Variations on the Larsen Truss – GWC Mag

In November of 1983, Popular Science published a short article titled “Superinsulated House Trusses” that briefly described John Larsen’s alternative to double-stud walls. John, a builder in Edmonton, Alberta, developed the approach in 1981. It is still being used today and is known as the Larsen truss. The idea is to build a house with conventional 2×4 walls, install airtight polyethylene sheeting around the building, and then attach to the wall shop-made trusses of long 2x2s joined with plywood gussets. When filled with fiberglass batts inside and out, John’s walls would hit R-35 with little thermal bridging.

“I came up with this idea while framing a double-wall house,” John wrote in an email. “Normally, experienced framers do the framing much more efficiently than ordinary carpenters like myself. They are, however, not so good at vapor retarder installation, which has to happen at the same time. By using the external truss system, they can frame the structural part of the house and I or a subtrade can come in and wrap the house externally with large sheets of poly, and then install the trusses. In the ’80s there was no internet, so we spread the idea by printing ads in building publications and sold manuals with blueprints for $15. We sold hundreds.”

In a 2011 interview with Martin Holladay, John said, “I was making the trusses until the late ’80s. After that, there wasn’t a big demand anymore.”

The Larsen truss may not have taken off as a product, but variations on the idea are still widely in use today. In fact, they’re a favored superinsulated wall assembly by some cold-climate builders, and for good reasons. They don’t necessarily have to be custom-built. Common manufactured products including I-joists and floor trusses can be used. They are a fairly straightforward way to add…