Building a net-zero house in Phoenix, Arizona? The city will be happy to provide you with a full set of plans at no cost.

The offer is part of the city’s plan to see all new buildings net positive by 2050, according to an article posted at Architectural Record, and follows a design competition in 2017 challenging architects to develop plans for a near net-zero house suited to the city’s hot, dry climate.

The winner was Marlene Imirzian & Associates Architects, which developed plans for a 2,185-square-foot, three-bedroom home. Home NZ, as it’s called, has a HERS score of 30, making it 70% more energy efficient than an average built-to-code home. With the addition of a modest solar array, the house would be net-zero.

The house can be built for an estimated $344,000 (not including the contractor’s overhead), and is designed to fit on a 60-foot by 110-foot lot. The design is orientation neutral, meaning it would perform as intended no matter which way the building lot is oriented.

According to the description posted with the plans, the single-story house takes its design cues from mid-century modern residential architecture. It includes a large front porch and a built-in garage. Operable, exterior shades can be left open when the sun isn’t shining and closed when windows are in direct sunlight, reducing cooling loads. Shades are designed to prevent 95% of direct sunlight from reaching window glass.

Other building features include:

  • Structural insulated panels (SIPs) for the walls and roof. Wall panels are 9 1/2-inch thick (R-45), OSB over polyisocyanurate insulation. Roof panels are of the same construction, 11 1/4 inches thick (R-70).
  • A reflective cool roof to minimize heat absorption.
  • Passive cooling by means of a solar chimney at the top of the house that can flush out stale air and introduce cooler, fresh air at night.
  • LED bulbs, a smart thermostat, and wi-fi enabled energy management.
  • Heating and cooling with a Carrier air-source heat pump. Whole-house ventilation with a Zehnder energy-recovery ventilator. The variable-speed air conditioner limits starts and stops for higher efficiency.
  • Double-pane windows.
  • Estimated annual energy costs include $286 in service charges, $240 for cooling, and $597 for lights and appliances.

The house plans are available free of charge, but they are provided without a warranty, and the homeowner assumes all liability. Also, the city requires that anyone taking the plans agrees to have them reviewed by a local professional architect and/or engineer as well as a licensed contractor before construction starts.

Winner almost didn’t enter contest

According to Architectural Record, Imirzian nearly ruled out her participation in the 2017 contest despite the $100,000 prize that would go to the winner. She wasn’t familiar with the HERS rating system—a key component of the competition rules—and she would be required to develop all of the construction documents and get them approved by the city’s planning office if she won.

“I said, ‘Forget it, I’m not doing more free work,’ ” she told the magazine. Later, she warmed up to the idea of entering, in part because it would give her a chance to learn more about energy-efficient design. Two weeks before the contest deadline, she contacted a HERS consultant in town, then rounded up an engineer, a construction company and a consultant.

Imirzian’s design was one of nine submissions. It took three people in her office 600 hours to develop construction documents and get city permits. She spent another $45,000 on consultant fees but didn’t get the prize money until the planning office had issued final permits. After the city uploaded the documents, Imirzian was asked to update them to conform with changes in the city’s building and energy codes. That took another three months of work.

The three-bedroom design is suitable for any climate zone, according to the architect who designed it. Floor plan courtesy of Marlene Imirzian & Associates Architects.

In a telephone call, Imirzian laughed when reminded of her initial hesitation to get involved. Although her practice includes residential design, commercial and institutional projects are the firm’s bread and butter.

“I probably shouldn’t have said that,” she said. “It’s a competition and it’s time, and I was just being honest about the evaluation. But the thing that was very compelling for me is that I was very interested in seeing what could be done to show that there are very reasonable and simple ways to do very cost effective, sustainable single family homes. It would be a very important message to get out.”

She said that to her knowledge no one has actually built the house she designed. That’s not as important to her as demonstrating that highly efficient houses don’t have to be complex, difficult to build, or expensive. The $344,000 price tag, she said, was probably in the lower range of a comparably sized custom house in the Phoenix market.

The $100,000 prize helped, but, Imirzian said, “it was not a profit maker, let me say that.”

“The level of ease, hopefully, of implementing this in someone’s new home is what I hope is the message people get,” she said. “Even if they don’t copy the house or use the house plans, what we’re trying to do with the graphics and the explanations is to show how simple assemblies and approaches to whatever project they do could lead to very significant performance results.”

On the road to carbon neutrality

The Home NZ project is the brainchild of Mark Hartman, the city’s chief sustainability officer who describes his job as helping to make Phoenix “the most sustainable desert city on the planet.”

Building houses that use less energy in Phoenix’s baking heat (it was 109° there the other day) is part of a city-wide effort to not only become carbon neutral but also provide a healthy ecosystem for its 1.6 million residents in many other ways. The package was approved by the Phoenix City Council in 2016. (You can read more about the effort here.)

“There’s this misconception that green homes cost a lot more,” Hartman said in a telephone call. “But if you design right from the beginning, the home could be efficient, and that means you need smaller [mechanical] equipment inside. The goal was to build a sustainable home that costs the same as current construction.”

The city used a $100,000 grant from the U.S. Department of Energy to establish the design prize, and set aside some additional money to cover permitting fees for people who decide to build Home NZ. Fees will be lower because the final plans—when they are posted—will not need plan review.

Plans posted now at the city’s website are preliminary. A last minute check of the HERS score after Imirzian & Associates updated the plans found it exceeded the 30 target by a couple of points, so architects went back and tweaked the specs for windows. Final plans should be available soon, Hartman said.

There have been 40 downloads so far, but Hartman had no idea yet how many of those people (which included reporters) will actually be interested enough to build the house.

“I would love to see 50 of them built,” he said. “That’s kind of my goal. That would be great, then maybe we could do other plans. I’d love to see homes like this become a standard, to have people realize that it’s possible.”

One obstacle is builder reluctance to try new things. For example, Hartman learned that windows with a lower solar heat gain coefficient are widely available and essentially the same cost as the windows that Phoenix builders typically use now. Ordering the windows would help cut cooling costs for homeowners. But there’s little demand for them. He hopes an upcoming publicity push with builders in the city to promote Home NZ will help change that.

Even some of the architects who took part in the design competition originally doubted it would be possible to get the HERS score down to 30, Hartman said. “No one thought you could hit HERS 30,” he said. “There was no way that it could happen. Even the participants said that until they actually started working on it and brought a team together. They were surprised—we really can do this.”

-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.