Kawahara, a former Silicon Valley tech industry professional, had been going on sustainable home tours. “I thought ‘green’ homes were just about solar panels and recycled materials and low-flow fixtures: Easy,” she says. Hurley, a clean-tech investor, had been researching energy-efficient systems and sustainable strategies in search of a “silver bullet to sustainability,” Kawahara recalls. “We liked the idea of creating a sustainable home and being responsible decision-makers. We talked with a sustainable-home consultant, and he mentioned insulation, windows, etc.,” she says, laughing. 

Then, the consultant mentioned the principles put forth by the Passive House Institute in Germany and, “His eyes lit up. He was animated,” Kawahara says. The couple attended a Passive House workshop, and “the idea clicked with us. Kurt is really into physics, and the Passive House principles are all based on science.”

Working with Graham Irwin, an architect and Passive House consultant with Essential Habitat, and Santa Cruz Green Builders, the couple transformed their bungalow into Santa Cruz County’s first certified Passive House. They named their home Midori Haus, which means “green house” in Japanese and German. They also wrote a blog in which they recorded the home’s transformation. They included posts on how they selected their house and designing for lifestyle and budget. They discussed their mechanical selections. They also wrote how-to articles on how to balance the airflow of the Zehnder HRV system and change filters to keep out dust, pollen, and smells.

After living in the home for several years, Kawahara wrote a book about the process: Midori Haus: Transformation from Old House to Green Future with Passive House. In his introduction to the book, Lloyd Alter, design editor of TreeHugger.com, wrote: “There are some who say that the Passivhaus or Passive House system is worshipped by a cult of data-obsessed nerds who design boxy buildings with no charm. One critic claimed, ‘Passivhaus is a single metric ego-driven enterprise that satisfies the architect’s need for checking boxes, and the energy nerd’s obsession with BTUs, but it fails the occupant.’ Others say it has become irrelevant; add solar panels to the roof and net-zero it out. The story of the Midori house puts paid to these arguments.”

A heat-recovery ventilator (HRV) moves exhaust air out and brings fresh air in, using the heat in the outgoing household air to warm up the fresh air. The unit’s filters keep dust and pollen from entering the house. “We love the indoor air quality—and the quiet,” says Kawahara.

“When we go to other people’s houses or travel, we’re never quite as comfortable,” she explains. “We rented an Airbnb in Santa Barbara, a 1920s bungalow advertised as green because it had solar panels on the roof and an edible garden. The house was so hot; we had to sleep with the windows open and the fans on. That experience validated that, had we just put up a solar panel on this old bungalow, we would be super-uncomfortable.”

The builders re-used the home’s existing materials whenever possible, including interior doors, fir floors, and baseboards and trims. They recovered the home’s original 1922 built-in buffet during the renovation, then refinished it after construction. An FSC-certified, post-consumer paper product bound with petroleum-free resin comprises the kitchen countertops. The couple hang-dries laundry in the house instead of using a clothes dryer. They installed a Murphy bed in the office to avoid building on an addition as a guest room.

With an experience like transforming a bungalow into a Passive House, she adds, “You’re faced with hundreds of little and big decisions every moment. Every decision has multiple angles to consider, with pros and cons. You can only choose one way, and you have to be okay with it. Still, I think our Passive House will be our forever home—unless we win the lottery and can do this again.”