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September - October 2003

The Rate Debate
Jim Benney

A New Angle on Skylights
New Procedure Closer to Real-Life Applications 
by Jim Benney 


What do you call a skylight that is installed vertically? A window, right? Until now, the National Fenestration Rating Council (NFRC) has tested and simulated skylights to determine U-factor just as it tests and simulates windows—as though they were installed in a wall (i.e., on a vertical plane or a 90-degree angle). This procedure did return an accurate rating for the skylight under those conditions, but as everyone knows, skylights aren’t installed in a vertical wall, they are installed on sloped roofs.

So NFRC set out to develop a new procedure to simulate skylights at an angle that approximates real-life applications more clearly. The key to the new procedures was the development of new algorithms, calculations and simulation software that could handle sloped skylights.

In June 2002, NFRC adopted two new programs developed by Lawrence Berkeley National Laboratories, Window 5 and Therm 5, which included updated techniques for how skylights are to be simulated. With the software in place, NFRC was free to adopt new procedures for rating skylights. Since NFRC still requires validation testing, skylight units will continue to be tested and simulated at 90 degrees for validation, then re-simulated and calculated for rating (and labeling) at a 20-degree slope.

The new procedure allows skylight manufacturers to promote energy-performance ratings that reflect how skylights actually are applied in the building envelope.

“This is a big step forward for consumers of skylights,” said Roland Temple, NFRC treasurer and skylight code and certification compliance coordinator for VELUX-AMERICA Inc. “The new standards will allow consumers to get a clear picture of how the skylight will meet their energy efficiency needs.”

According to Temple, the new 20-degree slope will result in different ratings for sloped products. He is concerned that this might cause some confusion in the marketplace. 
Because of the change from 90 to 20 degrees during the simulation process, the ratings will likely decrease, giving an impression that the skylight is less efficient. In fact, nothing about the product changes except that the ratings better reflect the efficiencies of skylights in relation to the building envelope. 

Avoiding Consumer Confusion
Consumers also can expect to see a mix of labels affixed to the skylights, as those simulated for 90 degrees continue to rotate through the retail system alongside skylights simulated for 20 degrees. To alleviate some confusion, NFRC will assist skylight manufacturers in developing and distributing educational materials with the products that explain exactly what the new ratings mean in terms of energy efficiency.

Since NFRC’s inception in 1989, its programs have stipulated that skylights were to be specified at 90 degrees until an appropriate method could be found to rate them at a more appropriate angle. Although the technology wasn’t yet available at the time, NFRC did not want to leave an entire class of products behind.

Change on the Horizon
Since last summer, manufacturers have been busy getting ready for the change. As the sunset date for compliance with the new procedure approaches, and April 1, 2004, consumers will begin to see more and more of the new labels. 

Energy Star® is getting ready as well. The program’s new four-zone criteria (see related article page 24) will require a U-factor of 0.60 to earn an Energy Star designation in the Northern, North-Central and South-Central regions to reflect the new sloped values.

Despite the fact that consumers might see some contradictory information on the labels for a brief time, Temple emphasized that the quality of the skylight is not changing–just the manner of simulation and rating. 

“Consumers will continue to receive the same high quality of skylights that they are accustomed to,” he said. “NFRC is merely providing them with the most current information using the best possible techniques available.” 

Clearly, this is a benefit for everyone.


Jim Benney serves as executive director of the National Fenestration Rating Council based in Silver Spring, Md. 

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