The Ultimate HERS Rating Guide Part 2: What Is The HERS Score Index?

Date Published: July 9, 2019

You may have heard the term “HERS rating” used regarding home construction and energy efficiency.  You may have even seen and read through a HERS report, or you may have a HERS certificate for your own home. Or you may have read our first blog in this series, What Is A HERS Rating.  Many have heard the term, but might not fully understand how to decipher the final product: The HERS Score.

To review, HERS is an acronym that stands for Home Energy Rating System. The HERS index, also known as the HERS score, is the industry standard that is used to measure a home’s energy efficiency.

A HERS rating can be used to compare a home’s energy efficiency to another home of similar shape and size.  Because the HERS index compares efficiency on a relatively “apples to apples” basis, the HERS score can be used effectively to compare one home’s efficiency against other similar homes.

So what does the HERS Index look like?

The HERS rating process is well defined for the building community as a solution to meet specific building codes. But as a consumer, the HERS score is the official product of the rating that you can use to compare properties. So, how do you interpret the HERS Score Index?

With a HERS score, the lower the score, the more efficient the home is.  The HERS index uses a scale in which a HERS score of 100 indicates a standard new home score, and a HERS score of 0 indicates a home that uses zero energy.

The HERS Score can be useful when purchasing a home or when building a new home, as it can give you an easy-to-understand expectation of what your energy consumption, energy bills, and overall comfort will look like.

Many counties in Colorado are accepting or requiring a HERS Rating on new construction homes. Are you in need of a HERS rating? Learn more here. 

Here are a few examples of HERS scores, and what they mean:

  • A home that scores a HERS  130 is 30% less efficient than a standard new home. You could expect that many existing homes older than 20 years old with minimal upgrades would score in this range.
  • A home that scores a HERS 70 is 30% more efficient than a standard new home.  This is a big improvement, but is not as difficult to attain as many might think.  Many newer homes or retrofitted homes with good insulation levels, higher efficiency heating and cooling systems, and higher efficiency lighting will score in this range.
  • A home that scores a HERS 50 is 50% more efficient than a standard new home.  This is where we’re really starting to see some extra effort put in toward green building.  This home will typically be designed with energy efficiency in mind, and will have above average insulation levels, high efficiency HVAC systems and water heating, and higher quality windows.
  • A home that scores a HERS 0 is 100% more efficient than a standard new home, and is a net-zero energy home.  This is typically only attained when the home has an energy efficient design from top to bottom, along with renewable energy such as Photo-Voltaic solar installed on the home, to produce as much energy as the home uses.

These are rough examples, but many factors go into a HERS rating to determine the end score. An improved HERS score can be attained with numerous combinations of energy efficient measures.

RESNET has created an interactive tool to help consumers and builders understand a HERS score at every level. This breaks down the efficiency/health levels of the home, energy cost savings, and carbon emissions.

What construction changes affect a HERS Score?

A certified HERS rater models a home from top to bottom in HERS software. Each energy design change will slightly change the HERS Score as well. The following are some of the key factors that will affect the final HERS score of a home:

  • The area and insulation levels of the thermal envelope, meaning all of the floors, walls, and ceilings of the conditioned space
  • The windows, including window area,  orientation, shading, and the insulation quality of the windows
  • The heating, cooling, and water heating equipment
  • The lighting and standard appliances that will use energy in the home, such as all kitchen appliances and the clothes washer and dryer
  • The overall air leakage rate of the home.  High air leakage results in more energy needed to keep the home heated or cooled.  Air leakage is measured on the finished home using a blower door test.

Changing one of the above items can have a range of effects on the outputted HERS Score. This is where an experienced HERS rater can help balance the inputs and help find ways to reach the desired HERS score on any type of property. This can also be used to a builder’s advantage as well by making trade-offs.

HERS Score factors in a blower door test

A Blower door test is used during the HERS rating process to test the air leakage. This number is factored into the score.

A trade-off is where a builder adds more of one energy efficient item to offset the lack of another. This is not used to bypass the energy codes, but it does help the builder when there are design issues that make energy compliance in certain areas more difficult to achieve. We will talk more about trade-offs in a later part of this guide.

You can see that having an efficiency score to assign to a home can be a very helpful tool.  We will explain in detail in upcoming posts how many municipalities are using the HERS index as the standard for meeting energy efficiency compliance in their building codes.  We will also show how a HERS rating on your home can be useful for resale or for determining the best ways to improve efficiency.

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About the Author: Luke Griess

Luke Griess is an ASHI certified Home Inspector and Certified HERS rater, with over 20 years cumulative experience in the home inspection, residential energy services, and construction industries.

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