HERS stands for the “Home Energy Rating System”. Essentially, it is a miles-per-gallon rating for a house. The index number is the score of the house which is calculated using all known factors affecting the energy use of the house such as insulation, heating and cooling efficiencies, window qualities, air-infiltration, and duct-leakage.
When those factors are all plugged into a very complicated mathematical formula, the software used to calculate the rating produces a number on a scale from zero to five hundred. On that scale, zero represents a zero net-energy home which produces as much energy as it uses, and a score of five hundred represents the worst possible construction and climate such as an uninsulated shack built in the arctic.
On that scale, from 0 to 500, a score of 100 represents a house built to meet the international energy code in 2004. Using that as the baseline allows us to say that each point on the HERS Index represents a 1% increase or decrease in energy use, cost, or efficiency. Therefore, a home which scores a 55 on the HERS Index would use 45% less energy than the code reference home.
It is important to understand that the HERS Index is just a number, not a code or set of standards. There are no requirements for a HERS Rating. There are however many programs and codes which make use of the Index as a means of compliance by setting a maximum or minimum number. For example, HUD uses the Index in a program called Enterprise Green Communities. In that program, multifamily projects undergoing renovation must be Rated by a third party rater and score an 85 or lower in addition to meeting other specific program requirements. Another example is the 2015 International Energy Conservation Code (IECC 2015) which allows builders to demonstrate compliance to the code by achieving a rating of 55 or lower (for zone 5A).
HERS Ratings and the community of professional Raters who are certified to conduct them are performed under the guidance of RESNET which stands for the Residential Energy Services Network. RESNET maintains a roster of certified individuals and standards of practice for the industry. To find a HERS Rater, you can visit the RESNET website. For advice on choosing a rater and questions you should ask, please read the post Working with a HERS Rater.