HERS Scores Hit New Lows: Marking New Energy Efficiency Highs

Ryan Meres -

Last year, 33% of all new single-family homes built in the U.S. were rated for their energy efficiency using the Residential Energy Services Network’s (RESNET®) HERS® Index, where lower scores mean less energy use. In total, more than 436,000 single- and multi-family homes were rated in 2024. 

Builders have continued to improve the efficiency of their homes, with a national average HERS score of 55, reflecting a 45% reduction in energy use compared with the 2006 International Energy Conservation Code (IECC) baseline. 

When breaking it down on a state-by-state basis, the clear standout for the highest percentage of new homes receiving a HERS rating is Massachusetts. The commonwealth saw 88% of all new homes receive a HERS rating. Arizona came in second place with 73%; while Texas surged 10 percentage points to 67%. The map below shows the percentage of new homes HERS rated by state. 

The RESNET National Buildings Registry collects energy modeling data on all HERS rated homes. With 99% of all homes in the registry being new construction, the data can reveal interesting trends in home building.

Same number of bedrooms, but smaller homes. The average number of bedrooms in single-family homes last year was 3.7. This number has remained nearly constant over the past five years. However, the average size of homes has continued to decrease. Last year the average HERS rated single-family home was 2,467 square feet; a 3% drop from 2023 and nearly 10% smaller than in 2020. 

Prevalence of slab foundations continues to increase. In 2024, nearly 97% of single-family homes were built on slab foundations. This is a 10% increase in just five years. Although slab foundations have always been common in warmer climates, colder climates are now seeing significant increases in this foundation type. From 2020 to 2024, the use of slab foundations in colder climates increased by 19%, with 90% of single-family HERS rated homes in these climates built on slab foundations. This statistic is likely reflective of cost savings associated with slabs.

Fiberglass batts used by most builders. The thickness (R-value) and quality of insulation installation have a significant impact on energy use and HERS Index scores. Last year, fiberglass batts were the most common insulation choice of builders. The chart below shows the types of wall insulation used by builders in 2024.

Reduced envelope leakage. Another metric that has a big impact on energy use is the amount of air leakage through the building envelope. This is measured by a fan, typically placed in an exterior door. The measurement is expressed in air changes per hour at 50 Pascals (ACH50). It measures how often the air in the home is replaced every hour (i.e., the leakiness of the home). 

Attention to detail and the use of foam, tapes, gaskets, and other sealing methods can all reduce envelope leakage, which can improve indoor air quality and occupant comfort. Most building energy codes require an envelope leakage rate that is less than 7 ACH50. The chart below shows the number of homes in 2024 with various levels of envelope leakage.

Tight homes need ventilation. When a home achieves low envelope leakage, typically below 3 ACH50, the code will require mechanical ventilation. This ensures appropriate indoor air quality is maintained through controlled ventilation strategies, rather than uncontrolled ventilation through the building envelope. 

Data on mechanical ventilation types shows that exhaust- and supply-only ventilation strategies are still the most common for HERS rated homes. For homes with mechanical ventilation, the third most common strategy is the use of the air handler (central fan integrated supply system) for ventilation. A less common but growing segment of ventilation types is the use of energy recovery or heat recovery ventilators. This strategy allows incoming outdoor air to be tempered with outgoing air from the home and typically leads to additional energy savings. The chart below shows the ventilation system types in HERS rated homes in 2024. 

Heating and cooling trends. Heating and cooling systems have a significant impact on energy use and choosing high-efficient heating, cooling, and ventilation equipment will reduce a home’s HERS Index score. Builders also can have the installation of the equipment graded, which can help lower scores and improve homebuyer satisfaction with reduced callbacks. More than 1,000 builders chose to have their HERS rating company grade the installation of their HVAC equipment in 2024.

More than half of all single-family homes that have a fossil fuel heating source used a high-efficient furnace; for multifamily homes it was over 80%. The chart below shows the number of homes with different furnace efficiencies.

Air conditioners are trending substantially toward 16 SEER or higher units for both single- and multi-family homes, as shown in the chart below. 

Trends in furnace fuel type continue to show a decline in the percentage of homes using natural gas for heating. When looking at the fuel type for furnaces, natural gas has the largest share of single-family HERS rated homes with 63%, but only made up 42% of the multifamily ratings last year. These numbers represent a drop of 3% for single-family homes and 2% for multifamily over the 2023 numbers and a continuing drop in the share of natural gas homes. Electric was the second most common heating fuel type, representing 58% of multifamily ratings and 37% of single-family ratings in 2024.

Last year, 27% of all HERS Rated homes were all electric, meaning the heating, hot water and oven all had an electric fuel source. The chart below shows the number of homes by heating fuel type for 2024.

Top 20 Most Improved States for Energy-Efficient Homes

With more than 10 years of data on HERS rated homes, it’s possible to look at the improvements builders have made in advancing energy efficiency in each state. The map below shows the top 20 states that had the greatest improvement in reducing average HERS scores from 2013 to 2024.

California, with its solar mandate for new residential construction, had the largest decrease in average HERS Index scores with 57 points. Although there are limited HERS ratings in Maine, the state came in second with a 37-point reduction, followed by Nevada and Alabama with an 18- and 17-point reduction, respectively.

Eight of these 20 states have energy code efficiency levels of 2009 IECC or less, according to the Pacific Northwest National Laboratory’s tracking of state code efficiency levels. Those states are Nevada, Arkansas, Georgia, South Carolina, Alabama, Tennessee, Oklahoma, and Arizona. Three states—Mississippi, Missouri, and North Dakota—have no statewide energy code. This is a good indication that builders doing HERS ratings are driving energy efficiency despite state energy codes. 

Final Thoughts

Last year marked the lowest national average HERS score ever recorded. With an average score of 55, builders have improved the energy efficiency of their homes by 45% over the 2006 IECC.

Building energy-efficient homes requires attention to detail. Increasing insulation, reducing envelope leakage, and installing high efficient heating, cooling, and ventilation equipment are just a few things builders can do to improve efficiency and deliver the high-performing homes today’s homebuyers demand.