Roof Truss Design

Wednesday, October 25, 2017

If you are designing a home with a vented roof assembly, it is critical that the design include details that will allow the home to be constructed for the best thermal performance. Re- member, all homes should provide the best possible shelter from extreme weather conditions, but a green home should strive to achieve this by using good applied building science, better building methods and sound, passive design strategies.  These efforts result in a durable, high-performance home that will continue to provide operational cost savings throughout its lifetime.

Roof Truss Design

Building better requires us to look at how effectively traditional building practices have performed and whether we can improve upon those practices. Roof truss design is a good example of this. In traditional wood truss design, the roof rafters sit right on the ceiling joist, so there is not room to take full-depth insulation all the way out to the edge.  is means that we end up with a fairly large gap between the end of the ceiling insulation and the top of the wall insulation, along the so t line. In a traditional wood-framed home, this is one of the most commonly missed areas of insulation, and since hot air rises, it provides a good escape route for heat to move outward from the conditioned space to the outdoors in the winter. In cold climates, the ceiling gets cold in this area, causing water condensation that leads to mold growth.

A better way to design trusses is with energy trusses or raised heel trusses. Energy trusses or trusses designed to cantilever out over the top plate provide an opportunity to get the insulation out all the way over the top plate of the exterior wall assembly. If your goal is net zero energy, the improved design and thermal performance work together to reduce loads on mechanical systems, i.e., building energy use.  e International Energy Conservation Code also recognizes the inherent energy performance bene ts of energy and cantilevered truss designs by allowing you to reduce the overall attic insulation level if they are used.

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