German windows, like German cars, have a very solid reputation for high performance and durability. U.S. interest in German windows has grown in recent years, especially among Passivhaus builders, leading several U.S. importers to conclude that the time is ripe to offer German windows to North American customers.

Double glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. isn’t good enough
To meet the Passivhaus standard, cold-climate builders need a very good window. In central Europe, Passivhaus designers insist on triple-glazed windows with a maximum U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. (in U.S. terms) of 0.14. That’s a standard that few U.S. windows can meet.

When I interviewed Dr. Wolfgang Feist in 2007, he told me, “The reason for the number which we now use in Europe is the comfort of the occupants. It is a functional definition. During the winter, the coldest surface temperature in the room will be the window. If you don’t have a radiator in your room, the difference between the surface temperature of the window and the mean surface temperature of the room should not be more than 3 degrees Celsius; that’s for comfort reasons.”

The colder the climate, the more important it is to use U-0.14 or better windows in a Passivhaus building — and not just for comfort. Low U-factor windows are necessary to meet the Passivhaus maximum annual heating energy standard of 15 kWh per square meter.

Calculating U-factors
The lower a window’s U-factor, the better it is at resisting heat flow. In the U.S., U-factors are calculated according to a standard established by the National FenestrationTechnically, any transparent or translucent material plus any sash, frame, mullion, or divider attached to it, including windows, skylights, glass doors, and curtain walls. Rating Council (NRFC). The U-factor shown on the familiar NFRC labels is a whole-window rating that takes into account the different U-factors of the window’s frame, sash, edge of glass, and center of glass.

Europeans calculate their window U-factors differently from North Americans. I’m not just talking about the units of measurement; it’s easy enough to convert W/ m2•°C to BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. / ft2•h•°F. (Just divide by 5.678.) I’m talking about the formula used to account for the varying U-factors of the window’s frame, the edge of glass, and the center of glass.

The issue is complicated and technical; suffice it to say that North American window manufacturers contend that the whole-window U-factors of North American windows, calculated according the NFRC requirements, are not as bad as a direct comparison with European window numbers might lead one to believe.

Using the Passivhaus software
Designers of Passivhaus buildings use a software program called the Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have 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 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. Planning Package (PHPP). For each window, designers must input four U-factors: a frame U-factor, a glass U-factor, a glazing-spacer U-factor, and a “junction” U-factor.

To meet the needs of Passivhaus designers, German window manufacturers provide these U-factors in their specifications. Many European windows are certified by the Passivhaus Institut in Darmstadt, Germany; in essence, the certification process gives an official stamp of approval to the manufacturer’s U-factor claims.

Although Passivhaus builders can use any windows they want, choosing Passivhaus-certified windows is the easiest option. A designer who chooses uncertified windows has to engage in a little detective work to obtain the crucial U-factor numbers from the window manufacturer. Although most Passivhaus designers do their best to determine accurate values for these U-factors, some uncertainty will always remain when uncertified windows are used.

North American window manufacturers haven’t bothered to have their windows certified by the Passivhaus Institut. Although the NRFC software used by window manufacturers requires the input of separate U-factors for window frames, edge of glass, and center of glass — U-factors that roughly correspond to the inputs used in the PHPP software — these numbers are not shown on the NFRC label. As a result, Passivhaus builders have to cajole U.S. and Canadian window manufacturers to provide the numbers. Since some window manufacturers subcontract their U-factor calculations to third-party labs, they may be unable to provide the requested information.

Window lust
Almost every North American builder who has attended a building-materials trade show in Munich, Freiburg, or Darmstadt ends up salivating over the quality of European tilt/turn windows. German and Austrian windows cost an arm and a leg; but they have hefty triple glazing and triple weatherstripping, and they close like a bank vault.

One U.S. building project that used German windows was the Waldsee Biohaus in Bemidji, Minnesota. (Built for a language school, the Biohaus was the first U.S. building to obtain Passivhaus certification). In order to maximize the chance that the building would meet the Passivhaus standard, the construction team decided to order a container full of Optiwin windows from Germany.

