The HERS Index is a number that gives you a measure of how energy-efficient a home is. We can debate how relevant that number is or how accurate is the energy model it’s based on, but the fact is that it’s being used.
The 2015 International Energy Conservation Code (IECC) includes a compliance path based on the HERS Index, which they call the Energy Rating Index (ERI). Here in my state, Georgia Power has been educating home buyers on the HERS Index since they created their EarthCents program a couple of years ago. But the HERS Index is based on the 2006 IECC. Is that a problem?
The HERS Index calculation
A while back I wrote an article on the calculations behind the HERS Index, and I showed the equation that appears at the end of this paragraph. The important concept contained in that equation is the HERS reference home. What the calculation is doing is comparing the energy use of the home being rated to the energy use of the HERS reference home. The rated home has whatever insulation levels, airtightness, and other efficiency parameters the HERS rater enters (which, unfortunately, aren’t always what’s in the home). The reference home is set up to meet the energy efficiency requirements of the 2006 IECC.
The result is that if the number comes out to be exactly 100 (for a home with no onsite power production and a PEfrac of 1), that means, roughly, that it would meet the 2006 energy code.
Shouldn’t we update the HERS Reference Home?
But wait! The energy code is updated every three years. Since 2006, we’ve had the 2009 IECC, the 2012 IECC, and now the 2015 IECC. Shouldn’t we update the reference home to use the most recent version of the energy code?
No. If we…