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A cutting-edge, energy-efficient affordable housing development is on the rise in Lake Oswego.

The 100-unit Mercy Greenbrae complex (formerly known as Marylhurst Commons) will encompass more than 100,000 square feet on the former Marylhurst University campus. It鈥檚 projected to consume 40 percent less electricity that a complex with a conventional design, Mercy Housing Northwest project developer Chris Bendix said.

Passive House certification, which prioritizes significant reductions in energy consumption for both heating and cooling purposes, will be sought for the all-electric project. If achieved, Mercy Greenbrae will become the largest affordable housing development in the Northwest to attain such certification.

Most apartments in the three- and four-story building will have two bedrooms, though some will have either one or three bedrooms. All units will be available to people earning 60 percent of the area median income or less.

The estimated project cost is $39 million. It鈥檚 being covered by Metro affordable housing bond dollars, Oregon LIFT funds, Oregon Multifamily Energy Program funds, 4 percent low-income housing tax credits, long-term debt via Freddie Mac and KeyBank, philanthropy from property owner Sisters of the Holy Names of Jesus and Mary, and a sponsor loan from Mercy Housing.

The Passive House route

This marks Mercy Housing鈥檚 first project in Oregon and its first for which Passive House certification will be sought. Though that increased costs by an estimated 4 to 5 percent compared to a conventional building, Bendix said the team would do it again.

鈥淭hat said, there are costs associated with the program,鈥� he added. 鈥淪o, as much as we will certainly strive to pursue aggressive sustainability goals on all of our projects, sometimes it鈥檚 going to be dependent on our ability to make sure we have the resources necessary to pursue some of the more aggressive sustainability standards.鈥�

The primary costs associated with this approach stem from the need for more advanced systems to meet rigorous energy and air handling standards. To meet the Passive House Institute US (Phius) 2021 CORE metrics, the building鈥檚 source energy demand (the total amount of raw fuel required to operate the building) target is 4,100 kilowatts per hour per person annually. That target is approximately one-third of what an average American household consumes in a year, according to the U.S. Energy Information Administration.

This stringent standard limited the range of potential systems that could be installed in the building, Bendix said. Working with O鈥橞rien360, a certified passive house consultant in Seattle, the team identified the Mitsubishi heat-pump system, a newer product in the U.S. market. The system employs a CO2-based refrigerant 鈥� ideal for producing hot water and maintaining overall energy efficiency. Installation required only four condenser units on the roof, simplifying equipment placement both inside and out.

Another critical component is the air filtration system. The one chosen, manufactured by CaptiveAire, reduces ventilation heating energy by an average of 79 percent, O鈥橞rien360 building performance analyst Ryan Guan said. The system does this by providing balanced ventilation with high energy recovery effectiveness and a packaged heat-pump coil that supplies fresh outdoor air and passive cooling.

Airtight envelope challenge

Without the advanced air filtration system, the building鈥檚 tight envelope could not exist, Carleton Hart Architecture principal Brian Carleton said.

The goal of passive design is to reduce active energy consumption by minimizing heat loss through the building envelope and air exchange. Passive House standards stipulate that any air leakage through the building鈥檚 envelope should not exceed 0.06 air changes per hour at 50 pascals of pressure. That is about 80 percent less air leakage than for a typical Oregon building, Guan said.

One of the primary challenges in designing for Passive House certification is creation of a tightly sealed envelope and to prevent unwanted condensation. Since the building is so airtight, any condensation that accumulates inside will not naturally be 鈥渂reathed鈥� out. Without the proper and robust ventilation, 鈥渢hen you鈥檙e creating a Petri dish inside the building and you鈥檙e going to grow things that you didn鈥檛 want to grow,鈥� Carleton said.

Approximately 93 percent of exterior wall penetrations (excluding the roof) were eliminated from Mercy Greenbrae to achieve the highly airtight envelope, estimated Rob James, HVAC director for Colebreit Engineering. This was accomplished via recirculating kitchen hoods, and bathroom and kitchen exhausts and ventless dryers that connect to just nine rooftop energy recovery ventilator (ERV) units.

Also, the Hunter Panel roof system will allow for open attic spaces to accommodate all air ducting services without breaching the building鈥檚 envelope and the worry of having any air leakage, Carleton Hart Architecture job captain Nate Latigue said.

Lessons learned

The project is Carleton Hart Architecture鈥檚 first foray into Passive House certification. While the firm is familiar with Leadership in Energy and Environmental Design (LEED), Earth Advantage, and other sustainable certifications, team members have been eager to complete a Passive House project, Carleton said.

鈥淚f you鈥檙e not pursuing this kind of rigorous certification,鈥� he said, 鈥淚 don’t want to use the term lazy, but you can get passive about it 鈥� pun intended.鈥�

Carleton Hart Architecture will look to design more airtight buildings in the future, Carleton added, as long as the necessary robust ventilation system can be funded.

The heat pump system, HVAC system and energy-saving lighting were takeaways for Bendix. Even lights that automatically dim to 50 percent power when spaces are unoccupied can significantly contribute to energy savings and cost reduction, he said.

What鈥檚 next

General contractor Walsh Construction is expected to finish work on Mercy Greenbrae in spring 2024. Upon completion, the anticipated Passive House certification would follow within three to six months, Bendix said.

Future developments on the ex-university campus and uses of existing buildings are still to be decided. However, Sister Maureen Delany of Sisters of the Holy Names of Jesus and Mary confirmed that no plans exist for additional housing projects.