Since German windows are rarely seen in North America, they have acquired a mystical aura. According to Edwin Dehler-Seter, the Bemidji institute’s environmental education specialist, the workers who opened the shipping container in Minnesota felt the same awe experienced by Howard Carter when he opened Tutankhamun’s tomb: “Unpacking and moving the heavy wooden-framed, triple-pane windows from Germany’s small windows manufacturer Müller required a lot of manpower, careful handling as well as patience. … The day before the cargo arrived, we all watched Müller’s video on how they had loaded and secured the windows in the container. Standing in front of the tightly packed and thoroughly secured pile of glass panes, window and door frames and complete windows, Zetah’s crew took the time to carefully investigate the situation to come up with a step-by-step plan on how to safely remove, carry and store this expensive and fragile cargo. Apart from the super energy-efficient, triple-pane glass filled with argonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows. gas, it was the large, solid hardwood window frames which had us all in awe. The superb craftsmanship and engineering of the custom made 6-inch frames along with the beauty of the Dreiholz (three-wood) construction from sustainable harvests, made us realize what a unique product we were carrying from the overseas container to the temporary storage container, where they will remain until Mr. Müller himself arrives from Germany to install his windows.”

The near-mythic reputation of Germany windows is only enhanced by the fact that few U.S. builders have used them. Importing German windows to the U.S. is time-consuming and expensive, so most North American Passivhaus buildings have specified fiberglass-framed triple-glazed casement or awning windows from one of five Canadian manufacturers (Accurate Dorwin, Duxton, Fibertec, Inline Fiberglass, or Thermotech). These excellent windows can be purchased with whole-window U-factors as low as 0.17.

Buying European windows in North America
Within the past couple of years, however, it’s gotten easier to buy Passivhaus-certified windows on this side of the Atlantic. North American importers now offer eleven brands of European windows:

• AlphaWin Windows from Holz & Form of Germany are being distributed by BuildingEvolution of Vancouver, B.C. (778-317-0035).
• Bieber windows (manufactured in Waldhambach, France) are distributed by Bieber Architectural Windows in Tustin, California.
• Energate windows (manufactured in Speyer, Germany) are distributed by H Window Co. in Ashland, Wisc. (800-843-4929).
• Heinzmann windows (made with Passivhaus-certified Veka frames) are available from European Windows in Gainesville, Georgia.
• Internorm windows are available from Holz & Form, a distributor in Vancouver, British Columbia.
• Intus Windows from Lithuania are being distributed by Intus Consulting of Washington, D.C. (888-380-9940).
• Optiwin windows are available from Peak Building Products in Watertown, Minnesota.
• Pazen EnerSign windows are available from Quantum Builders in Berkeley, California.
• Schüco Windows are being distributed by European Architectural Supply of Lincoln, Mass. (781-647-4432).
• Silber windows (manufactured in Mistelbach, Austria) are distributed by Red Cougar Enterprises in Cochrane, Alberta.
• Unilux windows — including their UltraTherm model — are available from several U.S. distributors, including Gilkey Windows of Cincinnati, Ohio (513-769-4527), Hawkeye Window and Door of Waterloo, Iowa (319-232-3220), and New England Fenestration in Massachusetts (978-530-7425).

A few facts about these windows:

• Internorm windows have been certified by the Passivhaus Institut. The best available Internorm windows have a U-factor of 0.14.
• Optiwin windows have been certified by the Passivhaus Institut. Optiwin Three-Wood windows have a U-factor of 0.125. According to Maine architect Jesse Thompson, “The Optiwin windows are just beautiful.” But Minnesota architect Rachel Wagner is bothered by Optiwin’s flawed window screens. “The screens are all slightly undersized,” said Wagner. “They’re loose and poorly fitted. Also, there are three slotted openings at the bottom of each window frame for shedding water, and the openings can let the bugs in.”
• Passivhaus-certified Pazen EnerSign windows are available with a very low (0.11) U-factor. EnerSign windows have much narrower frames than typical German windows, most of which are known for wide (3-inch) window frames and equally wide sash frames. “The EnerSign window is different from the usual big clunky German tilt/turns,” said Thompson.
• Unilux UltraTherm windows have composite aluminum-clad frames with thermal breaks and wood interiors. Unilux UltraTherm windows (U-0.12) have triple glazing with two low-e coatings. For south elevations, Unilux offers windows rated at U-0.21 with a solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. of 0.58. Although the Unilux Web site advertises the low-U-factor windows as “Passivhaus-Fenster” — Passivhaus windows — Katrin Klingenberg, the president of the Passive House Institute U.S., contends that Unilux windows are “just short of Passive House standards.”

