Walls framed conventionally with dimensional lumber have two big energy-efficiency problems: there’s not enough room for insulation, and there’s a good deal of thermal bridging—heat transfer through the framing. But modern techniques are addressing those issues.
The Passivhaus concept. Katrin Klingenberg found a way around both of those issues and designed a house in Urbana, Illinois, to the performance standards of Germany’s Passivhaus Institute. Instead of framing with dimensional lumber, she used 12-inch-deep I-joists for the walls and wrapped the outside with a 2-inch layer of rigid foam insulation. Wall cavities were filled with blown-in fiberglass for about R-60. Walls were balloon-framed, meaning the I-joists run uninterrupted from the foundation to the top plate, eliminating heat loss through a rim joist between floors.
I-joists are usually used in floor framing, not walls. But their thin webs of oriented strand board reduce thermal bridging, and their depth provides lots of room for insulation. I-joists, however, aren’t as stiff as dimensional lumber, and using them for wall framing requires a slightly different technique. Both sides of the I-joist, for example, need structural sheathing to prevent their flanges from bending or flexing.
Klingenberg, the executive director of the Passive House Institute US, no longer uses I-joists as weight-bearing components in walls. Instead, she builds structural 2-by-4 walls, sheaths them with OSB, and bolts I-joists to the outside. “It’s easier to get inspectors to sign off this way,” she says.
Larsen trusses. I-joist walls are similar to Larsen trusses, developed by Edmonton, Alberta, builder John Larsen in the early 1980s. Like the newer I-joist technique, Larsen trusses do not carry any of the roof loads. They can be made on site with 2-by-2s that are connected by small gussets of 3/8-inch plywood. When completed, the truss looks something like a ladder. Trusses are attached to the outside of sheathing that covers a 2-by-4 or 2-by-6 wall, and then sheathed on the outside before the siding is applied. Wall R-values can easily top 40.
Larsen says the trusses arose as a faster alternative to a double stud-wall design. Houses built with Larsen trusses are not only well-insulated but also can be very tight; one 4,500-square-foot house had an air-leakage rate of only 0.80 ACH at 50 pascals—almost “tight” enough to meet very stringent Passivhaus standards. An air barrier applied over the sheathing is less likely to have penetrations in it. Also, dense-pack cellulose at that thickness, while not technically an air barrier, is a lot more effective compared to something like fiberglass batts.
One advantage of this method is that trusses can be built on site from relatively low-cost materials. Blown-in fiberglass or cellulose costs less than sprayed polyurethane foam, but at 10 inches or 12 inches thick makes high R-values and low thermal bridging. On the down side, these extra-thick walls require special detailing for doors and windows.