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 glazing 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-factor (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.
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 Fenestration 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,…