Designing for the Extremes: Carleton College's 11 Phius Townhouses

By Jay Fox

Architect Kim Bretheim first started working with Carleton College about 40 years ago. At the time, his firm, Paul Madson & Associates (which was acquired by LHB in 2002), was hired to assess the school's off-campus housing and to develop a plan that preserved some buildings, renovated others, and saw the construction of new housing where appropriate. Four decades and approximately 15 projects later, the last phase of that long-running transformation is wrapping up. Bretheim, now retired, did not expect it to culminate in Passive House design, but he is not surprised that it did. Carleton has always had aggressive environmental goals (as documented in the Accelerator’s previous coverage), and the school's commitment to reducing its carbon footprint made the eventual adoption of Phius standards for two sets of student residences feel less like a leap and more like a logical progression in a decades-long story arc.

In total, Carleton has now built eleven townhouses to Phius standards. While this is an achievement in itself, the buildings are also remarkable from a technical standpoint because of the climate in which they have been built. Carleton is located in Northfield, Minnesota, which is around 45 miles south of Downtown Minneapolis and comfortably in Climate Zone 6. No surprise, the area sees winter temperatures plunge well below zero. However, what many may not realize is that the summer extremes in the Upper Midwest can be just as punishing. Daytime highs frequently climb into triple digits and can feel especially suffocating due to the humidity. Designing a Passive House in a climate dominated by either heating or cooling allows a team to focus its energy on one set of targets. When both extremes need to be considered during design, there is no such luxury.

Modeling as a Balancing Act

Molly Eagen served as the energy consultant for the townhomes, and she holds the distinction of having taken all eleven through full Phius certification. Eagen has an academic background in architecture and began her career in a conventional design role but soon found that her passion was doing environmental simulation work. She later founded Cause, a sustainability consultancy currently based in Oregon.

While modeling is an extremely technical skill, she says that she strives to teach design-integrated solutions to the architects with whom she works so that it becomes less siloed. “I really try to engage with design teams in a way that builds fluency and helps architects learn the pieces of this so they can take those [lessons] on to the next project,” she says. In this case, Eagen did not have to introduce LHB to the concept of Passive House because they are one of the few firms in Minnesota to have worked on a Passive House multifamily project before.

While Eagen has worked all over the country, she became very familiar with the climate of Minnesota because she spent ten years working there and “unintentionally” developed a specialty for cold climates. "In climates dominated by heating or cooling, you get to focus on making sure you meet one set of targets," Eagen explains. "But for this project—and all projects I've worked on in the Midwest—you never quite know which performance target is going to be the most challenging to meet. It varies project to project, and you have to have your eye on all of it.”

The challenge shaped nearly every design decision. If the team over-shaded or specified glazing with too low a solar heat gain coefficient, they risked falling short on heating targets by starving the buildings of beneficial solar gain during the winter. Tilting too far in the other direction and prioritizing winter warmth would blow past cooling demand limits during the summer. 

Eagen notes that strategies that are truly passive, like landscaping with deciduous trees that shade during the summer and allow solar gain during the winter, had to be calibrated precisely given the extremes between winter and summer design temperatures. Extended eaves and porch elements also helped optimize solar gains for the appropriate season, aided by glass specifications that Eagen says became the primary tool for maintaining balance.

Surprisingly, she had never taken a project through full certification until starting work on the Carleton townhouses, which also happens to be the largest Passive House project that she’s worked on. ”I feel like I entered the world of Passive House design and consulting through the Carlton project,” she says. She also notes that LHB had worked on a Passive House multifamily building prior to the Carleton project, which ultimately helped create solid foundation of trust among the team and with the client, even if their previous project was very different from the townhouses.

“ I think the architect having some experience definitely helped build confidence,” Eagen says.

The team used Alpen Zenith ZR06 thin triple-glazed windows with three tiers of low-e coating (high-performance doors were supplied by Therma-Tru). The solar heat gain coefficient (SHGC) ranged from 0.40 for high-gain glass down to 0.17 for solar control glass. All three configurations were deployed across the project, and Eagen's team ran studies on each building, testing every possible combination of low-e coating against the Phius thresholds to identify the optimal selection for each facade.

"We had to find a way to keep every other parameter the same and just change this one factor and really dial in the specifications of glass on each facade of every building," Eagen says. “If we had modified every parameter, such as selecting a slightly different wall assembly and slightly different mechanical design to optimize each unique condition, documentation would have been a mess.”

Though this approach simplified the modeling, Eagen says there were still 38 Phius review cycles to account for all eleven buildings and 14 separate energy models due to several mixed-use conditions.

Improving Upon the Familiar

Bretheim spent much of his career doing multifamily affordable housing, and that background shaped his philosophy for the Carleton assemblies. This philosophy was also shaped by a visit to Vancouver early in his exploration of Passive House. While there, he noted that the Canadian teams were designing envelopes that looked familiar to framers rather than anything radically new.

