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RESNET to Adopt Procedures to Address Calculating HERS® Index Scores for Smaller Homes

Oct 6, 2015

A criticism of the HERS® Index is that currently it is more difficult for smaller homes to achieve a low HERS® Index Score than larger homes.  RESNET has been engaged in discussion with stakeholders, including large production builders, and there has been an acceptance to have a standard that balances the scales for smaller homes.

Stakeholders have concerns with RESNET basing its adjustment on the ENERGY STAR size adjustment.  Builders in particular are concerned that the ENERGY STAR size adjustment factor provides no benefit for smaller homes and is overly punitive for larger homes.

To address this issue the RESNET Standards Development Committee (SDC) 300 is working on developing an amendment to RESNET’s National Home Energy Rating Standards.

To provide guidance to SDC 300, the RESNET Board of Directors adopted the following policy on future enhancements to the calculation of HERS® Index Scores:

The RESNET Board of Directors establishes a policy that all future modifications and adjustments to the RESNET standard for calculating HERS® Index Scores shall incentivize energy efficient design and construction.

The RESNET effort will take into consideration the research that Philip Fairey of the Florida Solar Energy Center has conducted for the ASHRAE SSPC 90.2 Committee.  Based upon this research the ASHRAE SSPC 90.2 has adopted the following motion:

The Index Adjustment Factor (IAF) applied to the results of compliance simulations shall account for comparative savings over the Reference Home achieved by a square, 2400 ft2, 2-story, 3-bedroom home having the same envelope attributes as the proposed design but using the Reference standard equipment, where the overall index adjustment factor is as follows:

            IAF = IAFCFA * IAFNbr * IAFNS


            IAFCFA = (2400/CFA)[0.372 * (%save2400)]

            IAFNbr = 1+ [0.075 * (%save2400) * (Nbr-3)]

            IAFNS = (2/NS)[0.14 * (%save2400)]

       and where:

            %save2400 = [100 – (Index2400)] / 100

Once the SDC has drafted a standard amendment on calculating HERS® Index Scores for smaller homes, it will be submitted to the RESNET consensus standards public review and comment process.