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Sep 9, 2019

What is the RESNET Index Adjustment Factor (IAF)

with Philip Fairey of the Florida Solar Energy Center


Taste as you go. When you taste the food throughout the cooking process you can adjust as you go.

-Anne Burrell


Feedback (tasting) and adjusting make a better recipe.

RESNET is all about feedback.


Join us as Philip Fairey, Deputy Director of the Florida Solar Energy Center and Secretary of the RESNET Board of Directors unravels the story behind the development of the Index Adjustment Factor (IAF), also called the Home Size adjustment factor in Addendum E-2018.


The IAF stemmed from anecdotal reports that unwarranted HERS credit was given to larger homes, or that smaller homes were penalized. 


These inequities in rating calculation were believed to be caused by the presence of two factors causing the HERS index to fall: larger conditioned floor area and greater number of stories. Additionally one factor, a greater number of bedrooms, was observed to be causing the index to rise.


After extensive research and detailed modeling, the IAF was developed and has helped to significantly reduce the range of variation for all three factors.


To learn more, including in depth technical details, see slides from Philip’s presentation at the 2019 RESNET Conference at this link:


The IAF is baked in to the modeling software and does not cause any change to the responsibilities of a Rater or RFI and does not increase the cost of HERS ratings.

NOTE: 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..


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