The lower the number the better. The Home Energy Rating System sets 100 as the mark for a standard new home using modern building practices. Each 1% decrease in HERS index corresponds to a 1% reduction in the energy consumption compared to the HERS reference Home (2006 IECC) Thus a home with a HERS index of 85 is 15% more energy efficient.

The HERS score takes into account several components of the home including:

Geographical location, climate data, square footage of the home, volume of the home, number of floors above grade, foundation type, and insulation properties.

Additionally it looks at several specific components of the home including:

Window and door properties, ceiling insulation properties, mechanical equipment and sizing specifications, duct system performance, mechanical ventilation, and air change/infiltration rate.