Appearance and Aesthetics: Low-Iron Glass

Of utmost importance to most architect, however, are two variables that European window products have addressed: performance and appearance. While the thermal profile of a window may be critical to energy efficiency, today's designers also demand excellent light transmission, clarity, and visual consistency.

One of the major improvements to architectural design in recent years has been the use of low-iron European glass, which has less of the naturally occurring tint found in standard float glass. As implied, this glass formulation has a lower iron oxide content—usually about one-quarter of the amount used in normal float glass—giving it what Mother Nature Network called “water-white clarity,” ideal for framing artwork or for floor-to-ceiling windows. The crystal-clear, highly transparent glass offers another bonus of improved acoustic performance.

In addition, low-iron glass is inherently more energy efficient than other glass types. The panels absorb less IR energy, so they stay cooler when exposed to direct sunlight, yet they also transmit light very efficiently, so there is some associated thermal gain in the building interior. The glass is favored for building-integrated photovoltaic (BIPV) systems, in part because of the good levels of visible light transmission (VLT). Yet the glass panes are similar to any float glass in material composition and recyclability.

Overall, low-iron glass makes a strong case for aesthetics, performance gains, and environmental friendliness. It is increasingly a popular specification for European window designs and projects meeting high-performance expectations.

Window Ratings: Understanding NFRC

Regardless of the glass, fill, and frame materials selected, most windows and skylights in the United States have a label indicating their baseline performance as verified by the National Fenestration Rating Council (NFRC). The labels list U-factor, SHGC, visible transmittance, and air leakage. While NFRC ratings and their familiar labels seen on new products help assure conformance with building codes and energy codes, they are similar to those codes themselves: They set only a minimum performance level.

More to the point, the NFRC values are based on a window size of approximately 4 feet by 4.5 feet. Given that glass will always outperform the frame, if the architect implements larger sizes in the building designs, the resulting window assemblies can achieve performance levels that are far greater than the NFRC stickers display.

European windows are designed and engineered to be made in very large sizes, so this unique opportunity to exceed the NFRC label performance levels has been seen by some architects and their clients as a good reason to use European windows at large sizes.

In addition to this strategy for using windows to achieve high performance, the NFRC identifies “beyond-code programs” in its literature, referring to green codes and “stretch” energy codes given in many jurisdictions. Some of these become new minimum codes in those states and cities, while some offer developers incentives or rebates to meet them.

Other beyond-code programs require more or better high-performance fenestration. These include LEED certifications, Living Building Challenge, and Passive House guidelines. Compare a window with an NFRC sticker to a European window product that meets PHIUS baseline performance:

Criterion (*typical values)	NFRC*	PHIUS*
U-factor	0.20 – 1.20	0.14 – 0.16
SHGC	0.25 – 0.80	0.48 – 0.62
VT	0.20 – 0.80	0.74
Air leakage	0.1 – 0.3 cfm/sq. ft.	0.03 cfm/sq. ft.

According to the trade group Efficient Windows Collaborative, the NFRC sticker displays ratings for the whole window, including glazing, frame, and spacers. This overall window value is important to know, and again, the window performance variables will vary greatly depending on how big the window is. Architects seeking improved performance and greater energy efficiency consider all of the components together, based on the size of the opening and the expected U-factor, heat gain, and VT.

European designed and fabricated windows tend to have better performance to meet the needs of Passive House or other “stretch standards.”