Passive house: A road map for radically reducing energy consumption?
June 30, 2015
This winter was one of the coldest on record in New York City, and many property owners saw major spikes in their energy bills. However, thanks to passive house technology and a glazed glass south-facing façade, the occupants of a recently retrofitted townhouse in Park Slope, Brooklyn, were able to leave the heat off even when temperatures outside fell below zero. According to the architect, the building’s cooling and heating systems consumed less than a fifth of the energy needed to keep neighboring townhouses at a comfortable temperature.
The project’s designer, a firm called Build with Prospect, bills itself as the first worker cooperative in New York City’s construction industry. Build with Prospect also is one of the first firms in the city to start doing passive house retrofits. And although construction costs for a Build with Prospect retrofit range from 4 to 7% more than conventional construction, the energy savings are so significant that a building can start yielding paybacks within as little as four years.
“The nice thing about a passive house is that the results are verifiable,” Build with Prospect architect Nate Priputen says, noting that the energy-efficiency standard, which was developed in Germany in the early 1990s, is a holistic system based upon strict measurements of total energy usage and air circulation. In contrast, the US Green Building Council’s LEED checklist system awards points for various other environmental benchmarks in addition to energy efficiency.
The passive house movement is strongest in Central Europe, where most of the more than 9,500 buildings certified as meeting its exacting energy efficiency criteria are located — in addition to the tens of thousands of buildings that have been built with passive house technology but not certified. A growing number of cities, such as Brussels and Frankfurt, are incorporating passive house standards into their building codes, and passive house–oriented masterplans are being developed for entire neighborhoods. And with the new European Union requirement that as of 2020 all new buildings meet “nearly zero” energy standards, meaning that they be built with a very high level of energy performance, the passive house model is destined to become even more prevalent.
With the growing concern about climate change, passive house is finally catching on in the United States.
In New York City, one of the leaders in the United States for this type of construction, only a handful of buildings have been certified as passive house. But with the growing concern about climate change and the burgeoning interest in “zero net” carbon emission strategies, the approach is finally catching on in this country. The New York City government’s recently published One City: Built to Last report, which lays out a road map for reducing the city’s carbon footprint, discusses passive house as a potential energy performance guideline for all new construction.
The passive house standard requires a tightly sealed and heavily insulated building envelope to ensure optimum energy efficiency. The minimum airtightness level allowed is 0.6 air changes per hour under 50 pascals of pressure. To ensure that a house is in compliance with this limit and that there are no leaks, the building’s designers conduct an on-site blower door test. “The biggest challenge is the sealing,” says Priputen, adding, “If you have a weak spot you have to make all of the other areas stronger in terms of insulation and air sealing.”
The other main pillar of passive house construction is a compact air and heat exchange system that conserves energy by transferring heat and/or moisture between incoming and outgoing streams of air. Designers specify one of two systems, depending on the site’s climate: heat recovery ventilators (HRVs), which transfer only heat, or energy recovery ventilators (ERVs), which transfer both heat and moisture.
We look at a solid new concrete wall in the back of the building, which is 18in thick as opposed to the 10in that Priputen says would be the standard for a new wall in a New York City townhouse. Accounting for part of the new wall’s thickness is expanded polystyrene (EPS) insulation, which removes the need for formwork made of plywood or metal. Not only does EPS insulation result in a more efficient construction process, it also eliminates the enormous amount of waste from the more typical plywood and metal formwork that is generally disposed of after the concrete has set in conventional construction.
Along with the superinsulated walls, passive house construction generally features windows with an R-7 insulation value or higher. (Insulation value calculations can be highly complex. For the sake of comparison, however, a typical single-paned window has an R-0 value.) These triple-paned windows, which are only beginning to be manufactured in the United States, lower the heat loads while keeping the inside face of the glass significantly warmer, greatly reducing cold spots within a room.
Although the passive house standard is much more prevalent in Europe, it evolved out of research conducted in the United States back in the 1970s, when the country was in the throes of an oil crisis. In fact, two of the prototypes for passive house construction were built in the 1970s by a team headed by the architect Wayne Schick at the University of Illinois. The University of Illinois team determined that most houses lost heat through cracks and thermal bridging. So to eliminate thermal bridging and air leaks, the team used double-stud walls and massive amounts of insulation, which Schick dubbed “superinsulation.” Of course, removing all of the natural ventilation made getting fresh air into the house a challenge, especially during winter months. To compensate for the lack of natural ventilation, Schick and his team developed one of the world’s first HRV systems.
While the University of Illinois team’s research failed to bring about immediate changes in the American building industry, it caught on in Germany, where engineer Wolfgang Feist used it as the basis for the original passive house standard, developed in 1991.
A number of factors have led to the passive house standard taking so many years to catch on in the United States. One of the challenges, according to Priputen, was the imperviousness of the insulating membranes available in the 1970s, which trapped water vapors with dire consequences. “It created mold and degraded entire buildings,” he says. “People moved away from it, and it has taken this long to embrace that way of building again.”