As Bretheim explains, this strategy of improving upon the familiar allows for a conversation. Conversely, asking a team to work with unfamiliar materials, build something that breaks with convention, and aim for performance levels they don’t normally strive to hit results in stonewalling. "People just add zeros to the cost," Bretheim says. "So, you design things to get the results of a tight envelope without…designing based on products built elsewhere that are pre-certified. Instead, we tried to do stuff locally that kind of looks familiar but results in a tighter envelope."

For Bretheim, the answer was double-stud assembly. The outer wall is built to be very tight, with fluid-applied membranes bridging cracks in the framing and rough openings. The interior side includes a service cavity that lets electricians and plumbers run their systems the way they are accustomed to doing without risking damage to the air barrier. Bretheim notes that this approach solves one of the most notoriously difficult aspects of coordination on a Passive House project: preventing accidental or unanticipated penetrations.

True, on a single-family Passive House project with a handful of people on site, it may be feasible to keep everyone vigilant about every penetration. Alternatively, on a multifamily project with an enormous budget, hiring a full-time person tasked with protecting the envelope (often known as an “air boss” or “skin doctor”) is feasible. Bretheim believes having that level of oversight on a mid-size project spread out across multiple buildings is unrealistic.

The above-grade wall assembly for the townhouses consists of Hunter panels—plywood bonded to four inches of polyiso—that form the exterior insulated sheathing, with a liquid-applied weather-resistive barrier on the plywood surface. Moving inward, high-density fiberglass batts fill a two-by-eight wood stud cavity. The balance between exterior continuous insulation and cavity insulation was calibrated to keep the dew point outside the stud cavity, a critical consideration in Climate Zone 6. The interior wall is metal stud framing and creates the service cavity. The interior finish is gypsum board, while the cladding is fiber cement panels with some metal accents. The effective R-value of the wall comes to R-43 (see Figure 1).

Below grade, insulated concrete forms (ICF) from Fox Blocks create the foundation walls with a total effective R-value of R-31. The ICF system uses modular layers of two-inch EPS on both sides of a six-to-eight-inch concrete core, with six inches of continuous XPS beneath the slab to create an unbroken thermal barrier around the partial or walkout basements (which do contain some student residences). Above, the roof assembly is R-70 and consists of 20 inches of blown-in cellulose within a vented open-web attic truss system, supplemented by two inches of XPS on the interior side to mitigate thermal bridging through the framing. Eagen notes that the air control layer is on the interior throughout the entire enclosure.

While this is a far more robust assembly than is standard for the area, it accepts the way people are used to building, Bretheim says. As an analogy, it’s a new dialect but the core language is the same. What does come as surprise to those working on the site, Bretheim says, is the amount of goo they’ll need for air sealing. “They'll use more goo than they'll ever think they could have to. It's just a lot of goo."

Learning by Building

Unlike LHB, the construction team from Terra Construction had no prior experience with Passive House. This was expected, as the building standard is quite rare for the region. To help build confidence, several members went through Phius builder training before work began, and Alpen sent representatives to walk crews through the window installation process on the early buildings. The triple-glazed units were heavy and the fluid-applied membranes around the rough openings reduced clearances, requiring the team to learn early that openings in the panelized exterior envelope needed to be slightly larger than they might have assumed.

What made the project unusual as a learning experience was its sheer repetition. With eleven buildings constructed in sequence, the first few served as live training exercises, and the team conducted intermediate blower door tests at various points during construction to catch and correct issues before they became embedded habits. What they were correcting were not catastrophic problems. Instead, they were learning the granular lessons that accumulate when a team works through a process for the first time, and they are precisely the kind of knowledge that does not get unlearned on future projects.

"The first building, you learn a lot of lessons," Bretheim recalls. "With the second building—not much to learn. Eventually, it just got very ordinary."

The data corroborates Bretheim’s observation, as airtightness improved steadily from building to building. By Phase II, two buildings needed to hit more stringent airtightness goals than the minimum required to meet their performance targets, and the crew achieved it without difficulty.

A Campus That Powers Itself

The mechanical systems in the townhouses benefit enormously from Carleton's geothermal district-scale energy infrastructure. The campus-wide system is one of only a handful among higher education institutions in the United States, and Eagen notes that it is the first of its kind in Minnesota. It uses a geo-exchange field paired with water-to-water heat pumps, and the system accounts for approximately 70% of the college's heating and cooling needs. Condensing boilers supplement the system during three to four months of deep winter, but the college is actively working to expand the geothermal field and eventually take those boilers offline.

“It’s really impressive,” Eagen says. “[Carleton] has been an incredible leader in demonstrating what these systems can look like on a campus.”

Every townhouse ties into this district system for space conditioning and domestic hot water. There are no gas-fired devices in any of the buildings (including the kitchens) and no connection to the public gas utility. Fresh air ventilation is provided by RenewAire EV Premium L energy recovery ventilators (ERVs), which deliver sensible heat recovery rates between 77% and 83%. Eagen adds that the performance rivals far more expensive systems while remaining a cost-effective solution for the region.

