What is the passive house movement?

One morning in December of last year, archi­tect Michael Ingui was up early, having his morning cup of coffee in his Brooklyn home. He looked out his glass window at the city, which was in the throes of a polar vortex, with average temper­a­tures barely reaching freezing. Inside, however, clad in only a T‑shirt and shorts, Ingui hardly noticed. In fact, he suddenly real­ized that he’d forgotten to turn on the heat in his entire home. 

“It’s 77°F in my house, the sun’s not up yet, and it’s holding the heat just fine,” Ingui said. 

He was able to be preserved in his cocoon of warmth because his home was performing exactly as he had designed it to — according to the stan­dards of passive house. 

Passive house (or ​“Passivhaus,” in its native German) is a set of volun­tary building perfor­mance stan­dards that helps build­ings reach extreme energy effi­ciency — to the point that they use little-to-no active heating and cooling systems to regu­late temper­a­ture (hence, ​“passive”). The philos­ophy emerged in 1970s Germany as a response to the energy crisis going on at that time; it uses an under­standing of building science and physics to heat and cool a home without using much energy. 

At its root, passive house design is rooted in five key prin­ci­ples: Thermal insu­la­tion, elim­i­nating thermal bridges, highly energy-effi­cient windows, airtight­ness, and air venti­la­tion with heat recovery. These passive house prin­ci­ples have proven adapt­able to many types of archi­tec­tural designs and loca­tions over the years, including in and around New York City. From suburban single-fami­lies to six-story multi­family build­ings in Brooklyn, New York owners, devel­opers, and resi­dents have taken advan­tage of passive house prin­ci­ples to make their homes work more efficiently. 

The first prin­ciple of a successful passive house is all about insu­la­tion. According to Marija Gjorgjievska, a Passive House-certi­fied archi­tect at Ryall Sheridan Carroll Architects, many certi­fied passive homes have ​“contin­uous insu­la­tion” throughout the building. This means no gaps: The insu­la­tion is an unin­ter­rupted unit, and often thicker than tradi­tional builds, acting like a down jacket to keep heat from escaping. Architects and builders use a range of mate­rials that are poor at conducting heat for insu­la­tion, including cellu­lose, mineral wool, or poly­styrene foam. 

This thick, contin­uous insu­la­tion also helps with reducing thermal bridges, another passive house prin­ciple. Passive House International describes thermal bridges as ​“heat high­ways,” or points in insu­la­tion where heat can easily move or escape from the building. Typically, thermal bridges are seen where there are gaps between insu­la­tion, such as where the floors meet the walls or parts of the walls that have windows, and a truly passive house aims to be free of them. 

But even for a house that isn’t 100 percent passive, it’s impor­tant to be mindful of windows and the gaps around them, warns Ingui. 

“A lot of times, people think it’s the window that’s failing, when it’s actu­ally all around the window. There’s a little space, and the cold just comes right in; cold, mois­ture, which creates mold. So better windows, better installed, with better insu­la­tion, it does so much.”

Well-insu­lated windows and doors are another key passive house prin­ciple. All of the experts Skylight spoke to were insis­tent about this point — Passive houses ​“need super-duper windows,” as Gjorgjievska put it, which are usually triple-paned for high perfor­mance. The extra layer of glass is not only a way to prevent conden­sa­tion that could lead to mold growth and dete­ri­o­rate the building struc­ture, but also offers insu­la­tion to prevent any heat leakage to the outside. ​“I look at windows as part of the heating system of a building, rather than just windows,” says Michael Robinson, an archi­tect who built a certi­fied passive multi­family home in Newburgh, NY. 

Windows and insu­la­tion also help a passive house to achieve the stan­dard of airtight­ness, key to temper­a­ture regu­la­tion. In addi­tion to contin­uous insu­la­tion, passive houses also have extra layers of membranes and sealants to keep outside air from seeping in as much as possible. Some of the best airtight membranes can be poly­eth­ylene- and polypropy­lene-based, and are often rein­forced with special tapes that can be stretched and folded, forming strong, long-lasting bonds with the membrane and construc­tion mate­rials. But Robinson points out that a low-tech, inex­pen­sive method called parging, or spreading a thin layer of concrete over brick walls, can also offer some level of airsealing. 

Finally, because passive houses are sealed airtight, they also need to have balanced heat recovery venti­la­tion — a way to venti­late the indoor space to main­tain healthy air quality, without sacri­ficing airtight­ness. To do this, passive houses utilize venti­la­tion systems called energy recovery venti­la­tors (ERVs) or heat recovery venti­la­tors (HRVs). These systems cycle in fresh air from the outside, and move out stale exhaust air from the inside. At the core of the system is a heat exchanger, which trans­fers heat (and, in the case of ERVs, humidity) between the inside and outside air to balance, or ​“pre-condi­tion” the fresh air’s temper­a­ture before it enters a home. ​“It’s basi­cally a way to exchange fresh air with the outside without losing energy,” Gjorgjievska says.

Altogether, these high-level concepts form the criteria against which a building is eval­u­ated to achieve passive house certi­fi­ca­tion. This includes the German PHI stan­dard and the US’s Phius stan­dard. 

But a home need not reach full certi­fi­ca­tion to reap the bene­fits of smart construc­tion and energy savings. 

“I would say more than three-quar­ters of our build­ings never get to Passive House [certi­fi­ca­tion],” said Ingui. ​“We’re just doing better stuff.”

Additional reporting by Camille Squires.