In April we attended the National Glass Association/Glass Association of North America (NGA/GANA) Energy Division meeting. During these discussions, it became clear that even though there appears to be a moratorium on reducing the prescriptive limit on window area in ASHRAE Standard 90.1, the battle for the wall is continuing, albeit on a different front. Tom Culp, Birchpoint Consulting and an NGA code consultant, reported that since the baseline efficiency requirements of other building systems have been reduced so much in ASHRAE Standard 90.1, there are fewer and fewer opportunities for trade-offs with façade performance. This means that design teams are finding it increasingly difficult to achieve compliance through the performance path, which most large buildings follow, for buildings with greater window area.

This discussion is not an isolated data point. Helen recently has heard architects protesting about the difficulty they are having in meeting energy targets with window areas of only 55 percent.

When architects ask how they can increase window area, yet still be code compliant, our answer is simple: Use higher performing window and curtainwall systems within a holistic systems approach to the building design. A recent example of what is possible with high-performing façade design is the LEED Platinum PNC tower in Pittsburgh, which uses a double skin curtainwall, automatically operated window vents and a central “solar chimney” for natural ventilation (fig.1). Even with such a high glazing area, it is reported to be 50 percent more energy efficient than the ASHRAE 90.1-2007 benchmark and 30 percent of the year, it is said to be able to operate in a net zero energy state.

Highly glazed buildings can achieve high energy-efficiency, but they need to use fenestration with great thermal performance and systems, which effectively manage solar gains. Lawrence Berkeley National Laboratory identified a net zero commercial window as one that has a very low U-factor (U=0.15 btu/oF.hr.ft2, R=6.7) with dynamic solar control and integrated lighting controls. Most commercial window manufacturers already have commercially available R5 (U=0.20 btu/oF.hr.ft2) windows.

So, what does this mean for code compliance today?

Since 85 percent of thermal bridging in a window occurs at its edge of glass and frame (fig.2), we recommend using thermally broken fenestration systems, that aren’t just minimally compliant with the National Fenestration Rating Council’s NFRC 100-2017[E0A1] definition of thermally broken metal system members separated by at least 0.20 inch (5.3mm) with a low-conductivity material. Effective, thermally broken fenestration systems should have much wider breaks to reduce conduction significantly. A well-known curtainwall consultant routinely specifies a minimum 1-inch-wide (25mm) thermal break to ensure both the needed energy performance and thermal comfort, and they are available up to 3 inches (75mm) wide. True warm-edge spacers, such as hybrid plastic-stainless steel or fully non-metal systems, that insulate the edges of insulating glass are also important for reducing thermal bridging.

Reducing conduction alone is not sufficient. Convection, which accounts for 35 percent of the energy flow at the edge, also needs to be prevented. Higher performing fenestration systems use wide thermal breaks, which also are shaped to break up convection currents using fins or attached foams (fig.3). Advanced window systems now are being augmented with low-emissivity coatings on the inside of the extrusions and thermal breaks to reduce the final 15 percent of heat transmission due to radiation.

Even though ASHRAE Standard 90.1 has held the prescriptively allowable window to wall area constant since the dialogue began in 2010, the real-world net result of other code changes makes it much harder to achieve building designs with higher glazed areas using the performance path, unless glazing systems with significantly higher performance are specified. These solutions come at a higher upfront cost than the business-as-usual, lowest-first-installed-cost systems. Achieving higher adoption rates of these products will require our industry to actively promote and demonstrate the longer-term economic value they deliver in terms of improved occupant health and comfort, building resilience and longevity, as well as energy-saving performance. While these are hard to measure attributes, many strides have been made in quantifying the impact of well daylit, and thermally and visually comfortable indoor spaces.

Based on recent conversations, it also is becoming apparent to us that the design community in general is not aware of the extent to which higher performing fenestration systems are already available. They need help from the glazing industry to find product and system solutions to meet code requirements. We have great opportunities to drive toward higher performing fenestration!