European Windows for Passive House Buildings

By Alex Wilson

What is it about European windows that makes them so popular for ultra-low-energy Passive House projects? The glass is largely the same; in fact, American companies have led the development of low-e glazings. In a nutshell, it is the frames, gasketing, and hardware that separate top-quality European windows from their American counterparts. And, ironically, the testing standard used for American windows has to some extent held American windows back.

The National Fenestration Rating Council (NFRC) was established in the U.S. in 1989 to standardize the testing and reporting of window energy performance. Prior to the formation of NFRC, window manufacturers used different methods to report their performance. Consumers and specifiers couldn’t compare apples to apples. Among the most important performance metrics to be addressed by NFRC was the U-factor, the amount of heat flow through a whole window unit under certain testing conditions. U-factor is the inverse of R-value, and until NFRC came along, manufacturers most commonly reported energy performance as the center-of-glass R-value.

U-factors differ depending on the temperature at which the measurements are made. These differences can be significant because of the thermodynamics of heat flow through windows. With a large difference in temperature across a window ( delta-T in engineering parlance), convection currents form in an insulating glass unit (IGU), and proportionally more heat loss will occur via convection compared with conduction and radiation. American window energy performance testing and modeling assume a delta-T of 70°F (0° outside and 70° inside). In Europe, however, a much lower delta-T is assumed for window energy performance modeling: just 35°F. Even in a reasonably cold Northern European climate, this much more closely reflects the average delta-T during the heating season.

American windows are designed to perform well with a 70°F delta-T, and European windows are designed to perform well with a 35°F delta-T.
This difference in assumptions affects the thickness of the air space in a window because a thinner window is less prone to the formation of convective loops. One can’t really fault U.S. manufacturers for building thinner windows. If they built thicker windows, they would perform worse using the testing methods that are required by NFRC. But this means that outside of northern Alaska, where you might actually see a 70°F average delta-T during the heating season, our windows don’t perform as well as European counterparts.

The same quirk of energy modeling assumptions may have indirectly played into another difference between European and American windows: the hardware. If you’re building thicker windows, those windows will be heavier and require stronger hinges and closures. European windows typically use much stronger, better hardware than American windows, and this contributes to the quality differences between them. The difference may also have to do with the European preference for inward-opening casement windows—and windows that can hinge at either the side or the bottom (tilt-turn operability). The hardware used for inward-opening tilt-turn windows has multi-point closures, which suck the windows tightly closed.

In addition to the windows listed in the table above, there are also many U.S. and Canadian windows that, while not certified to the Passivhaus standard, can be used to achieve Passive House performance. One of the challenges for non-European windows to be used in Passive House projects, however, is that the PHPP software used in certifying Passive House projects relies on the European method of U-factor determination, and most North American window manufacturers don’t provide that information.

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IMAGE CREDITS:
1. Photo: Alex Wilson
2. (no credit)

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