Software Consistency Frequently Asked Questions

I am seeing a very different HERS Index for the same home modeled in different software tools, who do I contact for an explanation?

RESNET’s Software Consistency Committee provides a form if you notice inconsistencies in the HERS Index for the same home across multiple software tools. The Software Consistency Committee will address inquiries based on the size and nature of the inconsistency, and will use this information to continuously improve consistency across RESNET Accredited software.

Why does the HERS Index change for the same home modeled in different versions of the same software tool?

The HERS Index calculation is defined by RESNET’s MINHERS Standards, which refers to ANSI/RESNET/ICC Standard 301. This standard is updated periodically to:

  1. Accommodate new technologies and incentivize better building practices, and
  2. Clarify areas of the standard that are ambiguous and different interpretations leading to inconsistencies among RESNET accredited software tools.

In addition, software tool developers will occasionally identify and fix defects in their software to make them more accurate.

Changes in ratings are not unique to the Home Energy Rating System. Other industry ratings, such as the EPA’s Fuel Economy rating for vehicles, also need to adapt to be more accurate and to accommodate advancements in technology.

Regardless of whether software tools are fixing defects or adapting to new standards, changes in a HERS Index between different versions of the same software generally mean that the HERS Index is becoming a more robust and reliable metric for home efficiency.

For questions about specific changes between different versions of the same software, please contact the software provider support.

RESNET also recognizes that changes can be disruptive to the industry. The RESNET standards development process is an open process with opportunities for the public to comment on and be aware of impactful changes coming down the pipeline.

Why is my HERS Index so different from the IECC ERI for the same building?

While the HERS Index is defined by MINHERS, the IECC ERI is defined by the ICC with their own amendments to the ERI calculation. Please see this article for more details: The IECC Energy Rating Index and HERS Index: What’s the Difference?

For additional questions regarding the HERS Index and the IECC ERI, please contact RESNET’s Program Director, Ryan Meres (ryan@resnet.us)

Why do seasonal and hourly engines produce a different HERS Index for the same home?

Many seasonal calculation techniques were born from a time when computation was slower, and more detailed calculations took a prohibitively long time to calculate. As more detail became  required to capture important aspects of building energy use, seasonal calculations were no longer seen as a viable approach to calculating a HERS Index.

Hourly calculations can provide a much more detailed and accurate representation of building energy use and, with modern computing, these calculations can be performed in a reasonable amount of time.

While changes in HERS Indices from seasonal to hourly calculations are disruptive to programs based on seasonal calculations, this change is intended to enforce greater accuracy and will be a one time change. As of January 1, 2021, seasonal calculations are not allowed for HERS Ratings.

For questions about changes between seasonal and hourly engines, please contact the software provider support.

Why doesn’t RESNET just use a single software tool (or a common engine) for ratings to eliminate inconsistencies?

Inconsistencies present a real challenge to raters, builders, and code officials. We know inconsistent HERS Indices can’t all be “correct”. While a single tool or a common engine virtually eliminates inconsistencies, it doesn’t actually solve the underlying problems that cause them. It simply hides the issues without understanding how the inconsistencies impact the accuracy and reliability of the HERS Index.

RESNET’s collaborative modeling approach to consistency relies on multiple implementations to provide accountability among software tool developers and expose areas where the standard is unclear or where building physics calculations are inaccurate. As uncomfortable and frustrating as inconsistencies are, this approach shines a light on the issues and forces resolution to make the HERS Index more accurate and reliable in the future.

How does RESNET’s software consistency process work?

The Software Consistency Committee objectives are as follows:

  1. Address user consistency inquiries
  2. Increase accreditation test coverage (e.g., to test a wider range of technologies, building types, and climates)
  3. Progressively increase stringency of accreditation testing criteria
  4. Standardize model reports for describing the rated home and the auto-generated reference home

How do I communicate with building code officials about changes to the HERS Index that impact compliance?

RESNET is committed to working with code officials to resolve issues. If a change in software impacts code compliance or other efficiency programs, please contact RESNET’s Program Director, Ryan Meres (ryan@resnet.us)

Link to 2021 Software Consistency Update Video

(This is one over a hundred sessions at the RESNET building performance conference. For more information and to register, visit resnet.us/conference)