RESNET Adopts Home Size Adjustment Factor for HERS Index Scores – Transition Period to January 1, 2019

July 12th, 2018 - Posted by under RESNET News

Previously, RESNET’s standard resulted in a discrepancy in the calculation of HERS Index Scores between larger and smaller homes that have similar energy efficiency features. Larger homes received lower HERS Index Scores than smaller homes.

To address this discrepancy RESNET adopted a standard amendment, ANSI/RESNET/ICC 301-2014 Addendum E-2018, House Size Index Adjustment Factors (IAF).  The standard amendment revises HERS Index Score calculations to address this issue. The amendment establishes a calculation methodology that compensates for size in order that similarly configured and equipped dwellings of different sizes will have equivalent ratings.

The Addendum went through several cycles of ANSI consensus standard development and public review before its adoption. It was submitted for ANSI approval and ANSI published its notice of acceptance of Addendum E-2018 on February 2, 2018.

RESNET accredited HERS software vendors have had their rating software updated to implement the Addendum. Certified RESNET HERS Raters should check with their software vendor for versions that will implement the new size adjustment factor.

HERS Raters will be required to use the size adjustment compliant software on homes that are issued a building permit on or after January 1, 2019.

Homes that were issued a building permit before January 1, 2019 may be rated without complying with the new size adjustment factor.

Builders can, however, use the new version of software now voluntarily.

For more information on the new size adjustment calculations, click on ANSI/RESNET/ICC 301-2014 Addendum E-2018, House Size Index Adjustment Factors (IAF). to view the adopted amendment.

RESNET hosted a webinar on this amendment and can be listened to at


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