What is a Larsen truss and does New Brunswick use them?

A Larsen truss is a site-built framing extension that attaches to the exterior of an existing wall to create a deep cavity for thick insulation — and yes, they are used in New Brunswick, primarily in high-performance new builds and deep energy retrofits where achieving R-40+ wall assemblies is the goal.

The Larsen truss was developed by Canadian builder John Larsen in the 1980s and remains one of the most elegant solutions to a persistent building science problem: how do you get a lot of insulation into a wall without thermal bridging through the studs? A standard 2x6 wall with R-22 batts has an effective whole-wall R-value of roughly R-16 because the wood studs — which run continuously from interior to exterior — conduct heat right through the assembly. The Larsen truss solves this by creating a secondary framing cavity on the outside of the structural wall that is filled with blown-in cellulose or fibreglass, with no continuous wood connecting the warm interior to the cold exterior sheathing.

How It's Built

The truss itself is simple: I-joist-style ladders or site-built frames made from 2x3 or 2x4 lumber are screwed to the exterior face of the existing structural wall (or new structural wall in new construction), projecting outward by 8 to 12 inches or more. These trusses carry no structural load — the original wall does all the work — so they can be made quite deep without engineering concerns. The cavity between the trusses is then dense-packed with cellulose, achieving R-30 to R-50+ in the wall alone, depending on depth. Combined with the original wall's insulation value, total wall assemblies of R-40 to R-60 are achievable. This is Passive House territory.

The vapour and air barrier goes on the interior face of the structural wall — exactly where it belongs in NB's Climate Zone 6. The Larsen truss cavity sits entirely on the cold side of that barrier, which means the insulation can be vapour-open without risk of condensation problems. This is one of the assembly's great building science advantages: it is almost impossible to get the moisture management wrong if the barrier is properly installed on the structural wall before the trusses go on.

NB Applications and Realities

In New Brunswick, Larsen trusses show up in two main contexts. The first is Passive House and near-Passive House new construction, where builders in Fredericton, Moncton, and the Saint John area have used them to hit the aggressive airtightness and thermal performance targets those certifications require. The second is deep energy retrofits of older NB homes — particularly 1960s and 1970s houses with original 2x4 walls and R-12 batts — where the homeowner wants to dramatically reduce a $4,000-per-year heating bill without gutting the interior.

The honest trade-off is cost and complexity. A Larsen truss wall retrofit means removing all the exterior cladding, building and attaching the trusses, dense-packing the new cavity, installing new sheathing and weather-resistive barrier, and re-siding the entire house. You are also moving the window and door planes outward, which requires extending all the bucks and trim. In NB's market, a full exterior Larsen truss retrofit on a 1,500 square foot two-storey home might run $40,000 to $80,000 or more when combined with new siding — it is a serious investment that makes the most sense when the siding needs replacement anyway, or when the homeowner is committed to a near-zero heating energy target.

A simpler alternative that achieves similar thermal bridging elimination at lower cost is continuous rigid mineral wool or XPS over the existing sheathing, typically 3 to 4 inches thick, before re-siding. This gets you to R-35 to R-40 effective whole-wall performance at a lower labour cost than building full trusses, and it is increasingly the preferred approach for NB deep energy retrofits where Passive House certification is not the specific goal.

Is It Right for Your Project?

If you are building a new high-performance home in NB and want walls that perform at R-40 or better with minimal thermal bridging, the Larsen truss is worth serious consideration — especially if you are working with a builder experienced in advanced enclosure construction. If you are retrofitting an existing home, get an EnerGuide evaluation first to understand where your biggest heat losses are. For most NB homes, the attic and basement deliver far better return on investment than a wall upgrade of any kind, and those should come first.

For high-performance new construction or deep retrofit projects, New Brunswick Insulation can connect you with contractors experienced in advanced wall assemblies through the New Brunswick Construction Network — find them at newbrunswickconstructionnetwork.com/directory?category=insulation.