Few phrases are batted around as casually, and with such little precision, as “high-performance windows.” If you’re building a net-zero house or seeking Passive House certification, high-performance windows will be a given. Taking on a major renovation of an existing home? High-performance windows are probably part of the plan.
In reality, there is no such thing as a high-performance window that’s suitable for all climates, all housing types, and all solar orientations. “Manufacturers are going to define ‘high performance’ as they see fit,” says Steven Urich, director of programs at the National Fenestration Rating Council (NFRC). “There’s no standard definition for it across the industry, as far as I know.”
As important as windows are to the overall building enclosure, they are but one of many components that have to work together. The ideal window for one part of the country may not work as well in another.
“It’s the whole-house concept, which is what Passive House is trying to get at,” Urich says. “This idea that windows alone, your HVAC system alone, insulation alone [will be the end-all solution is unreasonable; no single component can] determine the energy efficiency of your home.”
Performance ratings for windows are a combination of several factors. The glass, or glazing, is key because it takes up most of the overall area of a window. Insulating glass is amazingly diverse, ranging from simple two-pane assemblies of clear glass to multi-pane sandwiches with low-e coatings and gas infill, all with different performance characteristics that can be tuned for a specific application. Other key parts are the window frame, the spacers between the panes of glass, and the hardware that ensures a tight seal.
Reading the NFRC label
The NFRC is a non-profit industry group that tests window performance and publishes the…