How Is a Home’s HERS Index Calculated?
Anyone involved with the Energy Star Homes program has probably heard of the HERS Index, a method of scoring the energy efficiency of a new or existing home. A Web page maintained by the state of Arkansas, for example, explains that the “EPA requires a house qualifying for Energy Star to be built with best practices, tight ducts, and at least 15% more energy efficient than code as shown by a HERS Index score of 85 or less as determined by a HERS Rater.”
Knowing that the HERS Index measures a home’s energy efficiency is a good starting point. But it’s useful to dive a little deeper, to understand how the Index is calculated and exactly what it measures.
Defining the HERS Index
The HERS (Home Energy Rating System) Index was established in 2006 by the Residential Energy Services Network (RESNET), a California-based national association of home energy raters and energy-efficiency mortgage lenders. The lower a home’s HERS Index, the more efficient the home.
To calculate a home’s HERS Index, a rater uses a computer program — most commonly, REM/Rate. (RESNET has also approved three other software programs for calculating the HERS Index.) After the rater has entered data about the home into the program, REM/Rate compares the home being rated to a “reference home.” The reference home is an imaginary home of the same size and shape as the home being rated. In other words, the size of the reference house is not fixed; when a house is being rated, it is always compared to a reference house of the same size as the rated house.
The reference home does not have the same window area or window orientation as the home being rated; instead, the area of the windows in the reference home is assumed to be 18% of the floor area of the rated home. The windows of the reference home are assumed to be evenly distributed on the four orientations of the home.
The reference home is assumed to barely meet the 2006 International Energy Conservation Code; if the home being rated has thicker insulation or better window glazing than the imaginary reference home, those improved specifications will result in a lower (better) HERS Index.
Lighting and appliances are accounted for
The HERS Index accounts for on-site energy production, if any, and energy used for lighting and appliances. The reference home is assigned a budget for lighting and appliances; if the home being rated includes energy-efficient appliances or lighting fixtures, these items can help lower (improve) the home’s HERS Index.
The imaginary reference home is assigned a HERS Index of 100. Most existing homes have a HERS Index over 100, since most existing homes fall short of the requirements of the 2006 IECC. If a rated home gets a HERS index of 100, it can be expected to use the same amount of energy as a code-minimum home of the same size — a home equipped with “typical” lighting and appliances that are operated according to average American usage patterns.
If a rated home gets a score below 100, it will use less energy than a code-minimum home. A score of 0 corresponds to a net-zero-energy home.
Each 1 point decrease in the HERS Index corresponds to a 1% reduction in energy consumption compared to the imaginary HERS Reference Home. A home with a HERS Index of 70 uses 30% less energy than a code-minimum home of the same size and shape.
“The HERS Index measures the relative performance of your home with respect to a home of equal geometry that is constructed exactly as the HERS reference home is constructed, using a standard set of appliances that are operated according to a standardized set of operating assumptions,” explained Philip Fairey, deputy director of the Florida Solar Energy Center. “The reference house has the characteristics of the energy envelope requirements in the IECC. But the IECC addresses only heating, cooling, and hot water, while the HERS index addresses all energy uses.”
The HERS Index rewards efficiency, not conservation
The REM/Rate software assumes a certain pattern of appliance use. “A house without a dishwasher is rated as if it had a standard dishwasher,” notes Fairey. “The rated home has essentially the same appliances as the reference home, unless the rated home has more appliances. The reference home has one refrigerator. If the rated home has two refrigerators, that can hurt you. The reference home has one clothes dryer and one clothes washer. But you don’t get a credit for building a home without a clothes dryer. The software still assumes that a clothes dryer is there, even when one isn’t. You’ll just end up using less energy than the software would predict.”
Although this method of defining the reference house rewards the use of electrically efficient appliances, it does not give full credit to those adopting a simpler way of life. “The HERS Index does not encourage Americans to downscale their lifestyles,” says Philip Fairey. “It does not reward conservation; it rewards efficiency.”
For Richard Faesy, an energy consultant for Energy Futures Group in Bristol, Vermont, the fact that the HERS Index is conservation-neutral makes him a little uneasy. “It has always rubbed me the wrong way that you could have a 10,000-square-foot ‘energy efficient’ home,” says Faesy. “That’s possible because of the way the reference home is set up — in effect, it compares the house to itself. It’s an issue that we have tried to tackle in the Vermont green building standards and with LEED for Homes. The philosophy there — and it’s a philosophy that came out of Vermont — is, if you have a big home you should have to work harder to achieve the green label.” This philosophy is not reflected in the HERS Index.