Ordering windows from far, far, away
Although German windows have excellent glazing, the exact same glass (triple-pane argon-filled glazing with two low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. coatings and warm-edge spacers) is readily available from Canadian window manufacturers. The stellar performance of German windows is due mostly to the cork and foam thermal breaks incorporated into their frames. They also have redundant weatherstripping and high-quality hardware.

Although German windows are very well built and have very low U-factors, they do have a few disadvantages:

• Some U.S. architects prefer the narrow frames and sash of fiberglass-framed windows to heavy German windows. Given the same rough opening, most fiberglass-framed windows let in more light than German tilt/turn windows.
• Prepare to pay dearly for German windows — about $1,500 to $2,000 per window. Unilux windows cost about $90 per square foot. Stephan Tanner, the architect who founded Peak Building Products, said that high-quality Optiwin windows cost $100 per square foot or more, plus 6% to 10% for shipping. Canadian windows are much less expensive; according to Thompson, triple-glazed Inline Fiberglass windows cost $40 or $50 a square foot.
• Expect long lead times — about 10 to 12 weeks between order and delivery, with similar lead times if any warranty issues develop. “When a window comes with something wrong or broken, it takes a long to time to get a replacement over from Germany,” said Wagner.
• Since there are only a few distributors of European windows to cover all of North America, don’t expect to have a window rep show up at your job site quickly when a problem arises.

Considering the high cost of German windows, many Passivhaus builders have decided to stick with tried-and-true Canadian windows. “By the time Thermotech or Inline gets done building a narrow fiberglass frame and putting foam inside, it’s a very good frame,” said Thompson. “Everyone is using the same glazing and the same thermal-edge spacers. So I think that the fiberglass-framed windows can be as good as the best European windows.”

One more option
Instead of triple-glazed windows, some cold-climate builders purchase windows with Heat Mirror glazing. Heat Mirror glazing has only two panes of glass; between the inner and outer panes are one, two, or three plastic films that create separate air spaces. The main advantage these plastic films have over glass is lighter weight. Because Heat Mirror glazing allows for multiple air spaces, very low U-factors are possible.

Although Heat Mirror glazing has been around for decades, many builders are taking a wait-and-see attitude about the product. “I’m nervous about the Heat Mirror glazing,” said Thompson. “I wonder how long the plastic films will last. The Germans are very smart, and the Germans don’t use Heat Mirror.”

According to Robert Clarke, former president of Alpen Windows, window buyers shouldn’t worry about the durability of Heat Mirror glazing. “Some of the dirty laundry that followed Heat Mirror in the 1980s and earlier 1990s was legitimate. There were problems due to the perimeter seals — problems with the single-seal technology. All of that became history in 1995, when Heat Mirror switched to a new perimeter seal technology.”

The California manufacturer that acquired Clarke’s company, Serious Materials, offers a wide selection of fiberglass-framed windows with Heat Mirror glazing. Unfortunately, Serious Materials is not yet offering a low-U-factor window with a high enough solar heat gain coefficient (SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.) to attract many northern builders. The company’s Web site promotes a window (model 1125) said to “save more energy than any other window.” Although casement windows in the 1125 line have a very low U-factor (U-0.13), they also have a couple of Achilles’ heels: an unacceptably low SHGC of 0.20 and an unacceptably low visual transmittance (VT) of 0.30. (According to Clarke, any window with a VT below 0.40 “would not be ethical to sell as clear glass.”)

According to representatives from Serious Materials, however, their marketing department is currently rethinking the glazing options they offer.

More information
A useful discussion of Passivhaus windows has been published in the Fall 2010 issue of the Efficient Windows Collaborative newsletter.

Last week’s blog: “Martin’s Useless Products List.”