I-Joist vs Larsen Truss:
Does it make a difference for exterior high R-Value insulating?
Many high-performance buildings, when addressing the issue of super-insulation, will opt for traditional 2×6 stud framing with an added layer of exterior insulation to help reach the higher R-Values. One affordable and popular way to do that is with dense-packed, blown-in cellulose or fiberglass.
When considering the framing options for that exterior cavity, many builders will be inclined to use I-Joists and assume that any thermal bridging caused by that type of framing is negligible.
The thing is… the higher the performance, the more these seemingly small details can result in a gap between the expectation and the result.
Enter the Larsen truss option. A Larsen truss, with its point-to-point gussets (the small strips of plywood that connects the inside chord to the outside chord of the truss), greatly reduces the effect of thermal bridging compared to the continuous web of an I-Joist.
Our graph pictured here shows percentage, but you can think about it in dollars for effect. For every $100 you spend on a nominal R-Value, the Larsen truss gives you $92.50 of performance, while the I-Joist only delivers $81.10. So you basically lose about 10% of your expected R-Value.
The higher your goal, the more educated you need to be in making decisions like this. You may still decide to go for the I-Joist, for labor reasons or otherwise, but it’s important to arrive there as a well-informed decision. Every element of good design/build is an evaluation of trade-offs.
Go forth. Learn. Make informed decisions. And #RunWildBuildPassive !