Thermal Break Shear Wall at Sage Green

Oregon High Performance Homes Certified
Energy Star Certified

In support of Green One’s effort to develop the net-zero energy model for the homes, ODOE and the Energy Trust of Oregon initially suggested a high performance wall assembly with rigid foam sheathing added to the outside of the wall assembly to create a continuous thermal break. Yet, this method complicates numerous other related processes and is costly.

For example, adding rigid foam exterior to the otherwise conventional wall meant the foam would most likely be installed by the siding contractor after the walls were “tilted up” rather than being installed by the framer while the walls were still flat on the floor.  Building walls flat is a common practice that makes the process safer and more economical. Exterior foam was a complicated prospect with potential for mistakes and guaranteed higher cost.

In the course of working through challenges with exterior foam, the project team had a breakthrough idea of having the framer install 1.25” EPS foam board between the lumber frame and plywood sheathing  instead of on the outside, which resolved many of the problems with the earlier wall concept. In the builder’s own words, “I was just trying to get a thermal break into the wall assembly at the least cost. Having the framers put rigid foam between the frame and the ply looked like the quickest, simplest way: work is on flat walls, and it doesn’t disrupt established framing or other envelope practices.”

With this solution they could assemble the layers together on top of the framing, and then tip the wall up as a complete panel. Walsh notes  “The one exception is transference across floor lines: we had to use threaded rod and back-to-back hold-downs in the frame cavities instead of the typical flat strap on the outside.”

To prove that inserting foam between the framing and shear panel would not degrade the lateral capacities of the assembly, structural engineer Scott Nyseth was engaged to prepare structural calculations for the wall, who at the time worked for Miyamoto International.  For the system to work, Nyseth said sheathing would need to be nailed every 3” on the perimeter, rather than the standard 6”.  Then the team contracted with Dr. Rakesh Gupta at Oregon State University’s Knudsen Wood Engineering Laboratory in Corvallis to perform destructive testing of a series of wall panels of various foam thicknesses.  The Knudsen team performed ASTM-E72/CUREE-style cyclic lateral load testing (ie: commercial code standard), in conformance with ICC Evaluation Service Approval Criterion 130, on September 14, 2009

The Northwest Energy Efficiency Alliance Report #E16-296  includes a cost analysis comparing several different wall assemblies.  It concludes that “the 2×6 TBS wall assembly consistently offers better ‘bang for the buck’” as a strategy to improve thermal performance without disrupting framing contractors’ familiar construction process.  Compared to standard 2×6 framing, the 2×6 TBS Wall yields 32% lower heat loss (i.e. U-value) and 47% increase in insulative R-value.  These gains were achieved for about 65% less [incremental] cost than the exterior foam assembly which performs only slightly better that the TBS wall.  Labor to install the additional layer of foam added just $0.25 / square foot; and materials added $0.50 / square foot, adding up to an incremental cost increase of $1,200 per home.

Building the wall requires double the number of nails which increases heat loss by 2% compared to the exterior foam wall.  Yet because of the nails it also achieves greater airsealing of the assembly, stopping air leaks due to the slight compression of foam between the frame and sheathing.

• 2010 Oregon Structural Specialty Code