Passive house is strongest in Germany and Austria.
However, the political climate in the United States also appears to have played a part in retarding advances in building science. “The story in the US is that we took energy efficiency seriously for about eight years in the 1970s, and then Reagan got elected and it all got shut down,” declares Ken Levenson, a founding board member of New York Passive House, a nonprofit advocacy organization; and a founding partner of 475 High Performance Building Supply, which specializes in passive house building materials and technology.
In addition to politics, Levenson says that differences in national architectural education standards have led some parts of the world to adopt the system much faster. “[Passive house] is strongest in Germany and Austria, where there are the highest technical capabilities and the architects and engineers are much more aligned culturally with it,” he says.
Until recently, cost premiums also were a limiting factor to the passive house standard becoming a commercially viable alternative in this country. For example, up until about five years ago, there were only a couple of importers of passive house–quality triple-paned windows to the United States, says Levenson, but now there are more than a dozen importers and the cost has come down significantly. “Passive house windows were more than twice the cost of a decent American double-paned window,” he says. “Today, you would probably get it for a 25% upcharge.”
Although passive house products such as imported high-performance triple-paned windows still sell for a slight premium, they significantly pay off in the long run by eliminating thermal bridges, creating airtightness, and increasing comfort for inhabitants. “Consequently, you don’t need to have perimeter radiator heating or air conditioning — you can get rid of all of those mechanical systems and pull them back to the core of the building,” Levenson says. “Because of the optimization, which should be driving energy loads down by 85 or 90%, there typically is 75% reduction in size of mechanical systems needed for heating and cooling.”
One architect practicing in New York has even found a way to incorporate passive house technology into affordable housing. Chris Benedict, principal of Architecture and Energy Limited, has built two 24-unit affordable housing developments in the Bushwick neighborhood of Brooklyn that are on the way to achieving passive house certification. Currently, she is getting ready to break ground on a market-rate 40-unit apartment house, which will be the first passive house residential building in Manhattan.
According to Benedict, who has been designing energy-efficient buildings in New York City since 1996, the growing popularity of the passive house standard has made it easier to sell her specialized skills to clients. “I was in the energy-efficiency world prior to the arrival of the passive house standard in the US,” she says. “Before, if I was going to talk with people about energy-efficient buildings, I would have to talk about tons of different things. Now all I have to do is say passive house.”
In addition to being responsible for several of the largest passive house developments built to date in this country, Benedict has also played a role in getting zoning and building codes changed to make these types of buildings more cost effective for developers. For example, in New York City, prior to zoning code changes, developers interested in more energy-efficient buildings actually stood to lose the amount of allowable floor area because of the extra wall thickness that passive house requires. “In new construction it was a tough nut to crack for developers,” Benedict says. “What is nicer for developers than a glass building where the wall thickness is 2 inches? That is a lot more developable floor area because the wall thickness was counted as part of the floor area of the building.”
However, three years ago, Benedict successfully lobbied the New York City Planning Commission to allow floor area bonuses for extra insulation on both preexisting and new construction. As a result, on new construction in New York City, any building that has a wall thickness of more than 8in does not count as floor area so long as the wall assembly has a higher R-value than the current building code’s R-value requirement.
The change in the zoning code is already paying off for Benedict’s clients. On the market-rate passive house Manhattan apartment building that Benedict designed, she has been able to recover about 800ft2 of floor area for the developer in exchange for providing walls that exceed the New York City Building Code’s insulation requirements. This amount “was the cumulative extra thickness of insulation that didn’t have to count as floor area,” she said.
Currently, there is a debate within the US building science community as to which passive house standard to adopt. Most of the existing passive house buildings in the United States have received their accreditation from the original Passive House Institute (PHI), founded in Germany. And in order to receive PHI certification, an inspection is required by one of the institute’s accredited inspectors. However, there is also a Belgian Passive House standard, as well as a Swedish Passive House standard. In 2007, several architects split from the German passive house standard and founded their own organization called Passive House Institute US (PHIUS).
One of the big bones of contention between the German-based PHI and PHIUS is a difference of opinion over what a building’s maximum energy loads should be in order to qualify for certification. Benedict says that the PHI standards are impractical for very hot or very cold climates in the US that are unlike that of Germany, and that it would be more practical to have limits on peak energy loads rather than on yearly energy use. “One of the reasons we haven’t had the huge launch that could have happened in this country is because of the rift and the fighting that has been going on,” she says. “That has confused the marketplace a bit, and it also has confused practitioners who hear one thing and then another, so they don’t know which way to turn.”
It will doubtless take time for the competition between the different passive house institutes in this country to play out, and perhaps there is room for several different types of standards. Meanwhile, it has become much easier to build these types of buildings as a result of passive house technology becoming significantly more affordable and regulatory barriers being removed in places like New York City. With US cities tightening their energy codes, it seems clear that the building industry in this country will soon need to know a great deal more about passive house technology than it does now.