Lessons Learned: THERM and an Evolution of Wall Assemblies

By Mike Meade

R-value measures the ability to prevent heat transfer. The higher the number, the better.

I attended a training from Passive House Canada on THERM software, which was developed at the Lawrence Berkeley National Lab for evaluating heat transfer through building components. Using THERM, you can model 2-D heat-transfer effects in components at building interfaces like windows, walls, foundations, roofs, and doors. Heat-transfer analysis allows you to evaluate a product’s energy efficiency and local temperature patterns, which may relate directly to problems with condensation, moisture damage, and structural integrity.

After the training, I was hungry to share the software and give our project teams a better way to evaluate details for thermal performance. I also thought about buildings I have built over my career. I’ve worked on buildings with many types of walls — some just to meet code, some just to meet the budget, and some to create the highest possible performance. I used THERM to evaluate these assemblies to see how we fared.

R-value measures the ability to prevent heat transfer. The higher the number, the better.

(1) In the early 2000s, I worked at a firm where we built many buildings with wood-frame walls filled with batt insulation. These walls were built to the UBC code minimum, not for any specific high-performance goal. It turns out that this basic wall assembly performance isn’t bad!

  • Good: The overall R value of the wall is approximately R-19.7. This is largely because the wood framing and sheathing are not highly conductive.
  • Bad: The dew point is frequently within the wall, which could cause condensation on the back of the sheathing in the cold months.

(2) Eventually, our projects switched to metal stud framing to allow for quicker construction, flatter walls, and less wood product waste. These walls were also built to the UBC code minimum but provided a better wall for the contractor and owner to work with.

  • Bad: The R-value for this assembly drops significantly to R-14.7. This is due to the heat transfer through the metal studs. There is a significant cold area on the backside of the gyp, which can cause condensation deep into the wall and cold spots on the interior finish.

(3) At a new office in 2007, I learned about pushing the envelope both for cost and performance. My first experience with exterior insulation was a tower in downtown Portland. The assembly was based on 6″ metal studs with 2 1/8″ of XPS insulation outside the sheathing.

  • Good: The wall was quick to construct and led to a very good rainscreen construction. Thermal bridging was reduced inside the stud cavity due to being isolated from the thermal insulation.
  • Bad: The total R-value for the assembly continued to fall to R-12.3. While the studs are no longer the great thermal bridge, the total R value for the exterior insulation was only R-12.

(4) The next project took the lessons learned from the tower and expanded on them. The system was largely the same, but instead of supporting the exterior skin of the building on continuous “z” furring, we used a homemade thermally broken system. A neoprene washer was inserted between two L shaped pieces of metal to make our cladding support Z.

  • Good: This wall assembly picks up a small amount of performance over the straight furring approach. Total thermal performance ticks up to R-12.6. This is probably slightly unfair, as I modeled the detail at the connector. Further away from the connector, the wall likely performs even better.

(5) In 2012, I worked on an elementary school under the newly adopted Oregon Structural Specialty Code, which for the first time required continuous exterior insulation. This project had a very conservative budget and would be built to the code-minimum required performance. The wall assembly was wood-frame construction with fiberglass batts inside the stud cavities. We applied two inches of exterior mineral wool across the exterior of the building.

  • Good: Thermal performance rises quite a bit from my previous two high-performance wall attempts to R-19.7. A large part of this is due to the wood stud framing, as it largely reduced thermal bridging.
  • Bad: Having a hybrid wall carries the risk of bringing the condensation plane inside the stud cavity, where it could cause issues.

(6) In 2016, I started working on the PDX Concourse E Extension. We have designed a robust wall and are benefiting from the advancement in thermally broken cladding support systems. We are using metal stud framing with 4″ of exterior mineral wool insulation.

  • Good: This system is a robust wall — the fiberglass support system on the exterior transmits very little heat. Thermal performance for the system is up to R-23.8. The design of the wall places all moisture control outside the air barrier. The inside of the stud cavity is empty, which allows us to run systems in the exterior wall without a large detriment to thermal performance. If maintenance staff need to get into the wall, they don’t have to worry about moving and replacing insulation.

Over the years, there have been massive advances in science and technology that are allowing us to create better buildings for our clients and building occupants. It is great to be able to use software to better understand and evaluate the judgments we have made — and by better understanding our past, we can understand the buildings of our future.

Mike is a senior project architect with 18 years of experience and is a member of our in-house building enclosure committee, which provides envelope resources and technical support to our project teams. He sits on the board of the Portland Building Enclosure Council and is currently working on the PDX Terminal Balancing and Concourse E Extension project.