Sky High R-Value with no Foam – Larsen Truss Walls

by Erik North on July 6, 2012

Larsen truss wall


Sometimes the homeowner conversations during audits wander into what could be done with a home as opposed to the best ROI or greenest options. It’s difficult to significantly upgrade wall R-value without a major project.

Interior solutions involve gutting and re-insulating wall cavities or drilling holes for injection foam which wouldn’t substantially upgrade walls with existing fiberglass (at least not in any cost effective way). Some contractors in our neck of the woods have become pretty adept at stripping fiberglass out of the walls and replacing it with dense pack cellulose. In any case, not many homeowners want to tear up the inside of their house. So what external retrofit options are there which can substantially improve R-value and efficiency?

One approach is the Larsen Truss system. This is an exterior upgraded insulation approach that can be used in new construction and retrofits. It is greener than spray foam or exterior foam board approaches, and can achieve truly stellar R-values compared to standard framing.

What is A Larsen Truss Wall?

First off, I have to point toward this article by Robert Riversong on the modified Larsen Truss system he’s been building for 20 years. It’s a great technical breakdown of the advantages of this green super insulated construction.

There are many variations of truss wall construction which may contribute to its relative scarcity: lack of standardization makes it less repeatable, contractor to contractor. It’s not like standard framing where just about anyone worth their hammer has thrown up a few walls. But I’m getting ahead of myself.

Here’s the Wikpedia page on trusses, just to save a few lines. Truss system frames attach an exterior non-load bearing wood truss to a load bearing interior vertical wall with regularly spaced horizontal gussets. This extended space allows greater levels of insulation and minimizes thermal bridging. Larsen Truss systems attach pre-assembled trusses to the building exterior, allowing them to be used for retrofits of standard framed homes.

R-Value – Truss system walls are one of those approaches which allows truly spectacular R-value. The whole truss depth may be 8-12 inches, which when dense packed with cellulose means you’re likely north of R-30. Larsen Truss retrofits add about 7 inches of insulatable space to standard framing; again, meaning you’ll be over R-30.

Thermal Bridging – Truss framing systems have minimal thermal as the interior and exterior wood strips of the truss are spaced apart and that space is insulated. There is minimal bridging through the connecting horizontal gusset. Exterior retrofit Larsen Truss systems can further minimize thermal bridging by staggering the trusses and the house’s vertical wall studs.

This minimal thermal bridging allows a greater whole wall R-value than standard framing of comparable thickness.

Green – Larsen Truss and other truss systems are among the greener building options for retrofits and high R-value walls. The depth of the wall cavities allows tremendous R-value without resorting to petroleum-based foams. Cellulose use reduces overall embodied energy, and introducing advanced framing or modified concepts can control total wood material used.

Air Leakage – The truss framed wall doesn’t have an inherent air tightness advantage, other than being a really thick wall. But standard practices with its construction include air tight dry wall approaches and dense pack cellulose – both of which considerably tighten the building enclosure. Similarly, a retrofit exterior truss doesn’t necessarily produce a tighter building, but introducing 8 inches of dense pack cellulose would.

Structural – There are a few structural advantages as well. Truss systems don’t sacrifice any interior space to insulation. The very wide wall cavities make service installations a snap. Compared with the main retrofit option, exterior foam board, truss systems are much stronger, providing vertical studs for attaching siding.

Some green builder may not like to hear it, but there are drawbacks to Larsen Truss and truss framed buildings. All of these drawbacks can be addressed, but almost always that solution is : ‘experienced work crew.’ Leading us to …

Ease of Installation – Ease of installation may not be the exact correct phrase, but gets at the issue. Issues of cost, structure, moisture control and construction speed can all be leavened or eliminated with a work crew experienced in truss frame construction. It’s just that that is relatively uncommon compared to the army of guys who can toss up a standard stud wall. The green building advantages are obvious, but equally obvious is the comparatively small number of expert builders.

Material Use – Truss systems in their standard conceptions (I know I said earlier there was no standard; bear with me) use more material than standard framing; sometimes a lot more. This can mean added expense and added construction time. There are many variants that help address this: omitting exterior sheathing, adopting advanced framing concepts like 24 inch on center or engineered joists, all of which can reduce materials. Retrofit trusses would, of course, all be additional materials, but in the pursuit of high level wall performance.

Cost – The added depth and extra framing member (the exterior truss board) means extra insulation and wood. And that means extra cost. There are work arounds, like I’ve alluded to a few times. Omitting exterior OSB or plywood sheathing or adopting advanced framing approaches to minimize wood can take the edge off the construction and material costs.

Moisture Control – Like foam board exterior systems, truss and retrofit Larsen truss require a high level of attention to moisture control details. Tyvek or Typar or whichever weather resistant barrier you choose needs to be carefully detailed, along with window, door and joint flashing (or fully adhered peel and stick membrane, if we’re gettin’ fancy).

The bigger issue is if a leak or water infiltration occurs. A foam board retrofit would get damp, which might invite a few bugs (they like the wet). A cellulose insulated truss retrofit would handle water fine … to a point. At around 30% moisture content, cellulose goes to mush.

Structural (the bad) – Truss retrofits add 6-8 inches to the exterior of the house. This adds a ton of insulation, but you need a lot of roof to work with. If the overhang is short (or non-existant), you’ll need to extend the eaves and overhang to accommodate the added width.

As building and energy codes improve, alternates to standard framed houses are being explored. Larsen truss systems are one green option that produces very good insulation levels.

More Resources:
Build It Solar’s page on Larsen Truss systems

Green Building Advisor’s page on Larsen Truss Walls (Includes an interview with John Larsen)

One homeowner’s experience building a Larsen Truss wall

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