The college also generates a significant share of its own electricity. Two wind turbines (1.65 MW and 1.6 MW) supply approximately 40% of campus electricity, and the six Lilac Hill townhouses were fitted with photovoltaic (PV) arrays averaging 40 kilowatts per building (roughly 110 panels each) that produce an estimated 36,000 kilowatt hours per year. That output essentially matches the energy consumption of the Lilac Hill site, making it a net-zero development in practice if not in official designation. The Union Street buildings were not designed for rooftop solar, as the college preferred to treat renewable energy generation as a campus-wide initiative rather than a building-by-building mandate. Eagen adds that the PV generated at Lilac Hill is banked and shared with Union Street to meet Phius source energy compliance.

The Financial Logic for Institutions

The conversation about whether Passive House design is worth the upfront investment changes considerably when the owner intends to hold the property for 50 or 100 years, as is the case with hospital systems, municipalities, and colleges and universities. The owners of these buildings will be paying the heating and cooling bills for decades. An upfront cost premium that might give a speculative developer pause becomes far easier to justify when the operational savings compound over that kind of timeline.

Carleton's administration did not need to be convinced. As Eagen puts it, "It was an easy sell." The school's existing investment in a geothermal district system and its own renewable energy generation meant that every new building is plugged into an increasingly efficient infrastructure. The Passive House envelope ensures that the buildings demand as little energy as possible from that infrastructure, meaning more buildings can eventually be plugged into the grid without a need to expand it.

Enhanced durability is another financial benefit of high-performance building. Passive House buildings are more capable of adapting to a changing climate and are resilient in the face of extreme events. Futureproofing against more stringent codes is yet another consideration when framing total costs over an extended timeframe. Very simply put: That kind of prescience saves money in the long run.

“It’s partly responding to the goal in terms of building longevity and is it realistic to build residential buildings that can be modified over time,” Bretheim says.

Bretheim points to the University of Washington as a counterexample. Approximately 20 years ago, the school reached the conclusion that it needed to replace thousands of beds of 1960s-era concrete residence halls because the old buildings simply could not accommodate the systems and community configurations needed today. Concrete got them nothing in terms of adaptability and barely 60 years of use. The Brutalist student residences have already been demolished. In their place now stand (or will soon stand) more efficient and wood-frame residences. In the case of the new Haggett Hall, which is estimated to be completed in 2027, the 230,000-square-foot building will be all-electric and built using mass timber to avoid the mistakes of the past.

Advice From the Modeling Trenches

For practitioners considering their first Passive House project, Eagen offers a perspective shaped by the scale and intensity of the Carleton townhomes. With 14 energy models and 38 review cycles, efficiency in the modeling workflow was more an act of survival rather than mere efficiency.

Eagen credits Ed May's Grasshopper plugins, part of the Ladybug Tools ecosystem, with saving hundreds of hours over the course of the project. "WUFI modeling is time-consuming and clunky," Eagen says. "I used Ed May's Honeybee-PH Grasshopper plugins that are specifically designed to auto-populate WUFI inputs within Grasshopper, so I was able to export different versions of the geometry into WUFI without having to start over every single time. Those open-source tools are really game-changing."

Her other piece of advice is to identify the worst-case building early and use it as a benchmark. On both the Lilac Hill and Union Street sites, the smallest buildings presented the tightest margins because of the smaller internal heat gains. (Eagen notes that overheating due to internal gains may be a major concern within a Passive House high-rise in New York, but the same does not hold true for a townhouse in Minnesota.) By confirming early that the team could meet Phius thresholds on those worst-case scenarios in the smaller buildings, they were able to move through the design process for the remaining buildings with confidence.

Building Communities That Happen to Be Efficient

Bretheim is careful to distinguish between the Passive House performance of the buildings and the community design that gives them meaning. The spaces between the buildings, he says, are just as important socially as the spaces inside them. At Lilac Hill, front porches link the townhouses in an informal circuit reminiscent of an academic quad, with gathering spaces tucked into the landscape. At Union Street, the townhouses sit within a traditional streetscape, and many of them house culturally affiliated student groups alongside dedicated common spaces so that community activities do not have to occur within the residential portions of the buildings.

The goal was to build homes rather than dormitories and to create spaces that feel familiar (or perhaps familial) rather than institutional. Bretheim here returns to the idea of balance. On the one hand, students should have personal space and privacy. On the other, design needs to encourage community-building and discourage seclusion and isolation. Ideally, the students who live in the townhouses for a few years will carry with them the experience of what a comfortable, efficient, and thoughtfully designed community feels like. That some of those students might not even realize the technical sophistication of the envelope surrounding them is, in a sense, the highest compliment the design team could receive. The building is familiar enough to feel like home rather than a science experiment.

This effort to maintain something familiar reverberates throughout the project. Carleton's townhouses demonstrate that Passive House design in severe climates doesn’t require exotic materials or radical departures from conventional construction. It can rely on modified versions of familiar assemblies, provided those assemblies are backed up by careful calibration and hours of modeling work combined with a willingness to learn through repetition. For institutions weighing whether the investment is worth it, the answer from Northfield is clear: building for the long-term means building the kind of communities that people actually want to live in, especially when they feel like home.