous.

I have a number of concerns with this approach and that’s were my questions start. First if the relative cost of moving the second wall out 4″ is minimal (space is not a factor) what benefit would I gain from insulating the interior wall? (it would entail a second round of insulating and be relativity labour intensive). Second, what are the consequences for the shear capacities of both the interior and exterior walls as only the exterior wall is covered in plywood sheathing? Third, how high can I build a curtain wall without running into problems with buckling (assuming 2X4 lumber). Finally, Fourth, will I incur problems at the openings (Window and Doors) from the walls moving independently.

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Replies

  1. Lucas Durand - 7A | | #1

    Sasha,
    It is hard to come up with answers to your questions without a bit more detail (drawings)...

    It sounds like you are proposing an uninsulated service cavity within the interior (load bearing) wall...

    I can't say that I have seen too many double wall designs like the one you are describing.
    Here are some examples of wall systems that seem like they might be similar to what you are thinking of - all three are variations of a "double wall" with the interior wall being the load bearing wall:
    The SunRise home (ARTIC)

    The "Riversong Truss"

    The "House upon Moosehill"

  2. John Klingel | | #2

    Sasha: I am not a pro, but I am certain that having a load bearing wall and not bracing the studs from lateral deflection (your no plywood idea) will significantly reduce the load bearing ability of the studs. As the interior wall is where the load is, that is where you want to prevent racking, too. I don't think sheetrock is adequate for much of either of those. You gain about R-10 (ish) if you insulate then inner walls after you run pipes/wires. Is that worth it for you? Your call; it's done once, and lasts a lifetime. How high the studs can be depends on your loads (wind and weight); see a local engineer. I would lock the walls together. Do you have to use a vapor barrier? Canada? ("vapour" was a hint that you may be).

  3. GBA Editor
    Martin Holladay | | #3

    John,
    Your idea will work, as long as you get the details right. I suggest you use OSB as your vapor retarder; this will provide racking resistance. Your interior stud wall is a common detail known as a service cavity, as Lucas pointed out. If your outer 2x4s feel springy, just install a few plywood gussets or 2x4 tie-backs.

    Many builders choose a variation of what you are describing: namely, site-built wall trusses that bear on the interior side of the truss, with the exterior side of the truss cantilevered over the foundation. Usually, such wall trusses don't include a vapor retarder on the exterior side of the inner wall, however.

    Here are two articles to get you thinking:

    Service Cavities for Wiring and Plumbing

    All About Larsen Trusses

  4. Matthew Nolette - So Maine CZ 6A | | #4

    The guys at 475 recently posted a thought provoking and well detailed article along these lines. They solve the lateral bracing issue with diagonal 2x braces. I'm a little suspect at the cellulose installer's ability to get to every nook and cranny the way they have it but it's much in line with the original poster's thoughts.
    http://foursevenfive.wordpress.com/2012/04/18/double-stud-wall-optimized-for-cost-efficiency-performance/

  5. Sasha Harpe | | #5

    Thanks Everyone,
    Some Feedback from my point of view.

    "It sounds like you are proposing an uninsulated service cavity within the interior (load bearing) wall"
    -Yes
    "Do you have to use a vapor barrier? Canada?"
    Yes, northern Alberta and to the best of my knowledge the Building Codes says I need a vapour barrier and given that I think it would be simpler to only insulate once.

    I had initially avoided the Larsen Truss, because I assumed it was only avoiding part of the thermal break issue (ie the walls are still tied together) but I came across this: http://www.swinter.com/Collateral/Documents/English-US/CNAugust2009.pdf
    which seems to suggest that even by virtue of using separate 2X4's you would reduce thermal bridging, by extension the Larson Truss should be relatively Effective. Plus regardless of the spacing used on the Interior wall, I should be able to use 24" spacing on the exterior wall.

    So now the next hurdle will be to see if I can use 2X bracing on the interior wall instead of sheeting, and if that ends up actually being cheaper.
    Also can I hang a veranda of a Larson Truss?

    While I'm leaning towards the Larson truss system, It occurred to me that if the floor and roof spanned both walls. Then depending on the design, one should be able to consider both load bearing and move to wider spacing (ie 24" spacing with 2X4 studs on multi-floor homes instead of 2X6 on 24" or 2X4 on 16" )

    Thanks for all the input

  6. GBA Editor
    Martin Holladay | | #6

    Sasha,
    Q. "Can I hang a veranda off a Larson Truss?"

    A. Only if the details are reviewed and approved by an engineer.

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