To keep any house affordable, efficiency during the construction process is key. A high-performance wall assembly is no exception. There are multiple ways to construct a high-performance wall, but which one makes the most sense from a cost standpoint when building an affordable zero energy home?
Spray foam insulation has a higher R-value per inch than almost any other insulation product, but it is costly when compared to cellulose or fiberglass. On the other hand, a skinnier wall assembly can be used in combination with spray foam, which contributes to some savings.
Cellulose is the next best thing, but has approximately half the R-value per inch when compared with closed cell spray foam. Accomplishing the same result requires a wall twice as thick. In our experience, the best and most efficient way to construct this wall is with two rows of 2x4s with a 4 ¼ inch space between them.
In the diagram, you will see the details in full. Even though this approach requires building the exterior wall “twice,” the cost is still lower and there are other benefits as well for our affordable zero energy home(s). The space between the studs allows for a solid thermal break that a 2x6 wall with spray foam would not. Subcontractors can also use the space to run wires without drilling studs.
By creating a thermal break inside the double stud wall, it’s no longer important to add it on the outside with the use of rigid foams that are anywhere from 1” to 2” thick. This is a big time saver because exterior applications such as sidings and window trims can be installed without special padding to accommodate for the thickness of the foam.
We are always “tinkering” with the best approach to a high-performance wall assembly in an effort to increase efficiency and lower costs. The assembly in the diagram is the best we have come up with to date, but who knows, maybe we’ll think of something else in the future.