Proposed Amendment TECH-2005-09

Modify the HERS® score calculation and change its nomenclature from "HERS Score" to "HERS Index"

Proponent: RESNET Board of Directors

Proposed Changes: Changes rating score to "HERS Index" where "0" is a net zero energy home and "100" is the HERS Reference Home. Change all "HERS score" and "point score" references to "HERS Index" (various sections); add definitions for HERS Index and Rating Index (Section 302); and modify equation 2, Section 303.2 as follows:

HERS Index = (PEfrac * TnML / TRL) * 100

Justification: This proposal has been vetted with major national stakeholders (EPA, DOE, NAHB, USGBC and others). It was also the subject of a national survey completed by more than 430 individuals from all walks of life. Among the three options presented by the survey, which included maintaining the present score scale based on a Reference Home score of 80 points, the proposed option was the preferred by approximately a 2 to 1 margin. To review the results of the survey click on Results of RESNET Survey on HERS Scoring.

In light of these results, the RESNET Board of Directors has voted 15 to 2 in favor of submitting the proposed change for public review.

The justification for this proposed change is based on a confluence of circumstances that have converged at this particular point in time, as follows:

  • The newly adopted RESNET Standards, using the expanded scoring method; the IECC 2004 Supplement HERS Reference Home; and revised federal minimum standards for air conditioners, heat pumps and hot water heaters will result in a radical change in HERS scores as compared with scores computed using the 1999 HERS Reference Home standards.
  • The original Reference Home point score of 80 is arbitrary. It is also difficult to explain to consumers and others (including many important stakeholders) who are not intimately involved and familiar with the HERS rating process and methods.
  • The original scoring method "compresses" the scoring point available for highly efficient homes by confining the entire range to 20 points. The proposed equation expands this to 100 points, providing much greater perceived differentiation for the most efficient homes. Without such a move, as rating tools become more accurate and rated features expand to encompass more end-uses, the same level of differentiation would require scores on the old scale to be resolved to the hundredth of a point.
  • With the expanded HERS score, it will no longer be possible to establish linkage between a HERS score of 80 and minimal model energy code compliance. This occurs for two reasons: 1) because energy efficient lighting and appliances are not currently allowed as "trade-offs" within energy code performance methods, and 2) because the added energy uses that go into the expanded score comprise differing proportions of total home energy use depending on climate. Any HERS software used for code compliance would thus need to have code-compliance reports separate from the HERS report for any particular version of the code.
  • Using the expanded score and a Reference Home based on 80 points is likely to produce more confusion in the marketplace. It will be difficult to explain why the old 80-point reference and the new 80-point reference produce such different HERS scores. For example, try coming up with a simple answer for the following potential builder question: "Well, if the HERS score for the Reference Home has not changed at all, then please explain to me how the HERS score for my model home has dropped so dramatically?" Raters and their clients will need to re-calibrate their thinking about HERS scores anyway; why not avoid the inevitable comparisons between "old" and "new" scores by changing the scoring system more dramatically?
  • The proposed scoring equation yields scores that are much more intuitive and logical than the current scale, which effectively starts at 80 points and rates the relative efficiency of the home rather than the relative energy use of the home. Since "energy efficiency" is a more ambiguous concept than "energy use", the proposed point score is more intuitive.
  • The proposed scoring scale is very easy to explain - "A score of 100 means your home uses the same amount of energy resources as the standard American home (a code-built home with typical lighting and appliance use) and a score of zero means that your home uses no purchased energy resources. As a result, each point between 100 and 0 represents to an energy resource savings of approximately 1%."
  • Finally, there will likely never be as good an opportunity to make a change of this nature in the HERS scale. The scope of the changes that have been adopted represents the cumulative knowledge of a decade and hundreds of thousands of ratings. This proposal is designed to anticipate more inclusive and complete energy assessments of homes for decades to come, in a manner that is incremental and will not require further change in the fundamental rating scale - zero will always be zero.

Some Common Questions & Answers

Q. How will the proposed point score calculation impact the normalization that is used in the HERS score to achieve fuel parity between fuel types.
A. It will not. The load modification and normalization procedures are accomplished prior to the application of the scoring equation, so load modification and fuel normalization will not be affected in any way.

Q. How difficult will it be to "cross talk" between the original point score and the proposed point score (to compare results to the previous system)?
A. It is very simple. The following simple equations can be used to convert back and forth between the proposed and original scores:
          Proposed score = (100 - Original score) * 5
          Original score = 100 - (Proposed score / 5)

Q. Will software developers have to do major revisions on software tools to implement the proposed point score?
A. No, only a single line of code should need to be revised.

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