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Tstuds for Larsen Trusses

boxfactory | Posted in General Questions on

Hey all,

Two quick questions

Could anyone give me a reason to not use Tstuds as a Larsen truss?

I personally, with no data to back up my opinion, am leery of using OSB based I joists. My concern is long term durability related to rainwater, and how it seems to go wherever it wants, despite the best of our efforts. Any real world examples to prove me wrong would be welcome.

Also, in the BS and Beer about vapor barrier use with Christine Williamson, she advised against exterior cellulose use, as one would use in a Larsen truss. Would anyone have remodeled any of the early buildings constructed with this method? I would be interested in what was found.

I happened across Steve Baczek’s Larsen truss where outside of the trusses was a second layer of sheathing, board sheathing to be specific (with a WRB). With the second layer of sheathing, would the cellulose still be considered exterior?

Thanks a bunch!
Hope all is well,
Ben

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Replies

  1. AlexPoi | | #1

    Ijoist, Tstud, Larsen truss can all work with cellulose insulation but it'll depend on your climate zone and how vapor open is your outside membrane/sheating. Otherwise, you could end up with some condensation problems.

    Using OSB or plywood in a cold region on the outside is risky especially if you don't have an interior air barrier because it's semi vapor permeable. Wood fiber board is the ideal solution since it is highly vapor permeable but it's hard to find and expensive in North America. Proclima has a reinforced membrane as well that can kind of replace your outside sheating but some people prefer to have something solid.

    1. boxfactory | | #2

      I hadn’t even considered the wood fiberboard, but that is something well worth keeping in mind. Though I agree that some advanced planning would be necessary for two specialized products (Tstuds / wood fiberboard). Thanks!

  2. Expert Member
    Michael Maines | | #3

    I believe Christine was concerned about exterior cellulose in hot/humid climates. Is that where you are located? In a cold climate it works fine to have exterior trusses filled with cellulose. I don't see any reason T-studs wouldn't work for this but I doubt they would warranty their product for that application. You might consider Sonoclimat Eco4 wood fiberboard sheathing; it's somewhere between OSB and rigid wood fiber board in composition and it's made in Canada.

    1. boxfactory | | #4

      We are in the early stages of planning our build in VT, so it seems as though this is a viable option, given the colder climate.

      I wouldn’t expect to warranty something like a Tstud or an I joist, as I figure what we are discussing falls outside of their intended use. It occurred to me that the Tstud construction of solid wood dowels and dimensional lumber would be more robust than an I joist.

      I’ll be sure to look into that fiberboard sheathing. Thank you

      Ben

      1. Expert Member
        MALCOLM TAYLOR | | #5

        Ben,

        I joists aren't warrantied for use vertically, so using T-studs doesn't put you at a disadvantage. That said, I'm not sure I'd want to be the first to try it.

      2. Expert Member
        Akos | | #6

        Larsen truss is great for a retrofit, there are cheaper ways to build a high R value wall in a new build.

        Look at either standard stick build with exterior rigid insulation or double stud wall. If you have an open minded structural engineer, you can use even I-joists as studs to build a wide wall without having to deal with double studs.

        I've left off cuts of I-joists outdoors for many months to keep thing off dirt. They hold up surprisingly well, I would not worry about it inside a wall. If it gets bad enough inside the wall to damage the I-joist, you have much bigger problems.

        1. boxfactory | | #7

          Thank you for all the great info. The double stud wall was the other wall I was considering. I was just confused how after it’s use for decades, the only failures that I could find by google were a Journal of Light Construction article where the walls were not dense packed, and a Building Science Company report where the nails rusted. But still, the cold sheathing issue gets brought up with such regularity that I have to wonder if I am missing something.

          I did a similar google search for Larsen truss failures, and couldn’t find any so I figured that it was worth keeping in mind as well, but I thought I’d pose my earlier questions as there seems to be less info out there as compared to the double stud.

          Thanks again,
          Ben

          1. Expert Member
            MALCOLM TAYLOR | | #8

            Ben,

            Double-stud walls appear to have defied the modelling which showed they should experience moisture problems. With a good air-barrier, and the ability of the sheathing to dry into a rain-screen gap, they aren't prone to failing.

    2. andyfrog | | #11

      You have probably spoken with Christine more often, so can evaluate these two clips together?

      https://youtu.be/tU9ciWUfM0A?t=4752

      https://youtu.be/XfiRYQBU4_M?&t=1974

      I think the BS and Beer clip is the one that boxfactory is referring to, but there's another from a lecture Christine did which is a similar expression.

      Without asking her directly I don't know, but it seems like her reservation is with moisture sensitive materials outboard of the sheathing in general, not just in hot and humid climates--because she specifically mentions Switzerland in her example of synthetic stucco applied directly to wood fiberboard as a particularly risky example of that due to the level of craft required to detail that assembly.

      If you combine what she says in the two clips, to me it seems like she is talking about the general case of moisture sensitive materials outboard of the sheathing where the water control layer applied.

      And based on her comments about the wood fiberboard water repellent treatment, I don't believe she thinks people are putting exterior cellulose on without some kind of water control membrane.

      I could, however, be wrong. I had noticed for a long time that Lstiburek and Straub never mentioned or tested exterior cellulose or wood fiber board insulation. I have been searching for a study on the moisture content of exterior cellulose or wood fiberboard insulation, but the closest I can find are double stud wall studies for cellulose, and then a few studies on the vapor profile of various thicknesses of wood fiberboard insulation (which at sufficient thicknesses is actually not quite as vapor permeable as I would have expected).

      1. boxfactory | | #12

        I also had essentially zero success with finding any info about long term use of exterior dense pack cellulose / exterior wood fiberboard. Perhaps I looked in the wrong places, I was surprised that any info from Europe was very challenging to get, considering how long exterior wood fiberboard has been in use.

        Ben

        1. andyfrog | | #13

          http://cchrc.org/media/SafeAffordableRetrofits.pdf
          http://cchrc.org/media/SEAR_Report.pdf
          http://cchrc.org/media/CelluloseSnapshotFinal.pdf

          I did manage to find these reports, which was only for 2 years and is not in a particularly wet climate. It seems like exterior cellulose is actually quite favorable so long as there is no bulk water intrusion. What I was hoping to find is some study where they monitored what happened when water was injected into the cellulose.

          My main concern would be defects in the water control layer. For the low cost approach, the mechanically attached membranes can be quite fragile--I saw a video where someone with Solitex Mento on their roof was getting leaks during construction from pine needs falling on their roof. The more expensive approach is to put wood fiber sheathing outboard of the cellulose, and in theory a water control membrane is unnecessary but if it were my house I'd probably wrap that in a self adhered membrane. Some people might say to put OSB or plywood outboard of the cellulose, but that just seems like a double stud wall with extra steps to me.

          The second concern I'd have is settling, which is admittedly very minor in comparison. We know that unvented cellulose roofs are risky because of settling: https://www.youtube.com/watch?v=xZlnpQYdsuM
          &t=1551s but I don't know if this would happen in walls because there isn't so much moisture drive from the stack effect.

          Another option would be to look for studies done on double stud walls with mid-plane sheathing and no OSB/plywood on the outboard side.

  3. boxfactory | | #9

    Great to hear. That was my impression, just hearing it from someone else is reassuring.
    Thanks again,
    Ben

    1. Expert Member
      MALCOLM TAYLOR | | #10

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