Larsen truss durability question
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.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part