Volume 6 Issue 9 October 2005
To Energy Star Discussion
The September issue of DWM contained several responses to my column Energy Star® Loses Luster (see July DWM, page 16) about revisions to the Energy Star windows program. I would like to provide some clarification on a few points.
Much of the discussion has centered upon “equivalent energy performance.” Unfortunately, the previous U.S. Department of Energy (DOE) Energy Star prescriptive criteria do not reflect real world product performance. A review of frame and glazing type data published by the Efficient Windows Collaborative (EWC) shows that there are significant differences in thermal performance for Energy Star qualifying windows. The data from the EWC further suggests that frame and sash type also has an effect on solar heat gain performance. For example, the EWC states that a typical solar heat gain coefficient (SHGC) for aluminum-framed windows using double-glazed low solar gain glass with argon gas fill is 15 percent higher than non-aluminum framed windows.
In the Energy Star amendment announcement, the DOE maintained that the new performance trade-offs will allow “windows and doors with energy performance equivalent to the current prescriptive criteria” to bear the Energy Star label. A review of more than 10,000 individual records in the NFRC database (using a non-weighted analysis of available records) indicates the problem with that approach. Under the prescriptive criteria, windows that qualify in the South Central zone have an average U-factor of 0.31, and an average SHGC of 0.30. The additional windows that would qualify under the new performance based criteria have an average U-factor of 0.42 and a SHGC of 0.28. The net reduction of solar heat gain of 7 percent is offset by a 33 percent increase in U-factor.
It is in the Southern zone, however, that the NFRC data show a more disturbing trend. Windows that qualify under the prescriptive criteria have an average U-factor of 0.33 and an SHGC averaging 0.30. The new performance-based criteria would allow additional Energy Star windows with an average U-factor of 0.69 and an average SHGC of 0.35. The new qualifying windows will have double the average thermal loss, while allowing 15 percent more solar heat gain.
Whether or not the new windows allowed under the performance-based Energy Star method have equivalent energy performance to the arbitrary values assigned to the current prescriptive criteria is a subject for continued debate. It is clear, however, that the immediate effect to the Energy Star windows program is a reduction in the average energy savings from the use of Energy Star windows. How can this trend be seen as anything but a weakening of the Energy Star brand?
Michael D. Fischer
Director of Codes and Regulatory Compliance
Window and Door Manufacturers Association
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