Insulating TJIs in a Larsen Truss
If you’ve seen any of my other posts you already know I am on quite the odyssey in terms of trying to figure out the details of the house we will build zone 6A. My latest question involves insulating TJIs in a Larsen truss application. I am intrigued by the wall assembly employed by Chris Corson and the similar iterations employed by many other North American passive house builders (standard 2x stud wall with air-tight sheathing with TJIs acting as Larsen trusses wrapped in an over-engineered German WRB). The concept has an elegance and a robustness I find appealing.
However, I have one concern. In my research, the builders who use this system and the organizations like 475 that produce literature describing it take for granted that the TJI stud bays are insulated with dense pack cellulose. If I can find someone competent to dense pack the 12″ bays, then that’s almost certainly what I’ll do too.
But I am wary about the prospects of finding such a professional. I’m frankly not sure what the dense pack cellulose market is like in the Northern Michigan, but I would like to have some sort of contingency plan. In the event that dense pack cellulose is unobtainable, how wild of an idea is it to use R-49 Fiberglass batts between 11 7/8 TJIs spaced 24″ OC? Yes, I know that compressing a 14 inch batt will reduce the total R value from 49 to some lower but still acceptable number, but I’m more concerned with filling the irregular TJI stud bay created by the flanges. Would thick fiberglass batts insulate the TJI stud bays well? Would 2 layers of 8″ batts work better? Are there details I could add that would make this potential assembly more builder-friendly? Thanks for sharing your expertise and experience!
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John Larsen of the Larsen truss used fiberglass batts:
Fiberglass can also be dense packed, and may be more common in your area.
You can get batts for metal stud walls which are wider and will sit snug between the TJI webbs. For example, you can go with 2 layers of 6" R24 mineral wool batts. You would have to trim the edges a bit to clear the TJI flange.
For a new construction, one option to look at is using TJIs for the load bearing wall. No need for the interior stud wall, your engineer should be able to spec these to hold up the house. Construction wise, it is pretty simple, just build the equivalent of a TJI floor assembly and just tip it up in place.