Start with Glass Selection

For performance levels like those achieved with these certified Passive House windows, experienced architects tend to start with a general idea of performance needs and budgets for major building components. What allowances for European-style windows are needed for project planning? Are triple glazing or quad glazing allowed by the budget and overall client expectations? If they are consistent with performance needs, the answer is likely yes.

“We had done some significant investigation into the possibility of getting higher-quality, higher-performance windows than we could find in the United States for our Passive House- and energy-driven projects,” says Brian Fuentes, AIA, CPHC, principal of Fuentes Design in Boulder, Colorado. “We have been able to successfully help our clients install higher-performance Euro windows here in Colorado, even at high altitudes.”

In general, project design development should pay extra attention to glass and IGU selection variables and expected performance. This includes such functional needs as acoustics, interior daylighting, and exterior visibility and, if needed, security or ballistics performance. Second, zoom in on glass performance based on the raw measurements by the manufacturer: These are mainly U-value, SHGC (high solar heat-gain vs. low-gain glazings) and the related variables of light transmittance (VT), reflectance, and absorption. These three light variables will be compared to the SHGC values as part of balancing the value of interior natural light against anticipated heat gain—some wanted, some not.

Next, there are glass treatments and options that will affect the ultimate specification, such as low-emissivity coatings. Most coatings are thin layers of metal applied to the glass just after manufacture or during finished glass fabrication to improve solar performance or change the reflectivity of the glass surface. Mirrorlike reflective coatings were among the earliest coating types, and they helped cut heat gain simply by reflectance. This also reduced light transmission, often to unacceptable levels. Contrast this with low-E coatings, which reduce heat transfer but preserve higher levels of light transmission.

IGUs for today's high-performance windows are typically designed with multiple glass panels, and various coatings including low-E products are applied (or deposed) onto only the glass surfaces that produce the best possible results. In many cases, a combination of low-E coatings, sometimes with reflective coatings, will provide the best performance in terms of thermal and visual needs. Each glass panel has two sides, so a double-glazed IGU has four glass panel surfaces that could receive coatings; similarly, a triple-glazed IGU has six, and a quad-glazed window has eight. Each glass ply surface is designated by a number, with the surface at the exterior of the building always referred to as surface #1; the back of this ply is surface #2. The low-E and reflective coats, if any, are applied to the surfaces that provide the optimal physics of visible light transmission and heat gain.

Triple glazing is increasingly seen in North America, while the quad-glazed window is an emerging spec. The question is, does the fourth glass ply add enough value to bother? The answer may be yes, say experts like Speier. “Using triple glazing is a no-brainer because the additional cost is very small and the glass insulates nearly twice as well as double glazing,” he says. “Quad glazing is another big step forward; however, at current cost levels it typically only makes sense in colder climates.” In warmer climate zones, where solar heat gain is less desirable as enclosures are engineered to keep heat out most of the year, the lower SHGC and good thermal resistance of triple-pane windows are effective.

While the three cavities of quad glazing is inherently superior to two cavities—each chamber reduces convective currents and thermal transfer—the question in all projects is, “How superior?” Over time, the quad glazing pays for itself according to life-cycle analysis (LCA) studies; depending on project variables, a payback period of five to 12 years can be expected.

Gas Fills for IGUs: Argon and Krypton

The performance of the double-, triple- or quad-glazed IGUs that are used in the building project depend on how the cavities are filled. The use of inert gases—often called low conductivity gas fills—improves the performance over simple air fills. The low-E coating handles the radiant component of heat loss/gain, while the conductive heat movement or gas-phase conductivity remains; the gas fills help handle that second effect.

While air can reduce some conductive effect, its thermal conductivity is 0.014 Btus per square foot per hour for every degree Fahrenheit difference in temperature (Btu/h/ft²/°F); compare this to Argon, which boasts a conductivity that is about a third less, at 0.0092. Argon is a common gas fill, but Krypton is also used and its conductivity, at 0.0051, is about 63 percent lower than air alone, according to Green Building Advisor. Other gases may have a lower conductivity—such as the rare gas Xenon, which conducts heat at a rate about 79 percent lower than that for air—but their cost and practicality for use are not yet established.

In general, experienced architects view Argon as the more cost-effective gas, while Krypton can offer high performance in certain situations. For example, Krypton tends to be more effective in narrower spaces, such as in the smaller gaps between glass plies in some IGU designs of around 3/8 inch. Most double-pane windows have Argon fills, which works well with the relatively large gap between glass plies—around 1/2 inch between the panes. For these reasons, Krypton is used in some quadruple-glazed European window designs, as it is more effective in the narrower space. Experts note that if the window has a wider space between the glass panels, Argon is just as effective, and the wider space will give a higher IGU performance at less extreme temperatures.

Current-generation European frames can carry IGUs just over 2 inches thick, which accommodate either Argon-filled, triple-glazed glass at optimum 5/8-inch to 3/4-inch gaps, or quad glazing with smaller cavities, which then requires Krypton for optimum performance. Domestic, U.S.-made windows can typically carry IGUs of up to about 1 inch; to optimize performance, these tend to use Krypton fills.

Today's manufacturing technology has helped to improve the performance of inert gas fills. In some cases, for example, the design R-values and U-values of gas-filled IGUs have been compromised by fabrication and installation practices. Manufacturers shipping cross-country, for example, fit their windows with breather tubes installed, which allows the interpane spaces to reach equilibrium with the outdoor air pressure. This helps prevent breakage, bowing, and seal failures while traveling over high-altitude passes, but in some cases it has led to unacceptably high losses of the low-conductivity fill gases. Today, however, some European window manufacturers use more advanced pressurization systems to ensure the window performs at the rated performance even for very high-altitude locations and transport.

In addition to the use of inert gases for energy savings, the glass itself may require certain performance characteristics that affect specifications. Laminated glass—where two or more panes of glass in the IGU are bonded together by a durable plastic interlayer—offers strength and safety features ideal for resistance to hurricanes, missile impact, explosions, and forced entry. Perhaps more broadly applicable, laminated glass can provide excellent acoustical isolation and reduce sound transmission. Both laminated glass and hardened tempered glass, often called “safety glass,” may be required by code:

▶ for any doors

▶ where the bottom edge is less than 18 inches from the floor

▶ where any edge of the glass is within 36 inches horizontally of a walking surface

Also, windows adjacent to any doors and walking surfaces or near stairways, landings, and ramps may require tempered glass under certain conditions.