Why is the I-joist outrigger wall assembly a popular choice for Builders?
Tried-and-true wall assembly built in Canada for 40 years!
The I-joist outrigger assembly was not invented by Akira Living. Versions of this wall assembly have been used in super-insulated house construction in Canada for over 40 years. Today’s I-joist wall assembly coupled with a durable smart vapour retarder membrane (interior airtight layer) in lieu of 6 mil polyethylene, and a durable weather resistive barrier (exterior airtight layer) in lieu of building paper, and dense pack cellulose in lieu of batt insulation – creates performance attributes that make it the preferred wall choice for beginner and expert Builders alike in light-wood frame construction. Akira Living’s preferred recipe and solution to create super-insulated above grade walls designed for a Passive House in low-rise wood-framed projects. See HIRO and KAI to learn more about the building enclosure construction assemblies and detail packages available.
The evolution of the I-joist outrigger assembly starts with why it was invented.
1977 Harold Orr (Energy Efficient Building Engineer) designed the building enclosure system for the Saskatchewan Conservation House in response the radical increase in oil costs from the Middle East. The task was to build a house that could withstand cold Winters and hot Summers. The result was the worlds first house to combine super-insulation, airtightness, and a heat recovery system. The exterior wall was double-stud wall construction (inner wall is structural), with cavities filled with fiberglass batt insulation – effective thermal resistance of R-40. Polyethylene air / vapour barrier was installed on the exterior face of the inner wall and wrapped around the floor framing. Subsequently, the outer wood stud wall was installed (outer wall is non-structural). Harold Orr called it – “Prairie Double-Wall”. The Saskatchewan Conservation House was tested to achieve 0.5 air changes per hour (ACH) at 50 Pa (this complies with current Passive House requirement of 0.6 ACH). Harold Orr’s pioneering ideas led to the birth of the Passive House movement.
1979 John Larsen (Builder), was building a super-insulated home in Jasper, Alberta, using double-stud construction. This proved to be a labour-intensive wall system with extra lumber to assemble the outer stud wall, and difficult polyethylene air / vapour barrier installation. He wanted a different wall assembly to reduce labour onsite labour costs. He conceptualized a simple parallel-chord wood truss mounted vertically on the exterior wall as a cost-effective and lightweight alternative way of creating a double-stud wall assembly. The parallel-chord wood truss replaces the double-stud wall’s non-loadbearing outer wood stud wall and cantilevered floor framing.
1981 John Larsen (Builder), starts prefabricating parallel-chord wood trusses calling them ‘Larsen Truss’ from his shop in Edmonton, Alberta. The original Larsen Truss consisted of two parallel 2×2’s connected by rectangular gussets of 3/8 inch thick plywood spaced at 24 inches. Larsen Truss depth ranged from 8-1/4 inch to 12 inch deep, with cavities filled with batt insulation. The Larsen Trusses were designed to mount vertically on the exterior wall fastened into the wall sheathing and conventional 2×4 wood stud framing. The Larsen Truss System allowed the sealed polyethylene air / vapour barrier to be installed on the exterior side of the wall sheathing eliminating framing sequencing issues and complexity as experienced in double-stud wall construction. This system was effective because carpenters did not have to retrain on framing techniques as all air sealing was installed after the primary house structure was completed. Several magazines reported on the Larsen Truss System in the early 1980’s including the system’s effectiveness in full exterior renovations (retrofits).
2001 Trus Joist by Weyerhaeuser published a technical bulletin on balloon and platform framing details using I-joists for loadbearing wall stud framing. I-joists were invented in 1969 as a horizontal floor or sloped roof structural member, then repurposed for energy efficient construction as vertical studs in Germany. This is not a popular construction system in Europe to date.
2003 Katrin Klingenberg designed the first single-family Passive House in the United States. 11-7/8 inch deep I-joists were used as loadbearing wall studs, OSB installed on the interior side, and vapour-permeable fiberboard installed on the exterior side. The stud cavities were filled with blown-in fiberglass insulation. This wall construction type is known as the ‘Klingenberg Wall’.
2010 Dürfeld Constructors developed a prefabricated panelized version of the Klingenberg Wall for the Rainbow Duplex in Whistler, BC, Canada. 9-1/2 inch deep I-joists were used as wall studs, OSB installed on the interior side, and vapour-permeable fiberboard installed on the exterior side. The stud cavities were filled with blown-in cellulose insulation – effective thermal resistance of R-47. Prefabrication raises the bar for quality control. Later founders of Dürfeld Constructors started BC Passive House, opening a prefabrication facility in Pemberton, BC, Canada.
2011 Chris Corson (Builder with Ecocor) rethought the approach and combined the advantages of the Larsen Truss System with modern building materials used in the Klingenberg Wall – becoming an I-joist outrigger wall assembly. For a Passive House project in Maine, I-joists were mounted vertically on the exterior wall onto oriented-strand board (OSB) wall sheathing (sealed seams to function as the air barrier) and fastened into conventional 2×4 wood stud framing. Vapour-permeable fiberboard sheathing installed on the exterior side of the I-joists. In subsequent Passive House projects, the vapour-permeable fiberboard sheathing was replaced with a durable water-resistive barrier that can retain dense pack cellulose insulation. Today Ecocor has converted to a panelized prefabricated approach to the I-joist outrigger assembly.
2016 to Present Akira Living uses an I-joist outrigger wall assembly for KAI Exterior Wall intended for site-built Passive House projects. I-joists are a true engineered product to reduce onsite labour costs compared to the original Larsen Truss. I-joists provide strength and consistency to provide plumb and straight walls – making cladding installation simple. KAI Exterior Wall Assembly is the recipe and solution to create super-insulated above grade walls. Fast track your ability to deliver a successful Passive House with a proven wall system 40 years in the making.
Passive House building enclosures require two critical components:
- super-insulated construction assemblies (The Recipe),
- airtight & thermal bridge free transition details (The Technique).
Akira Living provides solutions to both components with KAI Assembly and KAI Detail Package. The designs are developed and reviewed by a Certified Passive House Designer, Professional Building Enclosure Engineer, and Structural Engineer licensed in Canada.
2022 What building exterior wall assembly will you use for your Passive House project?