Why use a Larsen truss wall?

As I explained in the Project Overview, this house employs an interior, site-made Larsen truss to create a double wall system that will virtually eliminate thermal bridging through the building envelope.

Green Building Advisor explains thermal bridging as follows: “Thermal bridging occurs wherever assembly components with low R-values relative to surrounding materials span from the inside to the outside of a building assembly. Thermal bridging takes place in wood-framed assemblies because, although wood is a pretty good insulator at about R-1 per inch, it is at least three times more thermally conductive than any cavity insulation, which start at about R-3.5 per inch.”

The Passive House Institute in Germany found that thermal bridging accounted for almost all of the performance discrepancies between modeled performance and measured performance in the early days of Passive House development. Despite this and other similar findings from North American building scientists, conventional construction here in the states continues to grossly underestimate the huge potential that thermal bridging creates for heat loss, and also for condensation and mold growth. (The foundation detail discussed in a previous blog post was also developed under the same reasoning.)

The site-made Larsen truss starts with the construction of simple cleats made from scrap lumber and plywood attached to 2×3 interior framing members. The 10” thick truss cavity sits inboard of the OSB air barrier as shown in the diagram. This allows for a continuous layer of cellulose insulation to be blown in, achieving an overall R-value of 61 for the walls when you add in the cellulose insulation in the 2×6 structural stud wall.

Walll Section

At the floor plate, the TJI framing does penetrate the Larsen truss, but stops short at the OSB air barrier where it is hangered into the 2×6 structural stud framing, in order to maintain the continuity of the air barrier. The space between the floor trusses is then baffled and insulated with cellulose.

In this photo you can see the space that will be filled with dense-pack cellulose insulation.

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Some may ask why we would go to all this trouble rather than using the sheets of EPS foam we used for the floor and foundation. The answer is to do with global warming potential, flammability, and health. According to an analysis by Alex Wilson on GreenBuilding.com regarding the global warming potential of various insulation materials, cellulose is the most benign by a substantial margin. It is almost 100% recycled and also out-performs other common house insulations for flammability. Of course, due to the capacity of cellulose to absorb moisture, we would never use it below grade. But, above grade, it is the best solution for addressing the multiple goals of reducing environmental building impacts in a cost-effective way.

The other part of the answer has to do with health. All foam insulation formulations contain toxic ingredients – bio-accumulative toxicants, carcinogens and endocrine disruptors. These substances can escape during manufacture into the environment and after construction inside the house. We use the correct grade of EPS (expanded, NOT extruded, polystyrene) limited sparingly to only where needed (below grade) because currently it is the most benign of the foams. For more, check out this blog.

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Location

Lanesboro, Minnesota
Climate Zone 6 (cold/moist)
Latitude: 43° 44' 18'' N
Longitude: 91° 54' 48'' W

House Size

Net Treated Floor Area: 1,514 SF
Gross Square Footage (House only): 2,210 SF

Building Envelope

Roof: R-99
Wall: R-61
Ground: R-53

Windows & Doors

Glazing: U-0.10 BTU / hour / sq. ft.
Solar Heat Gain Coefficient (SHGC): 0.48”
Frame: U-0.19 BTU / hour / sq. ft.

Modeled Performance

Specific Primary Energy Demand (Source Energy Demand): 12.1 kBTU / sq. ft. / year

Specific Space Heat Demand: 7.0 kBTU/sq. ft. / year

Peak Heating Load: 7,047 BTU / hour

Space Cooling Demand: 0.44 kBTU / sq. ft. / year

Peak Cooling Load: 3,625 BTU / hour

Pressure Test Goal: Whole House Air Changes Per Hour (ACH) = 0.4 ACH 50

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