Fixation on Window Performance

Why are windows such a critical focus of European standards? While architects have an intuitive sense of why glass apertures in an otherwise opaque enclosure mass would have an outsize impact on thermal conditions indoors, a simple illustration shows exactly how dramatic that impact can be.

Consider a subject vertical wall segment that has an insulating value of about R-50. Now add to the design around 15 percent glazing area. If the design employs ENERGY STAR-certified windows—with an insulating value of R-3 and meeting EPA-mandated performance levels that became effective in January 2015—the total wall R-value for this assembly drops from R-50 to R-15, a 70 percent reduction in performance, as shown in an analysis by the Seattle-based contractors Hammer & Hand. To compensate, let's say the architect designs an improved wall construction so that the opaque area reaches R-100. That's good news but rather ineffective, because it results in a total R-value of only R-17—only a 2 percent improvement over the R-50 wall example above.

It is such a slight improvement, in fact, that the designer's attention should have been on improving the fenestration performance instead. The fact that ENERGY STAR windows meet only modest energy standards is beside the point. Rather than meet ENERGY STAR, the project architects should exceed those performance levels and specify better windows that meet higher-performing European construction standards. With a better window with an insulating value of R-8, the original R-50 test wall segment can yield a whole-wall insulating value of R-38—a significant improvement over the R-15 one might achieve with lower-performing R-3 windows.

Key to the better performance levels? The R-8 product will have deep frame profiles with engineered thermal breaks using double- or triple-glazing, with gas-filled IGUs.

In addition, a range of opening types, design options, and functionality are available with recent European fenestration products. One of the most discussed in the last few years are tilt-turn windows. “These windows can both swing inward and tilt inward with a simple turn of an ergonomic handle,” says Florian Speier, founder of the Steamboat Springs, Colorado-based manufacturer Zola European Windows. “Tilt-and-turn windows can be very large, with each sash up to 5 feet wide, using multi-point locking mechanisms for both safety and effective air sealing.” While U.S. end-users are more familiar with basic sliding, casement, and double-hung window types, the tilt-turn is growing in market share. Unlike casements, tilt-turns don't have crank handles; common wood finishes include pine, oak, and meranti, which is similar to mahogany.

Tilt-turn windows and other European opening types, such as lift-slide doors, are engineered to accommodate moving sashes that are substantially larger than their domestic counterparts. Given that larger units are typically specified, the somewhat wider European frames become very slender in proportion to the window size. Nonetheless, some European manufacturers have pushed to minimize frames as much as possible in recent years.

For example, one concealed sash window—a tilt-turn type for openings up to 5 feet by 8 feet—is often seen in thin, aluminum-clad wood frames that completely hide their concealed hinges and sashes. The slender frame helps to maximize views out, and the hidden sashes help architects design continuous glass lines on façades regardless of whether the windows are fixed or operable. In this way, fixed windows and tilt-and-turn windows are indistinguishable from the exterior, keeping the look sleek and crisp.

Another related type of fenestration that is still popular in many U.S. markets is the casement window. “Casements are sometimes preferred because end-users want a window that swings out, so some European window makers are producing high-performance versions,” says Speier. “But they come with the limits of smaller sizes as compared to tilt-and-turn products due to design wind loads.”

There's no such concern for fixed windows, which present a very cost-effective method for implementing large glass areas. Some of the European-designed products offer fixed openings of up to 100 square feet or glass area that are covered by normal manufacturer warranties. In addition, European designed sliding doors and folding doors are finding more applications in the United States. These door designs have a unique hardware assembly: rather than sliding in their seals like many American types, the European sliding doors are lift-slides or similar constructions that move the operable element away from the seal before it slides. In this way, highly airtight compression gaskets can be used. Widths of up to 20 feet are possible with two large glass panels (and no dividers).

Even larger sizes are possible with more than two panels. For example, a triple-panel or quad-panel design can extend as far as 60 feet across.

Last, European designed window products include Passive House windows, as mentioned previously, which by definition are certified by Passive House Institute U.S. (PHIUS) or Passive House Institute Germany (or, in rare cases, both). The PHIUS certificates show climate-specific recommendations for the certified windows—on south-facing or north-, east-, and west-facing façades—as well as lists the whole-window installed U-values and the center-of-glass (cog) U-value, or U_cog-value. This measure describes heat gain or loss through the glazing without taking into account the effect of the frames or size of the glass. Lower values represent better insulation, and it is the reciprocal of R-value and R_cog-value, where a higher value is better performing.

Based on recent certificates issued by Passive House Institute U.S., a quadruple-glazed European window design with a Krypton fill and an SHGC of 0.456 can achieve a whole-assembly installed U-value of 0.11 or 0.12 Btu/hr.ft².F and a U_cog-value of from 0.056 to 0.064. “These installed U-values include the significant losses at the wall-to-window junction,” says Speier, “a loss that can be mitigated by the architect with careful installation detailing.” For comparison, the standard triple-glazed window with Argon fill (also PHIUS-certified) has an SHGC of 0.529 and achieves whole-assembly installed U-values of 0.14 to 0.16 a U_cog-values ranging from 0.096 to 0.119.

In all cases, the window products will allow building designs consistent with the requirements for thermal control and reduced air infiltration as required by the Passive House Institutes.