HERS Index and the “reference house”
I think that the HERS index is a pretty good barometer of good design choices and strategies.
There are some strategies that are not picked up by the index.
Part of this is due to the “reference house” concept.
Some design choices are not recognized because you are comparing your house to a geometric twin of YOUR house.
That means that a house with just a few corners may not Index much better than a house with way too many corners.
Same is true for homes with large wall surface area to floor area ratios compared to a more compact home.
The compact home will perform better … but may not INDEX better.
There are other examples…
Anybody have other examples or care to comment?
The good and the bad of the HERS index?
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A collection of one thousand construction details organized by climate and house part
Replies
I think we have to accept that HERS is a limited tool but likely the most efficient we have. I'm really excited that the green home examples being collected for this site are being asked to disclose energy bills and compare them to the HERS predictive model. The reality is that the predictions routinely vary from the actual performance and, as we all know, the HERS score can be gamed.
Henry Gifford, who spoke at Joe Lstiburek's Building Science Summer Camp this summer had a great discussion about this in reference to the LEED standard
http://869789182725854870-a-energysavingscience-com-s-sites.googlegroups.com/a/energysavingscience.com/www/home/BuildingRatingSystems.pdf?attredirects=0&auth=ANoY7cogXUXZdy87gI235YJDo1IsYiTWy86S6yrwEPrSRBosEXE_uZxxfTim1kCUlgOUpaJJyUOsmx_dWPw1LJB6beYa8GRlSjiT4X-RTCMB3Qmx9gsY58JCuluQ07B_ypP_NE3X7Kr0ru6Y1CKTqkjDyGoUSx73XeDh28ACl_hovyfKiqPgXIKUOf3D8JLWGS9U1VNhejNsVBKVFW3pzNHWqyzIqS5ZYTKQpp5hoP6hE1724JOSSzk%3D
But I think his point extends beyond LEED to Energy Star and the rest of the green certification systems. Basically if you use predictive standards and don't follow up with performance measurement and verification you're not doing "building science" you're doing "building theory."
I've committed to researching three of my past homes energy performance and posting them here along with the HERS ratings and blower door tests and green calculators but I haven't gotten around to doing the homework. Jim Sargent, who is a contributor here, routinely has his homes exceed their HERS ratings on performance. There is a lot to learn by comparing predictions against performance and looking for patterns.
Michael,
I really like the Gifford message myself..I had previously read his paper and watched a video from summer camp.(your link did not work for me)
Jim Sargent is my local hero/mentor.
I look forward to seeing your performance numbers v.s. HERS index.
Are all of the Energy use curves(on this site) based on actual metered performance?
John
I think the curves are based on actual energy bill data. in order to submit my homes for the site I have to be willing to provide the utility bills (obviously I'll be self-selecting for homes with no teenagers in residence) I think it will be interesting to compare with the HERS predicted usage to get a handle on what the HERS is favoring.
I agree that Jim is an amazingly generous resource to have as mentor and general good natured source of wisdom. The builder show in Vegas was filled with new condensing water heaters aiming to take a bite out of Rinnai's Business. I need to just do a blog on the new water heaters alone.