You hear about passive homes popping up everywhere from San Diego to Vienna, but what exactly is a passive house? And how are they built? Curbed spoke with passive house pros Ken Levenson of NY Passive House and San Francisco-based architect Bronwyn Barry to understand the green building trend that’s cutting carbon emissions and energy bills.
What exactly is a passive house?
"Passive house is the radical notion that you can reliably and consistently design a building that works for humans," explained Barry. "It’s a comfort standard and a methodology."
Essentially, a passive house is designed to be extremely energy-efficient so that it doesn’t take a lot of power to heat or cool. To be designated as a passive house, a building must embody a set of specific best practices that seal it from outside temperatures while maintaining a stable inside temperature and high air quality.
These best practices were developed over decades of research conducted by the Passive House Institute (PHI) in Darmstadt, Germany. Now, they’re used by thousands of architects, developers, and contractors all over the globe. When you call a house a "passive house" you’re saying that it was built to the PHI’s rigorous standards for insulation and energy use.
How does a passive house work?
"It’s sort of like building a thermos," said Ken Levenson, "but it’s a thermos with really good ventilation." When you want a space to naturally maintain its temperature—whether it’s as small as a thermos or as large as a home—you’re going to be following many of the same rules. Passive homes need to be air-tight, have continuous insulation, triple-paned windows, and a great system for controlling air quality.
The home’s design also needs to eliminate a phenomenon called thermal bridging which occurs when the temperature of one material transfers to another through physical touch, like a room feeling cold in winter because the steel beam supporting the floor is touching the freezing brick on the facade.
By thermally sealing off the interior of a space, a home’s internal temperatures are more stable by default. Implementing passive house techniques is enough to make a home 90 percent more energy efficient than the average house.
Why would someone want a passive house?
Because passive houses are so energy-efficient, heating and cooling them costs dramatically less than in other homes. And because internal air temperature is so consistent, passive homes are more comfortable than a house where the inside temps oscillate between sweltering and freezing.
"Once you live in one and hang out in passive homes, it’s incredible to realize how uncomfortable we are in conventional buildings," said Barry, admitting that one of her clients, a couple, told her that they stopped going on out-of-town trips because staying anywhere else felt miserable compared to their retrofitted passive house.
The air quality in a passive house will also be exceptional, eliminating any staleness or fumes. Air in a passive house is constantly circulated and filtered. Passive homes are also more resilient to power outages or other emergency situations. Even without electricity, the home will stay at a comfortable temperature for far longer than the average building, making it a popular choice for hospitals and senior residences.
But some of the benefits of living in a passive house are less quantitative. "On a cold winter day, you can sit right next to the window without a heater and without wearing two sweaters because the temperature of the window glass will be very close to the temperature of the room," said Levenson. "Comfort is the first and foremost benefit, particularly in New York."
How is a passive house built?
"It’s built like any other building," Levenson told us. "Ninety-nine percent of a passive house is made with the same materials, methodologies, workers, and schedules as a non-passive house."
Most of the passive house work takes place in the design stage because every element has to work together to produce the benefits of the methodology. It doesn’t make sense to have a fresh air exchanger if the home’s windows leak. So generally it’s a matter of beefing up the insulation and thermal isolation in the design. Actually executing a passive house build is fairly straightforward.
Also, a passive house doesn’t have to look like a hippy Earthship. The key principles of passivity can be tailored to fit very different styles of buildings from extremely modern homes to rustic cabins and historic apartment buildings.
How expensive is passive house construction?
Generally, the larger the house, the less its passive elements will impact the overall budget. A massive project like the dorm for Cornell University’s Roosevelt Island Tech Campus will be 2 to 3 percent more expensive to achieve passive house standards. Levenson says that building a more normal-sized passive home will typically add between 5 to 10 percent to the construction budget. But it continues to get ever more affordable as research into new materials and efficiencies evolve.
I want to build a passive house—what’s first?
"Find a passive house consultant or certified designer before you begin designing," says Levenson. "The worst thing you can do is wait for the house to be designed and then try to add the passive elements on top." For help finding someone in your area, check out the North American Passive House Network or PassiveHouse.com.