UPDATED on April 16, 2015
More and more designers of high-performance homes are buzzing about a superinsulation standard developed in Germany, the Passivhaus standard. The standard has been promoted for almost two decades by the Passivhaus Institut, a private research and consulting center in Darmstadt, Germany.
The institute was founded in 1996 by a German physicist, Dr. Wolfgang Feist. Feist drew his inspiration from groundbreaking superinsulated houses built in Canada and the U.S., including the Lo-Cal house developed by researchers at the University of Illinois in 1976, the Saskatchewan Conservation House completed in 1977, and the Gene Leger house built in 1977 in Pepperell, Massachusetts. Aiming to refine North American design principles for use in Europe, Feist built his first Passivhaus prototype in 1990-1991.
Feist later obtained funding for a major Passivhaus research project called CEPHEUS (Cost-Efficient Passive Houses as European Standards). Conducted from 1997 to 2002, the CEPHEUS project sent researchers to gather data on 221 superinsulated housing units at 14 locations in five countries (Austria, France, Germany, Sweden, and Switzerland).
The standard sets a strict bar
The Passivhaus standard is a residential construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. To meet the standard, a house needs:
- an infiltration rate no greater than 0.60 AC/H @ 50 Pascals,
- a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot),
- a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and
- maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot).
The standard recommends, but does not require, a maximum design heating load of 10 watts per square meter and windows with a maximum U-factor of 0.14.
The Passivhaus airtightness standard of 0.6 AC/H @ 50 Pa is particularly strict. It makes the Canadian R-2000 standard (1.5 AC/H @ 50 Pa) look lax by comparison.
Unlike most U.S. standards for energy-efficient homes, the Passivhaus standard governs not just heating and cooling energy, but overall building energy use, including baseload electricity use and energy used for domestic hot water.
Thick walls, thick roofs, and triple-glazed windows
Most European Passivhaus buildings have wall and roof R-values ranging from 38 to 60. Wood-framed buildings usually have 16-inch-thick double-stud walls or walls framed with deep vertical I-joists. Masonry buildings are usually insulated with at least 10 inches of exterior rigid foam. To meet the Passivhaus window standard, manufacturers in Germany, Austria, and Sweden produce windows with foam-insulated frames and argon-filled triple-glazing with two low-e coatings.
Although the Passivhaus Institut recommends that window area and orientation be optimized for passive solar gain, the institute’s engineers have concluded, based on computer modeling and field monitoring, that passive solar details are far less important than airtightness and insulation R-value.
In the U.S. and Canada, the phrase “passive solar house” was used in the 1970s to describe houses with extra thermal mass and extensive south-facing glazing. Because of the possibility of confusing Passivhaus buildings with passive solar houses, most English-language sources use the German spelling of “Passivhaus” to reduce misunderstandings.
Gotta have an HRV
Feist recommends that every Passivhaus building be equipped with a heat-recovery ventilator (HRV). Since the space heating load of a Passivhaus building is quite low, this load can usually be met in a mild climate by using an air-source heat pump to raise the temperature of the incoming ventilation air. Some European Passivhaus buildings include an integrated ventilation system with a heat pump evaporator coil in the ventilation exhaust duct, downstream from the HRV, to allow the heat pump to scavenge waste heat that might otherwise leave the building. In this way, the ventilation ductwork becomes part of a forced-air heating system with a very low airflow rate.
In Europe, most non-Passivhaus homes are heated with a boiler connected to a hydronic distribution system. Since residential forced-air heating systems are almost unknown in Europe, many Passivhaus advocates declare that their houses “have no need for a conventional heating system” — a statement that reflects the European view that forced-air heat distribution systems are “unconventional.”
Passivhaus comes back to the U.S.
The first building in the U.S. that aimed to meet Passivhaus standard was a private residence built by architect Katrin Klingenberg in Urbana, Illinois, in 2003. The home included an R-56 foundation with 14 inches of sub-slab EPS insulation, R-60 walls, and an R-60 roof. Klingenberg specified triple-glazed Thermotech windows with foam-filled fiberglass frames.
Klingenberg later founded a nonprofit organization, the Ecological Construction Laboratory (E-co Lab), to promote the construction of energy-efficient homes for low-income and middle-income families. In October 2006, the E-co Lab completed Urbana’s second Passivhaus building: a 1,300-square-foot home that resembled Klingenberg’s home in many ways.
As Klingenberg devoted more and more time to promoting Passivhaus buildings in North America, she decided to found the Passive House Institute U.S. — basically, a North American outpost of the Darmstadt institute — in Urbana.
In 2011, the European institute severed relations with its U.S. ally. The bitter divorce was accompanied by a series of accusatory press releases, and the two organizations remain estranged. For more information, see The Passivhaus Institut in Germany Disowns Its U.S. Satellite.
Although Klingenberg’s first and second Urbana homes were built to the Passivhaus standard, she didn’t bother to have the homes certified and registered. The first U.S. building to achieve that goal was the Waldsee BioHaus, a language institute completed in Minnesota in 2006. That building includes an R-55 foundation with 16 inches of EPS foam under the concrete slab, R-70 walls, and an R-100 roof. The building’s triple-glazed windows were imported (at a high cost) from Germany.
How do I learn more?
An easy way to learn more about the Passivhaus standard is to visit on online resource called Passipedia
Builders and designers interested in learning more about the Passivhaus standard may want to invest $315 in a Passivhaus software program, the Passive House Planning Package. Available from 475 High Performance Building Supply in New York City, the software is a spreadsheet-based tool that models a building’s energy performance to help designers fine-tune the specifications of a building aiming to achieve the Passivhaus standard.
Here are links to three interviews I conducted with Wolfgang Feist:
Last week’s blog: “Thinking About Net Zero Energy.”