To Faesy, conservation is not a dirty word. “As we push down towards zero-energy homes, we need to be addressing this issue,” he says. “It is not only about how much are you using compared to how much you could use — it’s also about making some choices to do without.”
The HERS Index can’t predict energy use
The HERS Index is a useful metric; however, it doesn’t tell you how much energy a home will use. Of course, it’s a good sign if a home has a low HERS Index — but just because your house has a low HERS Index doesn’t mean that your energy bills will be low.
For example, if you have a very big house with a low HERS Index, all you know is that your house will use less energy than the typical very big house. But a huge HERS 70 house can still use more energy than a small HERS 100 house.
A 1997 Home Energy magazine article, “Home Energy Rating Systems: Actual Usage May Vary”, looked at the HERS rating, a precursor to the HERS Index. Under the old (obsolete) HERS rating system, high scores indicated that a house was energy-efficient, and low scores indicated that a house used a lot of energy — the inverse of the current HERS Index scale. In spite of the fact that the current HERS Index has inverted the scale, the two scoring systems share the same fundamental approach to rating homes.
In the Home Energy article, author Jeff Ross Stein wrote, “As a research project for Lawrence Berkeley National Laboratory, Alan Meier and I compared home energy ratings with actual utility billing data for about 500 houses. … In general, we found that HERS can be remarkably accurate at predicting average annual energy costs for groups of homes. Predictions for individual homes were less impressive. Some individual ratings significantly overpredicted or underpredicted energy costs, especially for older homes. Furthermore, there was no clear relationship between the rating score of an individual home and actual energy cost. … Technically, rating scores only measure a house’s individual potential for energy improvement; they are not designed to be used to compare different houses in the same way miles-per-gallon ratings are designed to compare cars. However, many consumers and HERS-related financing programs assume that houses with higher [better] scores will have lower energy costs. Unfortunately, houses with higher [better] scores, even when compared to houses of similar size, did not tend to use any less energy than houses with lower [worse] scores. … Occupant behavior is probably the single most significant determinant of actual energy use. HERS have the difficult task of making assumptions based on typical occupant behavior. Reality can easily diverge from these assumptions; predicted energy use or energy cost can be off by 50% or more due to occupant behavior.”
According to Philip Fairey, it’s unfair to criticize the HERS Index for doing a poor job of predicting energy use. After all, there’s more to a home energy rating than the calculation of the HERS Index. “The HERS Index is NOT the only requirement of a Home Energy Rating,” Fairey noted in a recent e-mail. “The RESNET Standards quite explicitly specify what is required to be included in a a Home Energy Rating Report, as follows: ‘303.3.2.3: The estimated annual purchased energy consumption for space heating, space cooling, domestic hot water, and all other energy use, and the total of these four estimates; 303.3.2.4: The estimated annual energy cost for space heating, space cooling, domestic hot water, and all other energy use, and the total of these four estimates.’ So, you see, a Home Energy Rating requires not only an estimate of the energy use of the home but also and estimate of the cost of that energy use.”
The HERS Index doesn’t take occupant behavior into account
Researchers who study residential energy use have long known that occupant behavior explains much of the variation in energy use from one house to another.
Speaking at the March 2011 NESEA conference in Boston, building scientist John Straube noted, “Energy use variation between the lowest and highest energy users — the bottom 10% of users and the top 10% — varies by a factor of 2. A study of 17 identical homes in Oklahoma showed a factor of 3 variation in electricity used for purposes other than space heating or cooling. The high users consumed 3 times as much electricity as the low users. Even in Germany, where we expect everyone to always follow the rules, the ratio of domestic hot water use varies by a factor of 4 from the low-end users to the high-end users. That’s why REM/Rate and HERS Index numbers are just predictions.” Your actual energy use may vary.
Limitations of the HERS Index
Many energy raters have noted that it’s easier for a large house than a small house to get a low HERS Index.
Duncan Prahl, an energy researcher at IBACOS in Pittsburgh, described the problem in a technical paper, “Analysis of Energy Consumption, Rating Score, and House Size.” Prahl wrote, “One artifact of this method [HERS] is that as houses of a given occupancy get smaller, the energy consumption for space heating and cooling is lowered, and domestic water heating becomes a larger relative component in the final rating score. This is due in part to the fact that as houses get smaller, the ratio of envelope area to floor area increases. This has the tendency to make it more difficult for smaller homes to achieve the same score as a larger home, provided both houses have the same number of bedrooms.”
