What's Happening from Environmental Building News
Use of Home Energy Rating Index Grows
While the LEED for Homes rating system from the U.S. Green Building Council has garnered a lot of attention since its 2007 launch (see
EBN
Vol. 16, No. 12), a less complex and comprehensive rating system, the HERS Index, has quietly been applied to hundreds of thousands of homes and is referenced by a growing number of programs.
The HERS Index, or “Home Energy Rating System,” is a system established by the Residential Energy Services Network (RESNET), a California-based nonprofit. RESNET updated the old HERS Score system with the new HERS Index in 2006. The HERS Index compares a home under construction with a reference home meeting the minimum requirements of the 2006 International Energy Conservation Code. The Index, which is calculated from a computer model using RESNET-accredited software, also factors in actual measurements from a home: results of a blower-door test, an insulation inspection, and a duct-leakage test for homes with ducts in unconditioned spaces.
Steve Baden, executive director of RESNET, explained that the HERS Index allows more latitude for recognizing reductions in energy consumption. “The old reference home was an 80,” he said, on a scale going up to 100 (higher being better), so “you didn’t get many points for increasing the performance of a home.” Now, the theoretical reference home scores 100 (see graphic), and a theoretical net-zero home scores a zero. Lower is better, and index scores correlate with percentage improvements compared with the reference home: an Energy Star home scoring 85 uses 15% less energy than the reference home. In addition to considering heating, cooling, and hot water, the new HERS Index also takes into account the energy use for lighting and major appliances as well as renewable energy produced on site. The inclusion of the latter has led builders to use features like photovoltaic panels to lower the HERS Index of homes—in some cases to zero.
The HERS Index is used to assess whether a home meets the standards of Energy Star for Homes, the energy efficiency program run by the U.S. Environmental Protection Agency (EPA). According to Baden, 168,000 homes earned a HERS Index good enough for Energy Star certification in 2007. Baden said that extremely low scores are rare, but some builders have set their targets at zero. Jay Walsh, an energy analyst with the Center for Ecological Technology in Massachusetts, has recently scored two homes with a HERS Index of zero: one in Williamstown, Massachusetts, and another in Esopus, New York. Both use plenty of insulation, Energy Star appliances, compact-fluorescent lighting, ground-source heat pumps, and on-site photovoltaics.
Although principally identified with Energy Star, the HERS Index is gaining wider use. It has also been adopted—and rebranded—by the Department of Energy (DOE) in its “Builders Challenge.” In launching the challenge in February 2008, DOE called for U.S. homebuilders to build 220,000 homes that exceed Energy Star standards. Although DOE and EPA cooperate on some aspects of the Energy Star program, EPA runs Energy Star Homes, and DOE’s move appears to one-up EPA in that area. Whereas an Energy Star home would score 85 or lower on the HERS Index (or 80 or lower in colder climates), the Builders Challenge calls for a score of 70 or lower. While the Builders Challenge uses the same HERS Index, DOE refers to it as the “EnergySmart Home Scale,” or “E-Scale” for short. According to
Energy Design Update, which quotes a source at DOE, the change in name boils down to a negative and possibly sexist attitude among homebuilders toward “HERS.” The Builders Challenge does not offer incentives for builders—unlike Energy Star, which often helps homes qualify for tax credits or utility incentives—and it so far lacks administrative support from DOE. The program promises a prescriptive pathway to compliance, tailored for different climates, but the guidelines for that pathway haven’t been delivered.
Despite possible confusion about the use of the HERS Index, the Builders Challenge highlights the room for improvement below the Energy Star threshold of 85. “If you’re building a good quality building, and you’re putting in good insulation and a good heating system, you’re probably already at the minimum Energy Star level,” said Walsh, suggesting that Energy Star is fairly easy to achieve. But Walsh argues against lowering the Energy Star threshold. Discussing the Builders Challenge benchmark of 70, Walsh said that he tries to help homes achieve 70 or better, but “I don’t see that as an entry point for Energy Star,” he said. “The inclusion of as many homes as possible should be the real goal.”
The 2030 Challenge, the campaign for radically improved building performance driven by targets to reduce greenhouse gas emissions, has also adopted the HERS Index, using it to define aggressive benchmarks for residential energy performance. Architecture 2030, the group behind the 2030 Challenge, has called for homes built after 2007 to achieve a HERS Index of 65. That target drops to zero in 2030.
LEED for Homes also recognizes the HERS Index, using it as one of two possible compliance paths for its energy optimization credit.
The widespread adoption of the HERS Index suggests that it is both relatively robust and that it communicates results effectively, and Walsh confirmed that “the index is being received well.” There have been some recent studies, though, showing that actual energy consumption in HERS-indexed homes doesn’t reflect their HERS Index scores. “The primary reason is that there are people living in it,” Walsh said. “They can change the dynamic of a home’s energy use greatly. Just because there’s a setback thermostat doesn’t mean that they use it.”
– Tristan Korthals Altes
For more information:
Residential Energy Services Network
www.resnet.us
Energy Star Homes
U.S. Environmental Protection Agency
www.energystar.gov
Builders Challenge
U.S. Department of Energy
www1.eere.energy.gov/buildings/challenge/
June 1, 2008
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Photo: Jay Walsh
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