A new amendment to the HERS rating will take effect on January 1st, 2019, after which all HERS raters will be required to use an index adjustment factor to reduce HERS score variations between larger and smaller homes.

Due to a quirk in the HERS formula, homes with large floor areas or multiple stories have been found to have 20% lower (better) HERS scores than than smaller homes with the same material properties and energy measures. RESNET’s new index adjustment factor is designed to “even out the difference” between homes constructed with the same envelope, reducing the variance in HERS scores from 10.8 points to 2 points, according to EnergyGauge USA.

The timing of the amendment coincides with the overall decrease in new home size. New homes had been decreasing in size over the last two years as builders added more entry-level homes for millennials and other first-time home buyers. Although new single-family home size increased at the beginning of 2018, NAHB economists expect it to drop again as additional moderately priced homes are constructed for market newcomers.

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