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Larsen truss durability question

David Argilla | Posted in Energy Efficiency and Durability on

Hi,
I’m reading some BSC literature on high R value walls and insulation retrofit strategies.
Document http://www.buildingscience.com/documents/reports/rr-1012-residential-exterior-wall-superinsulation-retrofit/view?topic=resources/retrofits states that Truss wall systems have durability risk based in part on on a perceived danger of condensation on exterior sheathing. Reference is Straube, J., & Smegal, J. (2009). Building America Special Research Project – High-R Walls Case Study Analysis, (which is a dead link on the site, at least for me).

I did find a document showing details of truss wall were the same concern is detailed (http://www.buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/high-r-wall-05-truss-wall-construction/?searchterm=Straube%20and%20Smegal%202009).

So looking at the detail linked above, the OSB structural sheathing is placed on the outer chord of the truss, were it would be cold and in danger of condensation forming. The location of the sheathing is not correct though, at least not in areas where structural sheathing is required for for seismic or wind lateral bracing, or am I missing something? Shouldn’t structural sheathing be placed on interior or exterior of 2×4 wall studs, not the truss? Sheathing is then kept warm by truss insulation, and sheathing can be detailed as air barrier, which helps protect truss insulation and sheathing from interior vapor movement to exterior.
Anything wrong with this analysis? I am not in the construction trade, so worried that I am missing something.
I guess if sheathing was on exterior of stud wall, then truss would have to be nailed/screwed through sheathing to the studs, can this connection be reasonably detailed to be strong enough to support the weight of insulation(~8 inches), truss sheathing (thinking 1/4 or 3/8 inch plywood or fiber board), and wood siding? Or would be advisable to support truss by pouring small footing or pads under outer chord?

Also, is there any empirical evidence that truss walls (correctly flashed and with rainscreen) have durability issues vs. exterior insulated foam sheathing stratagies? Looking to upgrade insulation in our 1906 house in region 4C (Seattle), since we are taking siding off and adding sheathing to the building for structural and heating reasons. I have a bias against insulated foam sheathing, and would rather use cellulose, though my mind is not made up yet.

Thanks,
Dave

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Replies

  1. TJ Elder | | #1

    David,

    There is some debate on this issue but I'll give you one perspective. First, I would not place OSB sheathing at the exterior of a truss wall, but for that matter I wouldn't use it at the exterior of any wall. Second, not every application of "sheathing" is structural, and the term could apply to materials much more permeable and/or less prone to moisture damage than OSB. For example if you already have structural sheathing at the outer face of your interior frame, then there's no need for structural sheathing over the truss frame and you could use gypsum sheathing (e.g. DensGlass) or Homasote, or 1x board siding. These materials would not trap moisture in the outer insulation, and would survive periodic wetting without damage.

  2. Albert Rooks | | #2

    Hi Dave,

    Good job on the research. I'll agree with Thomas on all points. I can say that using the Homosote, Fibre Board or other vapor permeable material on the exterior chord is a current norm for larsen truss Passive House projects with R50+ walls here in Western Washington. The lateral loads are picked up by OSB on the inner chord. Builds out just fine and WUFI's well in climate 1 (Seattle). There are many in the area that pursue the same plans because they would rather not place a "foam product" where a cellulose product would serve the purpose.

    Depending on your truss thickness, try and maximize the exterior permeable material thickness as dense pack in a thick wall can cause a "belly" between framing members. It's not too big of an issue if you rain screen.

    Best of luck !

    Albert Rooks

  3. David Argilla | | #3

    Ok, thanks to both of you for your recommendations, I like the idea of using gypsum or homosote for truss sheathing. Encouraging to see others have used Larsen Truss in the PNW.
    As far as bulging issues with dense packed cellulose, I was considering using webbing on the truss bays so that there would be a small air gap between the cellulose and the outer sheathing. I was thinking that it could reduce possibility of wetting cellulose if there was an exterior leak by allowing water flow down the backside of the sheathing. Don't know, am I over thinking this? Perhaps overkill since initial plan is also to use rainscreen approach to siding, as this is a 2 story house, and the SW walls are very exposed to rain.
    Thanks
    Dave

  4. Albert Rooks | | #4

    Hi Dave,

    In my opinion, I would not net the bays. I would be looking to avoid any air within the exterior sheeting. It will cause wind washing and/or thermal loops in the insulation cavity. Just use a really high quality house wrap like Siga Majvest or equivalent, well taped and the wall will be great. The taping is to establish a wind-tight layer.

    Good luck,
    Albert.

  5. Matthew Amann | | #5

    Response to Thomas: Make sure that Dave understands that you mean paperless drywall, not standard, because this would be a disaster. Also, besides lessening the possible air tightness of the assembly, there is nothing wrong with "washing" or having spaces of air at the edge of the insulated cavity, as this is the concept of every vented roof in the country. If the air is not warm and rich with vapor moving from the interior space(which with R-50 it would not be warm), the air will do no damage IMO, again, every vented roof is built to use this principle.

  6. GBA Editor
    Martin Holladay | | #6

    Matthew,
    There is nothing wrong with having moving air in a rainscreen cavity to "wash" the vent channels -- as long as you have a good air barrier between the vent channels and the insulation.

    The same principle also applies to vented cathedral ceilings, although many builders forget to install an air barrier between the top of their insulation and the ventilation channels. Air barriers matter!

  7. David Argilla | | #7

    OK thanks
    points taken. Not sure how much wind washing to expect, as I only planned to have opening for water to flow down and out, no opening at top of truss bay. Correct term would be similar to drainscreen? Regardless, vent channels sound more complicated and time consuming than stapling netting, so probably better to spend time and effort on detailing the WRB and exterior truss sheathing to prevent leaks. Thanks for your help, I'm sure I'll be back with more questions.

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