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Exterior Cork Insulation at Emerson St. House

Case Study by Chris van Daalen, Mary Stewart
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Exterior Cork Insulation at Emerson St. House
photo credit Birdsmouth Construction
Building Envelope
Fire Safety
Exterior Insulation
Exterior Cork Insulation
City of Portland, OR
Christopher Pagnotta, Development Services
Benjamin Valentin, Birdsmouth Construction
Josh Salinger, Birdsmouth Construction


From the living roof to its permeable driveway and attached accessory dwelling unit, this new construction, PHIUS+ certified home beautifully promotes Passive House and net-zero energy building. The home’s highly energy-efficient wall assembly includes 3” of expanded cork insulation board on the outside which was initially rejected by the code official as a non-conforming assembly required to be 1-hour UL listed fire-rated assembly. The cork insulation was approved through a code appeal.

Permitting Process

The owner wanted an exemplary net-zero home with an attached accessory dwelling unit (ADU).  The exterior wall assemblies are advanced-framed 2x6 walls surrounded by dense-pack cellulose, covered by a proprietary sheathing system on the outside, 3 inches of cork insulation, and clear cedar rain screen siding (ADU has aluminum siding). Initially the building official said the addition of cork insulation within the UL-fire rated assembly meant it could no longer be deemed a fire-rated wall.  The builder hired a fire protection engineer and appealed the decision, citing Hamarthy’s 2nd rule of fire endurance:  Fire resistance does not decrease with additional layers.  After providing analysis of the fire resistance of the entire wall, and filing a code appeal arguing the alternative assembly will provide equal or better fire resistance, the building official found the UL-fire rated assembly with the addition of 3” of cork insulation is still a 1-hour fire resistant wall (from both sides).

Code RequirementCompliance Path
2014 Oregon Residential Speciatly Code section R302.3 requires 1-hour fire rated walls in all locations which support floor ceiling assemblies which are also required to be fire rated. Code appeal with fire protection engineer report showing equal or better fire resistance. Building official ruled the alternate wall assemble was still a 1-hour rated assembly.

Compliance Details:

Emerson Wall Assembly DiagramTo reach the required R-value for the energy saving standards of Net Zero/Passive House, they designed the alternative 2x6” wall assembly with 3” of additional exterior expanded cork insulation.  It will add R-12 of insulation value to save heating and cooling energy without losing the integrity of its structural capacity and providing equal or better fire resistance.

The assembly consists of 2x6 advanced-framed (i.e. 24” on center) walls with dense pack cellulose insulation in the stud cavity at 3.9 lb / cu. ft. covered with 5/8” Type X gypsum wallboard on the inside. On the exterior side of the stud, they applied 5/8” fiberglass mat sheathing, followed by ½” OSB sheathing with integrated air barrier, vapor control layer, and weather-resistive barrier to provide structural shear strength.  A 5/8” rainscreen batten and siding were attached to the studs through the cork insulation.

In their analysis, the fire protection engineer found the cork insulation would likely contribute some fire resistance but assigned it a value of zero to be conservative.  Because of the attached ADU below the main dwelling unit, the building was considered two units for building code analysis, which meant it had to meet the 1-hour rule from both inside-out and outside-in.  Upon review, the building official found that the 1-hour rated wall assembly with the added insulation met the code requirement of a 1-hour fire resistant wall from both sides.

The Administrative Appeal Board found that the information submitted by the appellant demonstrated that the approved modifications were consistent with the intent of the code; and would not lessen health, safety, accessibility, life, fire safety or structural requirements.  They also ruled and that the unique special conditions of the Net-Zero standard being pursued by the builder make strict application of those code sections impractical.  Thus the code-appeal was approved.

According to Tad Everhart, certified Passive House Consultant, “This was an achievement: addition of eco-friendly insulation does not impair the fire resistance of the other materials in the assembly or the assembly as a whole.  The absence of a negative is still a positive achievement.”

Project Narrative

Initially designed as a retrofit, the original plans had to be scrapped; The black mold coating all the roof sheathing meant the roof had to be replaced.  There was no rebar in the foundation, so that had to be rebuilt.  Even with careful deconstruction, there wasn’t much worth saving.  What did have to be preserved, given the small lot size were the original orientation and footprint, which weren’t helpful.  The long axis runs from north to south.  With these challenges, meeting the Passive House standard required strict adherence to the home’s airtightness goal and optimizing the mechanical equipment’s efficiency.  Thanks to careful design and execution, Birdsmouth Construction succeeded in surpassing its airtightness target.  The exterior wall assemblies which are almost all wood, reflect the owner’s commitment to environmentally friendly building
An advanced heat pump water heater that uses CO2 as the refrigerant delivers both the heat and hot water with an overall coefficient of performance (COP) of 4.5. The home’s new and very efficient heat-recovery ventilator (HRV) achieves a sensible efficiency of 93%. A 6.8-kw solar photovoltaic array on the roof offsets the homes energy usage.

Design / Build Process

The owner bought the property from a local farmer to help the farmer finance improvements they were planning, and to invest in a project she could be proud of. She operates Rooted Investing in Portland, and believes “we need to invest in our own local communities and bring capital back to earth.”  She herself resides in the Emerson Street House ADU, but wanted a full-size home where she could welcome family from out of town, host community meetings, create an art gallery, and give tours to the public to demonstrate cutting-edge green building techniques.  To that end, the builder designed an exemplary project and installed energy and thermal monitors throughout the home so it can continue to serve as an educational resource for the public and builders alike.

Cost / Benefit

Compared to a traditional home, a net-zero energy passive house like this one costs more to build.  However, because it produces as much (likely more) energy than it uses, those added expenses for insulation, advanced triple pane windows and ultra-efficient heating and ventilation systems will pay for themselves in a relatively short period of time. This home was also more expensive than other passive houses, in part because the cork insulation is fairly expensive.  However the builders felt this wall assembly is the simplest, most durable and most effective way to build a passive house in the Northwest’s maritime climate.  They argue this home will achieve “payback” more quickly than other houses because of the 450% efficient heat pump water heater that provides space and water heating to both the ADU and the main home (see other case studies on Sanden CO2 Heat Pump Water Heater).  With temperature/moisture sensors in ceiling, walls and floor, and monitors feeding real-time data on energy use and system efficiency into an online dashboard, they plan to demonstrate a positive cost-benefit while using the home for learning to advance the state of the art. They will use the data to show potential customers, persuade competitors, and even educate their own subcontractors why and how a passive house design delivers benefits and return on investment


The home’s performance achieved an air leakage rate of 0.25 ACH50, and is projected to use heating energy of 6.7 kBtu/ft2/year, with a total source energy of 32.5 kBtu/ft2/year.

Project Contacts
Designer: Ben Valentin, Architect Birdsmouth Construction Approving Offcial: Christopher Pagnotta City of Portland Bureau of Development Services Consultant: David Gessert, P. E., Consulting Engineer Fire Protection Consulting 503-531-8717
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