With glass performance for energy efficiency being comparable in the U.S. and Europe, window frames on this side of the Atlantic have been eyed as underperforming on the U-value front as compared to those in Europe. But the reason for—and even the very premise of underperformance—is complicated, say experts.

As for the why, one general consensus that industry members point to is that codes are more stringent in Europe. Interestingly, in addition to U-value benchmarks, differences also may involve requirements that are not directly related to energy efficiency.

“[Europe doesn’t] have the same air-water structural requirements that we have in the U.S.,” says Patrick Muessig, vice president of global technical operations at Azon USA. “So if you can get away with a lighter frame, then it’s easier to get a better thermal rating.” Adding such factors as requirements in regions that may have earthquakes or hurricanes, for example, further complicates the equation.

Moreover, comparisons between the two continents can be of the apples-to-oranges variety. To start, the European market structure is set up differently, with frames and glass often marketed and sold as separate products. Therefore, if, say, you go to a European trade show, don’t expect a straight U-value for the whole window system. Rather than see a label for a product with a comprehensive U-value for a unit, you may see a U-f value, referring to the frame alone.

“It’s important to note the market differences,” says Chad Ricker, market team manager for Technoform Bautec. “It’s not necessarily comparing apples-to-apples. It’s not as clear as one would think.”

Those market differences spill into the technical. Explains Helen Sanders, vice president of technical business development at Sage Electrochromics, “In the U.S., the code requirements are based on whole window U-factor values, whereas in Europe there are specific requirements for both frame and center of glass.”

And then there’s the technology itself. Proprietary products notwithstanding, any high-performance technology available in Europe is available in the U.S. But with these constantly evolving technologies, coupled with the multiple variables that make up window systems, comes the need to make everything work together toward a client’s end goal, says Mark Silverberg, president of Technoform North America. His company, therefore, places a strong emphasis on the comprehensive consultative relationship with their clients, he says.

What are a client’s overall performance and energy efficiency targets, for instance? Technoform, for example, works with a client from early in the design process to bring all the variables together, given that, according to Silverberg, mere use of a polyamide insulating strip does not, in itself achieve a goal. The placement of the insulating strips is dependent on optimizing each design, as are other variables.

Silverberg also points out other market-related elements that increase the complexity of the issue. It’s more than the codes in Europe simply being more stringent; it’s a matter of market economics at work. In Europe, for example, building owners often are required to have energy certificates that stipulate energy consumption. If energy rises above caps, the building owner, not the tenant, may be responsible for the overages, giving the owner economic motivation to achieve high energy efficiencies.

In the U.S., meanwhile, developers are concerned most with lowest first-use installed cost and simple payback calculations. Energy costs are typically passed on to tenants, while developers may plan to flip the building within five or seven years, bringing such factors into play. It becomes a situation in which, “Due to the market structure, there’s no way to recapture their investment in the high-performing building envelope,” says Silverberg.

Exceptions to that generalization: public buildings, wherein the long-term operating and maintenance costs of a building will be a priority, and large corporations with a strong corporate-responsibility ethos and brand. Both of those segments are driving the use of high-performance technologies, Silverberg notes.

And then, of course, there is the code—and even that factor gets more complicated than simply stating that they are more stringent in Europe. “My sense is that the code requirements have been the main driver for fenestration developments in both continents,” Sanders says. “In Europe, because of the 1.1 W/m2K center of glass requirement in most areas, this requires a good low-E and argon filled for all units.  In some of the northern climates [approximately a] 0.06W/m2K is [necessary] which requires triple glazing with argon fill.  The other requirements force European [insulating glass] manufacturers to use warm-edge spacer and frame manufacturers to have very good thermally broken frames.”