musingsheader image
0 Helpful?

Passivhaus Windows

Cold-climate builders look for the best available windows

Posted on Oct 2 2009 by Martin Holladay

UPDATED on June 11, 2013 with new information on European Passivhaus-certified windows available in the U.S.

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 German 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. Because labor costs in Germany are much higher than in eastern Europe, an increasing percentage of European windows are now manufactured in low-wage countries like Lithuania, Poland, and Slovakia. North American importers now offer 13 brands of European windows:

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 Energy, offers a wide selection of fiberglass-framed windows with Heat Mirror glazing. Unfortunately, Serious Energy 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 Energy, 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.

May 2013 update: Two U.S. window manufacturers will sell Passivhaus-certified windows
In early 2013, Alpen Windows and Marvin Windows announced that they will begin selling Passivhaus-certified windows that are manufactured in the U.S. For more information, see Marvin to Offer Passivhaus Windows.

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

Tags: , , , , , , , ,

Image Credits:

  1. Jesse Thompson, Kaplan Thompson Architects
  2. Unilux

Nov 10, 2010 1:40 PM ET

More information
by Martin Holladay

Check out an interesting article, Windows for North American Passive Houses, in the Fall 2010 issue of the Efficient Windows Collaborative newsletter.

Dec 6, 2010 7:04 PM ET

choosing windows
by Kevin Stamm

I've seen the damage done and the drug is water. For me the first thing I look at in a window is water management, if something starts leaking, ooh say in a corner, where does the water go? There has to be a weep system to get the inevitable water down and out, it can't sit in the frame and rot it if wood or cause mischief was glazing seals. Does one expect the caulking on the glazing to sash joint or the corners of the sash to last forever? I look at corner samples I just wish it was easier to see them from all the world over choices brought up in these blogs. Choose a window first on durability then crunch all those U and SHGC #'s . Would like to hear from someone who has looked at a lot of corner samples w/ this goal and would like to see this as part of window "rating system" Kevin Stamm

Oct 5, 2011 10:13 AM ET

Passiv Haus publication # 37 ?
by tim koelle

New to this blog. Great discussion. Where can I linked to the publication, Passiv Haus publication # 37, mentioned by Thorsten Chlupp?
We are looking for high-performing Entry Doors; any ideas?
thanks, Tim

Oct 5, 2011 10:24 AM ET

Response to Tim Koelle
by Martin Holladay

I think this is the document that Thorsten was talking about:

How's your German?

Jun 23, 2012 7:32 PM ET

Part of being green - produce locally
by Christopher Chan

Thank you for an excellent article and what a great resource to not just the products but the knowledge!

And part of being green is the source locally as much as possible to avoid transportation footprints. We don't live in an isolated world; trade and exchange among countries and communities are important. Yet, we can surely find benefits in job creations and carbon footprint reduction by building these European windows and doors right here in North America.

Since 2010, I am part of a few small business owners who have been bringing the technologies and education in North America. The Alpine Technical Workshops(c) teaches the production and construction method of German-style wooden tilt-and-turn window and lift-and-slide doors, from preparing blank stocks to profiling, from glue-up to hardware construction, and from glass to gasket installation. This small business caters to mostly other smaller to medium size shops all around North America. Before any woodworker invests in the machines and tooling on these products, a few of them likes to come to our live workshop in Colorado to experience it first hand on how it works and study to business opportunities before expanding their current shop products.

Again, I am glad to see more community building articles like this one as it shares knowledge and products that move our business and desires to be greener forward. And it's a bonus that manufacturing locally in each community (or at least right here in North America) produces more jobs and business opportunities.

Nov 30, 2012 11:15 PM ET

Buyer beware
by Jacob Staub

For those interested in certified Passive House components this is an excellent link:

The above link is a primary source for up-to-date information on who's making what Passive House International certified component where. Hint: Northwin of Canada has a certified window frame.

Based on my own hunt for Passive House windows the best piece of advice I can give is to ask for a THERM file for the window. If the company doesn't have a THERM file and you can't make one yourself based on company supplied information you might as well buy more "expensive" windows for all the time/money you'll spend to verify that "cheaper" so called "Passive House" windows will perform as advertised. If verification isn't your bag then Passive House isn't really either.

Full disclosure: I am not affiliated with any Passive House agency, group, or manufacturer. I am developing a Passive House and the marketing tactics that have been deployed to secure market share make the process unnecessarily challenging.

Jake Staub

Dec 1, 2012 6:19 AM ET

Response to Jacob Staub
by Martin Holladay

Thanks for the useful advice.

I'll leave it to the readers to determine whether the high hurdles that North American Passivhaus designers face when choosing windows are due to useful criteria set by the Passivhaus Institut in Darmstadt -- hurdles which assure quality and performance -- or are due instead to an obsession with paperwork that is unjustified by results.

May 14, 2013 7:57 PM ET

Passivehaus windows
by Karlyn K

Canadian Manufacturers have made significant progress in matching style and performance of European windows. See the people at - they make a very good fiberglass window and offer doors as well.

Jul 10, 2013 12:17 PM ET

New window in US certified by Passive House Institute
by Christopher Chan

Casagrande Woodworks Inc. has received the first Passive House Institute (PHI)’s window frame certification in the United States for its window called Synergist. This window has a thermal transmittance (Uw) of 0.79 W/(m2K) with Passive House Efficiency Class A (advanced component). Synergist is also the only PHI-certified solid all wood, out-swing casement window in the world and will be manufactured by Casagrande Woodworks in California later in July 2013.

Casagrande Woodworks Synergist Press Release Final.pdf 26.69 KB
zd_casagrande_synergist_en.pdf 458.67 KB

Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!