Building Envelope Efficiency: Understanding Fenestration

Fenestration is defined as the arrangement of windows and doors on the elevations of a building, and can have a significant impact on building envelope efficiency.

The green roof atop The Republik Building in Durham, NC

Credit: Xero Flor
The green roof atop The Republik Building in Durham, NC

Editor’s note: This article first appeared in the July-August 2015 issue of Business Energy.

Ulrike Passe, associate professor of architecture, and director of Iowa State’s Center for Building Energy Research lists a building’s fenestration components as doors, windows, and skylights. “You need to seal every window and opening because that’s a critical factor for energy efficiency these days,” advises Passe. “The better the windows and doors, the more holistic the performance of the envelope.”

In fact, the performance of energy-efficient windows can be significant, as in the example of 625 North Michigan Avenue, a 25-story office tower located in downtown Chicago. It was constructed in 1970 and has approximately 350,000 square feet of conditioned space. Even though the building had previously been retro-commissioned to perform to higher energy efficiency standards, an interior curtain wall retrofit system increased energy savings by 16%. The windows, supplied by Thermolite, headquartered in South Bend, IN, were comprised of the company’s RetroWAL interior curtain wall retrofit system, and included three models: ¼-inch laminated glass with low-e hard coating, 1-inch insulated glazing unit with low-e soft coating, and ¼ inch laminated glass with low-e hard coating, plus 1-inch blinds in the air cavity.

The fenestration industry has made great progress in energy efficiency, according to Tom Herron director, communications and marketing, National Fenestration Rating Council, Greenbelt, MD. “One of things that makes this industry so exciting is the smart technology,” says Herron. “For example, we’re seeing window shades, sensors, and lighting controls. Eventually, we’ll have smart windows that adjust to your preferences or energy requirements. It’s important because people don’t realize that there is a tug-of-war between the sun’s heat and the glass, so the HVAC system has to work harder.”

One of the main goals of the NFRC is to help building designers and facility managers in meeting the growing list of regulations. “There are fenestration energy codes for commercial and residential buildings,” says Herron. “But the main thing to keep in mind is the overall performance of the glass, and the NFRC’s ratings provide the whole range of product performance. They take everything into account, including the framing, glazing, and spacers, and we rate doors and skylights. So the NFRC label on a product is your assurance that it’s going to perform the way it’s advertised.”

When it’s time to consider a building’s energy load, performance specifications are important for architects, designers, and engineers who need to know the ratings of a building’s fenestration components. Says Passe, “When an engineer sizes an air-conditioning system they include the building envelope in their calculations. The envelope is the starting point for all system decisions. For example, if you have solar gain in summer, it’s detrimental to your cooling load. So a wall with unshaded glass exposed to sunlight would require you to turn up the air conditioning, and the same with black roofs. A black roof heats up more than a white roof, and the heat going through the roof is determined by the surface temperature of the roof. So the material for the roof is critical, and that’s why in California we see so many white roofs.” BE_bug_web

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