Building a net-zero, solar home in Fraser, Colo. — one of the coldest places in the contiguous U.S. — students at the University of Colorado, Boulder, incorporated foot-thick, hyper-insulated walls, sealed on the outside with pine tar, which was originally used for waterproofing on skis, according to student Charlotte Mitchell.

The SPARC (Sustainability, Performance, Attainability, Resilience and Community) house is one of nine net-zero homes currently being showcased online as part of the Department of Energy’s virtual Solar Decathlon. The homes — all designed and built by student teams at universities in the U.S., Canada, Chile and the Netherlands — provide ample evidence that building super-efficient, comfortable homes that generate as much, if not more, energy as they consume is now possible, affordable and replicable.

“Buildings play a huge role in meeting the [Biden] administration’s goals for addressing climate change, because we can’t transition to this [clean] economy or a carbon-free power sector without reducing the carbon footprint of the building stock,” Kelly Speakes-Backman, principal deputy assistant secretary of energy efficiency and renewable energy at DOE, said at a media preview of the decathlon on Monday.

Since its inception, more than 20,000 students in the U.S. have participated in the biennial decathlons, producing a growing cohort of green architects, engineers and designers, decathlon Director Holly Carr said.

This year’s decathlon, DOE’s ninth since 2002, was originally scheduled for June 2020, with individual college teams building their homes as part of a “solar village” on the National Mall in D.C. Past decathlons have also brought solar villages to California and Colorado, with international spin-offs later this year in China, India and the Middle East.

The COVID-19 pandemic forced a delay and rethinking of the U.S. event, Carr said, with teams building their designs locally — a change that will become permanent and has perhaps allowed the homes to have a greater impact. For example, the SPARC house may become a model for affordable housing in Fraser, said Kristen Taddonio, who is now living in the house and is a board member of the local electric cooperative.

“Because of the cold climate, the build season is really short, and with a universal skilled labor shortage, the focus of development has understandably shifted to high-return luxury second homes,” said Mitchell, who is in her fourth year at UC majoring in architectural engineering. “Additionally, building materials are often less accessible and far more expensive in remote mountain towns, and these factors together drive land and property prices way up and push people out.”

Similarly, in Ogden, Utah, students from Weber State University partnered with the city to build a craftsman-style, net-zero home on a vacant lot. The 2,450-square-foot, six-bedroom home is now occupied, and the city has asked the university to work on net-zero homes for two more vacant lots, said Jeremy Farner, the faculty adviser for the school’s decathlon team.

Green Building Trends

Green and net-zero building no longer occupies the architectural niche it once did. President Biden’s $2 trillion infrastructure plan includes $213 billion to be used for affordable housing, including building or retrofitting “more than a million affordable, resilient, accessible, energy-efficient and electrified housing units,” and more than 500,000 homes for low- and middle-income buyers. (See Biden Infrastructure Plan Would Boost Clean Energy.)

California now requires all new residential construction to be net zero, with commercial buildings to follow in 2030. Denver in January released its guidelines for new buildings in the city to be net zero by 2030. The guidelines define net zero as energy efficient, all electric, powered by renewable energy and providing demand flexibility for the grid.

The local homes built for the Solar Decathlon almost all fit that description. They are judged on a broad range of criteria, including architecture, affordability, performance, occupant experience and, for the first time this year, their carbon footprint, not only of the structures themselves but all their materials as well. The level of thought, detail and creativity that goes into each house is impressive.

The team at the University of Nevada, Las Vegas designed their Mojave Bloom home specifically for military veterans recovering from either PTSD or traumatic brain injuries. An open layout provides clear sightlines inside and outside the home, team member Ryan Mantei said. Acoustically absorptive ceilings with extra insulation cut down on sudden noise or echoes that could be triggering for the resident, and an internal hydroponic green wall improves air quality and brings nature inside.

The University of Denver is working on the decathlon’s first net-zero retrofit of a 1950s ranch-style house. The project is located on a 100-year flood plain, so the rebuild is mixing energy efficiency with Federal Emergency Management Agency flood plain guidelines.

The variety of styles and locations notwithstanding, the decathlon homes share some common design features, providing a glimpse of evolving trends in green building. Several teams used prefabricated components, like insulated walls, to cut material and building costs. The prefabricated walls of the SPARC house were installed at the site in two days, Mitchell said.

Electric heating and cooling using heat pumps and electric, ductless water heaters were also installed in a number of the projects, along with finished concrete floors that retain heat to help keep homes warm in the winter and cool in the summer. Rooftop solar panels, generally around 7 kW, were ubiquitous, often coupled with storage. The Weber team’s house pairs its solar with a programmable heat-pump water heater that can be used for load shifting, Farner said.

While events like the Solar Decathlon create significant buzz for green architecture, a full transition to net-zero building still faces significant obstacles. Efforts to make net-zero construction the norm in Maryland by 2033 failed in the state legislature this week, with critics arguing that technology and building codes are not sufficiently advanced to support such a goal.

The decathlon provides real-life examples that undercut such arguments.

Green building is a critical piece of “having to deal with our climate crisis in a way that doesn’t prevent people from living the lives they want but rather empowers them to do it,” said David Nemtzow, director of the DOE’s Building Technologies Office. “We cannot succeed with our environmental and global energy policies unless we can also deliver comfort and aesthetic appeal and productivity and durability to the buildings around the country and the world.”

The DOE’s virtual Solar Decathlon is online now at www.solardecathlon.gov/virtual_village/.