Do Europeans Make Better Windows Than We Do?
Differences in testing protocols yield different U-factors, but do European manufacturers have a ‘secret sauce’?
It should come as no surprise that Europe, home of the Passivhaus standard, produces some outstanding windows. Some builders of high-efficiency houses in North America turn to European window manufacturers for their 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., even though some U.S. and Canadian producers also offer high-performance products of their own.
Is there a way to compare the performance data on windows from these two sources? That’s what Steve Young, now planning a 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. in Climate Zone 5, would like to know.
“I have read many blogs and Q&A pages from this web site and I am still somewhat confused about European windows,” Young writes in Q&A post at GreenBuildingAdvisor.
First, he says, there are the different testing standards between North American and European manufacturers, which makes Young wonder how he should view data on U-factors, solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. (SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.), and visible transmittance (VT).
“So should I pretty much ignore the U-factors, if I am looking at the ‘best’ glass and frames and compare the SHGC and VT only?” he asks. “The Optiwin figures (SHGC 0.52) seem to beat anything that Thermotech Fiberglass has (SHGC 0.42), or is that somehow measured differently in Europe, too? Transmittance has to be measured the same?”
Young points out that European windows don’t have nailing flanges like North American windows, and that while fiberglass windows are available on this side of the Atlantic, they don’t seem to be available from European makers.
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How can the two be compared?
That’s the subject of this month’s Q&A Spotlight.
Testing protocols are different
Don’t ignore published European U-factors, says GBA senior editor Martin Holladay — just keep in mind the fact that you can’t compare them directly to those published in North America. “Lower is still better than higher,” he writes.
In the window industry, adds Greg Smith, “it's something of a given (depending on source, of course) that translated European U-factors are at least 10% ‘better’ than the equivalent NFRC [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] rating. Some folks will suggest the ‘advantage’ is closer to 20% when you start getting down into the low teens and better.”
One factor is how the testing calculations are actually made, Smith says. “The lower delta-TDifference in temperature across a divider; often used to refer to the difference between indoor and outdoor temperatures. used for European calculations versus the greater delta-T used for [North American] U-factor calculations is an ‘advantage’ for European designers resulting in the apparent U-factor advantage of the European glass package/window,” he writes. As Smith's subsequent comments made clear, when a European manufacturer reports a lower (that is, better) U-factor than a North American manufacturer, the lower number doesn't necessarily indicate that the European window will perform better than the North American window.
“Additionally, European windows are more likely to be fabricated with a thicker glass package (heavier glass with wider airspace) than are North American window makers,” Smith adds.
Finally, European manufacturers are more likely to use low-iron glass in high-performance windows rather than the conventional float glass used by North American manufacturers, Smith says.
Do European windows actually perform better?
Mike Eliason argues that while there are “slight differences” in how U-factors are calculated, “the numbers don’t appear to be off by a significant factor.”
More important is the apparent difference in performance. European makers “have been pushed by both policy and demand” for glazing with high solar heat gain coefficients and very lower U-factors, Eliason writes. As a result, there are some “spectacular glazing units” available from European makers, particularly from companies such as Glas Trösch and Guardian.
“We’d love to use [North American] windows and glass manufactured in North America for the PH [Passivhaus] houses we’re working on,” he says, “but no one seems willing to step up to the plate and produce what we can get from EU for cost, performance and quality.”
“In our research, the glass in North America isn’t quite up to par with what’s being produced in the EU,” he says, “even if you take a 10% difference based on the alternative means of calculating U-factors.”
Glass packages are ‘pretty much identical’
But Smith doesn’t agree. “If we do an apples to apples comparison, then glass packages are going to be pretty much identical whether the glass package is from North America or Europe,” he writes. The use of thicker, low-iron glass and wider air spaces gives European glass the “slight performance edge” that Eliason cites in a number of examples, but that’s not due to any technological advantages, he says.
“When comparing European and North American glass packages using the same basic glass substrate, comparable (high solar heat gain, low solar heat gain, etc.) low-e coatings, and keeping air space depth and fill consistent, you aren't going to have any real differences in energy performance, no matter where the glass package was produced,” Smith says.
“Bottom line is that glass is glass, doesn't matter if it is floated in Europe or in North America (just as long as it's low-iron versus low-iron or standard soda-lime float versus standard soda-lime float). European argonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows. and North American argon come from the same source no matter where the IGU comes from. 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 are pretty amazing -- both application and performance -- but there is no one in either Europe or North America who has cornered the market on some new miraculous low-e coating technology.”
What’s really different, he says, is a “matter of what people are demanding from window suppliers and what the window suppliers want from their glass suppliers. We might spell the difference between Europe and North America as ‘education.’ ”
The European ‘secret sauce’
Having the details spelled out is helpful to Jesse Thompson, who says it “never smelled right” to him that European manufacturers had some “secret sauce” giving them an edge.
“Especially when they seem to be all multinational corporations selling products across the globe,” Thompson says. “It seems to all come down to good old-fashioned sales demand, regulation, and market movement.”
In response, Eliason notes: “Ah, but there is a secret sauce of sorts.” European calculations favor thicker spacing between panes, whereas the NFRC calculations favor thinner ones. And the Europeans seem to have figured out how to make low-iron glass without it costing too much.
“We had it [low-iron glass] priced out by two North American [manufacturers] and the increase in price was phenomenal,” he says. “With the EU Passive House windows we imported, there was no cost increase for the ultra high SHGC glass. In actuality, it was a cost decrease -- as the windows were less expensive than the North American windows we would have otherwise used, and then because of the high SHGC, we were able to shed a few inches of insulation as well.”
Our expert’s opinion
Here’s what GBA technical director Peter Yost has to say on this issue:
Alex Wilson wrote a great piece for BuildingGreen on European windows for Passive House. And Mike Duclos, a Passive House consultant and energy rater who deals with differences in European and North American windows all the time, reminded me that the wider frames of European windows also make “over-insulation” easier.
Our industry needs to address these differences at the international level. But the real problem of comparing North American and European windows runs pretty deep. For years, scientists at the Lawrence Berkeley National Laboratory have been lobbying the International Standards Organization (ISO) to adopt a single set of standards for the testing and computer simulation of window performance. And yet two very different standards (ISO 15099 and ISO 10292) are still being developed and pursued. This is NOT how global commerce is supposed to work, especially when less and less actual testing of windows is being conducted and more and more modeling is.
For more information on issues with ISO window standards development, see these two references:
- “Discrepancies Between ISO Window Simulation Standards” by Dr. Charlie Curcija, University of Massachusetts - Amherst, November 23, 2005. (Charlie Curcija is now a senior scientist with LBNL, and they will soon be posting this reference on the LBNL website). This paper references the work cited below.
- CENT/TC89/WG7 ad hoc ISO 15099: “Comparison of standards on thermal, solar and/or light properties of windows and window components” by Dick van Dijk, Delft, The Netherlands.
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