To encourage residents to build greener houses, Phoenix is offering free design and construction plans for HOME nz, a three-bedroom sustainable residence designed by Marlene Imirzian & Associates Architects.

HOME nz was the winner of Sustainable Home Design, a 2017 competition by the City of Phoenix and AIA Arizona that called on architects to develop a design for a near net zero house that would be adapted to the hot, arid climate and be attractive, affordable, and have the potential for widespread adoption. The winner took home a $100,000 prize. The competition fits into the city’s goal to require all new buildings to be net positive, generating more energy than they consume, by 2050.

Imirzian’s design was the favorite of all nine judges, but she almost didn’t enter. Participating in any competition is a gamble: submissions take time and resources, and the odds of winning are slim. This one was onerous because the design had to meet a scoring system that measures a house’s efficiency known as the Home Energy Rating System, or HERS, and Imirzian had never heard of it, despite her extensive experience in sustainable design. The winner would then have to develop the construction documents, and get them approved by the city’s planning office.

“I said, ‘Forget it, I’m not doing more free work,’ ” says Imirzian, whose firm has offices in Phoenix and in Escondido, California.

But she began to wonder if this exercise would provide an opportunity to learn about simpler efficiency guidelines that clients might more willingly embrace. “I was intrigued,” she says. “The strategies are quite feasible and have major impact.”

Two weeks before the deadline, she contacted Desert Skies Energy, a HERS consultant in Phoenix that explained how the rating system works. A HERS rater assesses the efficiency of a single-family dwelling, assigning it an index score. The lower the score, the more efficient the structure. A typical new home, for example, has a HERS index of 100, a figure determined by the U.S. Department of Energy. To determine that score, a HERS rater evaluates features like exterior walls, the heating and cooling system, and measures air leakage. For the competition, a design had to score a HERS index of 30, making it 70 percent more efficient than a home with a score of 100.

Although HERS is designed to evaluate existing homes, the city chose it because “it was a common measuring stick, so we could compare buildings,” says Phoenix chief sustainability officer Mark Hartman.

Imirzian decided to take on the challenge, enlisting Jay Atherton, a designer who works at her firm, along with Desert Skies, Henderson Engineers, SCL Consulting, and Furcini Construction, a Phoenix contractor.

The three-bedroom house fits within a 60 by 110-square-foot lot.
Rendering courtesy Marlene Imirzian & Associates Architects

After an intensive two weeks, Imirzian delivered the design for a 2,100-square-foot midcentury–style house, with a living room, family room, and porch, that can be built for $344,000. The structure uses high-performance glass to reduce the transmission of heat from the harsh Arizona sun. Retractable fabric screens keep out 95 percent of direct sunlight, leading to passive cooling and allowing for the house to be oriented in any direction. A solar chimney has a damper that automatically opens and closes depending on weather conditions. Structural insulated panels are used throughout the house, including the roof. By housing the ductwork below the insulated roof, the mechanical systems capture cooling energy inside. The result is an envelope so tight and efficient that the house does not need to produce much energy to achieve net zero.

“Because of the efficiency that the engineer brought to the project, we only used a third of the roof for solar panels,” Imirzian says.

Hartman points to the success of the design as evidence that the city could eventually reach its net positive goal. “If today we can build one that is net zero, then I’m sure we’ll hit that target” in the next 30 years, he says.

Once she won, a team of three people in Imirzian’s office spent 600 hours developing construction documents and getting permits approved. Although the city waived the permitting fees, Imirzian spent about $45,000 on consultant fees, and did not receive the prize money until after the planning office issued final construction permits. Soon after the city uploaded the documents, it asked Imirzian to update them to reflect changes to the city’s building code, requiring another three months of work.

Looking back on the process, Imirzian says the project was worthwhile. “I learned a ton,” she says. “We were shocked by the simplicity of what you could do to get a high-performance building.”