HERS index

How does it Work

A home energy rating is a measure of a home’s energy efficiency. The HERS index (see graphic), which is where the rating comes from, is a nationally recognized measure of energy performance for houses similar to the miles per gallon measure used for automobiles. The rating score provides homeowners and potential buyers as well as mortgage industry experts an insight into how well a particular home ranks in terms of energy use and comfort.

Home Energy Ratings are performed by certified RESNET Home Energy Raters. HERS is the most in depth energy assessment in the industry today. It uses computerized simulation analysis software to calculate a rating score which is then ranked on the HERS Index.

Some of the criteria gathered in order to generate a HERS rating is as follows:

  • How prone is the home’s thermal envelope to air leakage under natural conditions.
  • How prone is the air distribution system (ductwork for heating and or cooling) to leakage. Just as important is the location of the air distribution system which can either be within the thermal envelope or outside (i.e. unconditioned attic, basement or crawlspace).
  • The amount of insulation and crucially, the quality of installation within the walls, floors and ceiling. This requires a site visit and a visual inspection by a HERS Rater or field rater representative during construction before the sheetrock is installed.
  • The number, type, size and U value of windows and exterior doors as well as the direction each faces (i.e. North, South, East, West)
  • The efficiency of the water heating system as well as the fuel type.(gas, oil, electric)
  • The efficiency of the heating and cooling system as well as the fuel type and whether or not the thermostats which control the unit are programmable.
  • Type of mechanical ventilation.
  • The number of LED light fixtures as a percentage of the total.
  • Water usage efficiency
  • Appliance efficiency for washer, dryer, oven, dishwasher and refrigerator.
  • The presence of on-site power production such as solar panels is also taken into consideration.

HERS Index Score

After compiling the necessary information a certified HERS Rater will calculate the index score and compare the data gathered against a design modeled home of the same size and shape. The modeled home is generated by the HERS Rater as part of the HERS rating process. The lower the index score the more energy efficient the home.

For perspective, a typical home built before 2006 which is considered an existing home would be expected to have an index score of approximately 130. The reference home, which is a home built in the year 2006 is considered the standard home and would be expected to measure 100 on the HERS index. These provide the basis for rating new homes going forward. For example, a home with a HERS index score of 70 would be 30% more efficient than the standard home while a home with a HERS index score of 130 would be 30% less efficient than the standard home. Where will your new home rank?