IECC ERI Paths vs. HERS Index

Modified on Thu, 20 Dec, 2018 at 9:27 AM

Various IECC versions (and locally amended IECC versions) include an ERI path for compliance which reference ANSI/RESNET/ICC Standard 301 (we’ll call it 301 here). While the HERS Index and IECC ERI both reference 301, the HERS Index and ERI are not always the same. Amendments are often made to 301 and are applied towards the calculation of a HERS Score as they are approved. Most ERI Paths are "frozen" at the time of their adoption, meaning amendments made to 301 after adoption are not included in the ERI for that particular code, which can lead to a divergence between the HERS Index and the ERI. Also, some ERI paths (notably in IECC 2018), make significant modifications from 301 for how the ERI should be calculated. This means that you could see a failure for an ERI code path saying that the ERI is too high even though it appears that their HERS score meets the threshold.

Listed below are some of the ERI paths in Ekotrope that have diverged from the HERS Index:

IECC 2018

1. The IECC 2018 adopted a modified ERI which diverges from ANSI/RESNET 301. Equation 4-1 in section R406.3 modifies the ventilation rate of the reference home, which can have a significant impact on the ERI. The impact is generally to raise the ERI score at least 2 pts, and sometimes up to 10 pts.

2. The IECC 2018 adopted ANSI/RESNET 301 as it was when the IECC 2018 was published. Any ANSI/RESNET 301 changes after that time are therefore not included in the IECC 2018 ERI score. Most notably, this means that the House Size Index Adjustment Factor of Addendum E and the LED lighting changes in Addendum G are not included in the IECC 2018 ERI.

These two differences can combine to generate very large discrepancies between the HERS Index score and the IECC 2018 modified ERI.

North Carolina 2018 Energy Code

North Carolina 2018 Energy Code adopted ANSI/RESNET 301 with only Addendum A and Addendum B. Any ANSI/RESNET 301 changes after that time are therefore not included in the North Carolina 2018 ERI score. Most notably, this means that the House Size Index Adjustment Factor of Addendum E and the LED lighting changes in Addendum G are not included in the NC 2018 ERI.