According to a Boston architect and building envelope consultant who posts comments on the JLC Web forum using the name Ted S., “As the house gets bigger, the HERS rating goes down. I get failing small houses in my state’s energy efficiency program all the time, but the bigger ones pass with the same insulation, windows, mechanicals, etc. I’m not supposed to promote that little fact.”
Another criticism of the HERS Index concerns the way that REM/Rate conjures up the reference house. Posting on the Q&A page at the GBA Web site, architect Jesse Thompson wrote, “If we design a house that has 18 bay windows and dormers (chock full of difficult-to-air-seal, difficult-to-thermally-break and difficult-to-insulate construction transitions), that house then only gets compared to a better-built version of that same 18-bay-window-and-dormer house, not a simpler, less-surface-area-to-volume, easier-to-build house that will use less energy by design. So, two buildings with HERS 50 can end up with vastly different energy use, size, constructability, and final energy use. They won’t, in the end, be comparable to each other, even though that is the goal of the tool.”
Other energy rating systems
While the HERS Index attempts to provide a score to describe the energy-use characteristics of a home’s envelope, appliances, and lighting fixtures — an approach sometimes called an “asset value” scoring system — other home-scoring systems use historical energy bills to get a handle on a home’s energy consumption — an approach called an “operational value” scoring system. One example of an operational value scoring system is the EPA Home Energy Yardstick.
In an article discussing the merits of the two different scoring approaches, Philip Fairey and David Goldstein explain that the EPA Home Energy Yardstick is “a web-based statistical analysis tool that compares the reported energy use of a given home against data provided in the 2001 Residential Energy Consumption Survey (RECS) to determine the home’s percentile energy use compared against the reported RECS data, adjusted for climate, number of occupants and house size. … The Yardstick and the HERS Index have completely different (and complementary) uses and purposes. Neither one is necessarily correct or incorrect – they simply measure different things. The Yardstick uses utility bills to characterize the energy use of the home as it is used by its occupants. The HERS Index provides a measure of the relative energy use of the home based on its physical characteristics and a standardized set of operating characteristics. … An energy rating label based on the previous occupants’ energy use habits would say little to nothing about the physical energy attributes of the home and would make it virtually impossible to compare the physical energy attributes of one home against another.”
Energy consultant Allison Bailes has reached a similar conclusion to Fairey and Goldstein. In a comment posted on the Q&A page of the GBA site, Bailes wrote, “The HERS protocol looks at the physical parameters of the house and mostly disregards occupant behavior. It uses a set of standard operating conditions so that one house can be compared accurately to another. … The Home Energy Yardstick proposed by the EPA does the opposite. … It’s great information to have, but not the same as the HERS rating at all. The two are complementary systems and would go well together. It’s not an either-or situation.”
Recent changes to the HERS Index
Because new data on residential energy use for appliances and electricity have become available, RESNET has voted to change some elements of the algorithms used to calculate the HERS Index. “There’s an amendment to the RESNET standards that has recently been adopted by the board of directors,” Fairey explained. “It changes a number of things in the current standards, which change the results some. Larger homes don’t have as big of an energy budget for appliances and miscellaneous energy use as they used to, but reasonable-sized homes — 1,600-square-foot homes and 1,800-square-foot homes — are about where they were before.”
The issue of whether the HERS Index should be tweaked and recalibrated as energy use patterns change is contentious. “The HERS standards assume that 10% of a home’s lighting energy use is from fluorescent lighting — tubes, CFLs, whatever,” Fairey noted. “If incandescents become illegal, I don’t know where RESNET would go. These are difficult balancing acts that RESNET has to concern itself with. If you change the basic lighting budget now, within a few minutes, a builder will call you up and ask, ‘Why is my house design no longer compliant with Energy Star?’ or ‘This house used to get a HERS 60, and now I only get 65.’ ”
According to Fairey, it makes sense to keep changes to the HERS Index to an absolute minimum. “RESNET has not seen a need to increase the efficiency of its reference case,” said Fairey. “If anyone wants to raise the bar, it is very easy to do: just aim for a lower HERS Index.”
Last week’s blog: “New Air Sealing Requirements in the International Residential Code.”
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