A Bungalow in Santa Cruz Is Transformed into a Passive House

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.”

Passive House, Quiet House

The 1,569 square-foot Midori Haus was remodeled to meet Passive House’s rigorous standards. That meant a roof, floor slab, and exterior walls super-packed with cellulose insulation and mineral wool insulation; an air-tight envelope; and triple-pane, argon-filled fiberglass windows. In addition to energy- and water-efficient appliances, the house uses LED lights. A solar thermal system provides water and space heating.

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.

Poster Couple for Rainwater Storage

“One thing we love that’s not a Passive House feature is our rainwater tank,” says Kawahara. “PV or solar is a way of making use of a resource falling freely on your roof. A water tank is a way of capturing another: rain. Most people use rainwater for landscaping or their garden. We captured rainwater to provide our home with indoor non-potable water for cold water laundry and flushing the toilets.” The couple’s rainwater tank was a demonstration project in the city and funded via rebates from a local nonprofit.

The tank, which was installed in 2014, holds 5,000 gallons of storage. “Twenty percent of indoor water use goes literally down the toilet, which is a lot of waste,” she says. “Because we’re in drought-stricken California, we do a good job of making use of our water efficiently.” The house was already dual plumbed in anticipation of the California plumbing code changing. When it did, the couple started storing rainwater. They captured enough water to last a year. “We were the poster child for this rainwater system,” she says, which reduced indoor water use by 625 gallons per month.

Granular Decision-Making

When asked what they might have done differently, Kawahara gets down to the nitty-gritty. Take the fiberglass door selected to optimize thermal performance. The couple chose a less expensive version they could stain with a low-VOC product. But when the time came, the manufacturer recommended a different product that wouldn’t flake or peel, but which “was so stinky,” she says.

“In retrospect, we should have paid more attention to finishes and indoor air quality. Even the low-VOC paints are smelly. With a very tight house, even with ventilation, the air was still stinky. We had to air out the house for a month with the fans going because my husband is a super-sensitive canary, and the air quality had to be just so.”

Kawahara also says she would have selected a low-maintenance, recycled plastic material for the deck rather than redwood. “It’s beautiful, but it requires maintenance. Those exterior stains, even if rated low-VOC, are still stinky and wafted into the house through the HRV intake.”

She says, “We should have pushed for electrification.” The property was already plumbed for gas, “and we decided to make use of something already existing.” While the couple’s cooktop is induction, they have a gas boiler. Gas, charcoal, and propane grills are all bad for air quality, she says, but “We would miss our gas barbeque on the deck.”