Larsen Truss thermal bridging

Thermal bridging of I-Joist vs Larsen Truss

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.

I-Joist vs Larsen Truss R-Value performance gap

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 !

4 comments

  1. Thanks a bunch for sharing, very interesting. The % gain here makes total sense. My biggest question is one that you nod towards: what is the dollar impact delta between the two approaches? If the labor cost means that the TJI is a significant savings, an increased savings in energy will quickly not be worthwhile in terms of dollars saved for energy consumed (even in an NZE project where PV is required to create that energy we find the line of diminishing returns kicks in at some point). Potentially the impact of increased terms of human labor with the Larsen truss is a bigger concern than the dollars. It seems labor is a crucial resource in the construction industry which is often not viewed in any term beyond cost, but skilled labor is a finite resource. We should also consider the diminishing returns of increased R-value in all of these conversations. If the loss in the wall assembly is down to some very small amount of Btus/ft2 of the building, 11% may not be worth the effort, even with Passive House or Living Building Challenge goals. Did you model an example building and look at the change in overall heating/cooling load per building area? Would love to hear your thoughts on that. Always happy to see people saving energy, just always want to be mindful of when we should turn from the building envelope to investing in some other resource reduction.

    Additionally, it would be great to consider another important factor that may well lean strongly towards the Larsen truss: what about the savings in terms of embodied resources, or embodied CO2? I would expect this is a metric under which the Larsen truss shines– less lbs of material used, therefor less resources and a lower CO2 footprint. This % savings is potentially greater than the % increase in R-value, and may even be the primary justification. Any chance your existing model can be easily adapt to study this savings as well?

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