It’s the law of unintended consequences: With greater emphasis on daylight and views comes increased heat and glare inside. Blinds, curtains, and sunshades negate the benefits of expansive glazing, as do higher HVAC costs. In recent years, tinted electrochromic glass—also called smart, smart-tint, dynamic, or switchable glass—has become a more accessible option.
Electrochromic window products typically consist of an insulated glass unit (IGU) with a laminated outer light. A double-glazed IGU product by Fairbault, Minn.–based SageGlass, for example, comprises six lite surfaces, in which surface one is the outboard face of the exterior-most lite. Surface four, the inboard face of the laminate immediately adjacent to the IGU’s sealed air space, hosts the electrochromic coating, which itself is made of five micro-thin layers: a transparent conductor, counter electrode, ion conductor, electrochromic electrode layer, and transparent conductor. When a voltage is applied to the sandwiching transparent conductor layers, lithium ions hop from the counter electrode across the ion conductor and power the electrochromic electrode. The more voltage applied, the more tinting occurs, redirecting sunlight and solar heat angling to come inside the building. The transition from clear to fully tinted—and vice-versa—can be fast, from under three minutes to upward of 15 minutes.
The Dallas office of Corgan has used electrochromic glazing on office projects and airport terminals. “It is a very worthwhile system when eschewing traditional fabric shades on large areas of curtain wall glazing,” says senior associate James Adams, AIA. The system is particularly useful, he adds, “in tandem with circadian lighting design.”
Buildingwide, the use of double-pane electrochromic IGUs or windows can save 15% to 20% more energy than a double-pane, low-E IGU across climate zones, and up to 26% at peak load times, according to SageGlass senior channel marketing manager Jordan Doria. The electrochromic window itself, he notes, uses less than 5 volts of electricity to operate.
Performance Expectations
Electrochromic windows provide “greater benefit in more cooling-dominated climates because of [their potential to achieve a] very low solar heat gain coefficient (SHGC),” Adams says. Typical SHGC values for electrochromic glass range from 0.41 to 0.09, where the latter value indicates that 91% of the incoming solar heat is blocked.
For projects in hot climates, look for windows with lower SHGC values, around 0.25, to help reduce cooling loads in the summer. For projects in cold climates, look for products with higher SHGC values, around 0.40 to 0.50, to allow more solar heat gain and reduce heating loads in the winter. This highlights one inherent advantage of smart glass: its SHGC can vary with its tint, ideal for a region with a mixed climate.
Common U-factors (the rate of non-solar heat loss) for electrochromic IGUs range from 0.07 to 0.40. In hot climates, look for a U-factor of 0.20 to 0.25 for a double-glazed unit. When a lower value is preferred—for example, in an extremely cold climate—a triple-glazed window may be more appropriate.
In the Clear
The visible transmittance (VT) of smart glass can be adjusted based on daylight and glare conditions. The wider the VT range, the more the glass can mimic the performance of shading and the better suited it is for mixed climates. Regardless of location, Doria says, sometimes a VT of 60% is valuable and sometimes 1% is valuable: “You need 1% to control glare, but when glare or heat control are not needed, you want 60% to maximize natural light.” A product capable of a VT between 28% and 60% can offer clear views, good for when glare is not an issue. A VT of 18% to 21% corresponds to low-level tinting, good in times of overlighting without no direct glare. A VT of 6% to 10% indicates a midlevel tint, which can control some direct glare. Finally, a VT of 1% or less indicates full tint and glare control.
Common tint hues include blue, green, and gray, though custom color options are possible.
Somewhere in Between
In-pane zoning can be an elegant solution for mixed climates. It works well in curtain wall applications because multiple tint zones can occupy a single pane—up to 5 feet by 10 feet in size—and effectively manage the incoming sunlight since only part of the glazing may be subject to glare at times. In lieu of in-pane zoning, distinct IGUs stacked or butted together can be set to different tints.
Retrofit Alternative
While electrochromic glass is often specified for new construction, existing windows can be reglazed with smart glass as well. A less intrusive option is to apply switchable film to the original glass. While switchable films don’t offer smart glazing’s broad range of options—acoustical mitigation (from a sound transmission class rating of 35 to 42), fire resistance, and lamination—and may not be appropriate for the application, they can offer a quicker and more economical route.
Cost Comparison
Electrochromic glass might cost three times as much as a standard low-E IGU, but the comparison is akin to weighing the benefits of a smartphone against a flip phone. When compared with a low-E IGU plus an automated mechanical shading system, the upfront cost can be surprisingly similar.
