Housing Policy Explainer

Energy Codes and New Home Efficiency

Energy codes are model building codes that establish the minimum energy efficiency standard for structures. Included in the scope of the model code are requirements tied to the reduction of energy consumption and emissions related to air and duct leakage, exterior and shared walls, floors and foundations, ceilings, lighting, windows and doors and insulation. 

Energy Code compliance

There are two paths for energy code compliance: 

Prescriptive Path: Under the prescriptive approach, a builder will follow the specification of the energy code just as they would any other building code. The prescriptive approach does not allow a builder to deviate from the specific requirements for any reason, including cost, material availability, or unique project characteristics. 

Performance Path: Under the performance path approach, flexibility is granted to a builder to achieve the efficiency requirements of the code. This approach allows a builder to deviate from the more rigid, prescriptive approach provided they meet certain efficiency requirements. From an affordability standpoint, builders have widely embraced the performance path because of its ability to achieve affordability while still meeting the efficiency requirements. Performance path requirements are tied to the efficiency in the codes adopted at the city, county or state level. 

The model IECC currently contains several performance path options: Total UA alternative under R402,  Total Building Performance under R405, and the Energy Rating Index (ERI) under R406. 

These alternative paths provide builders with appropriate flexibility on a variety of measures, however, they do require compliance with certain provisions, such as insulation of heated water pipes and are tied to the efficiency requirements of the prescriptive path.

The purpose of energy codes tied to operations of established minimum efficiency requirements for new and renovated buildings to achieve reductions in energy use and emissions over the life of the building. Measuring efficiency, or performance, is critical because it is separate from energy consumption. Two of the exact same homes built to the same standard can and will have different energy bills because of consumption tied to building occupant needs and behaviors. Appliance and light usage, thermostat settings and the number of occupants all influence energy consumption. 

How is Home Efficiency Measured?

Efficiency for new homes is often measured by the Home Energy Rating System (HERS®), which is a standardized method for assessing the energy efficiency of residential buildings in the United States. HERS®, a program of RESNET, involves a comprehensive evaluation of a home’s energy performance, considering factors such as insulation, windows, efficiency heating and cooling systems, and overall airtightness. A HERS® of 100 is the baseline, tied to a reference home built in 2006. A HERS® of 0 would be a net zero home. In 2023, the average HERS® in the United States was a 53. To gauge efficiency, the HERS® score for a home is subtracted by 100, the reference home score and the remaining value is how more efficient a new home is than the reference home.

Example: New homes in 2023 were, on average, a HERS of 53, meaning that new homes in 2023 are 47% more efficient than in 2006. 

Air leakage is another method for measuring energy efficiency. This metric measures the rate at which the air within a defined space is replaced by fresh air. An Air Change per Hour(ACH) of 0, means that in one hour there was no new and an ACH of 1 means the entire volume of the structure’s air is changed in one hour.  ACH is calculated by:

Cubic Feet Per Minute (CFM) X 60

Structure Volume

ACH is measured by obtaining the CFM rate for a structure via a blower door test, a diagnostic tool used to measure the airtightness of buildings. It involves fitting a powerful fan into an exterior doorway and depressurizing or pressurizing the building to measure the rate at which air infiltrates or exfiltrates, aiding in identifying areas of air leakage and potential energy inefficiencies. 

Some airflow is required for both durability and health reasons. From a durability standpoint, air leakage allows the home to breathe, preventing the buildup of moisture. The American Society of Heating, Refrigerating and Air-Conditioning Engineers recommends a minimum ACH of 0.35, but not less than 15 CFM per person for health reasons. 

Policy Issue: Home Size and Efficiency 

As ACH depends greatly on the volume of the home, smaller homes such as slab-on-grade homes, single-level homes or homes without high ceilings naturally have a higher ACH than homes with basements. Attached housing will also have more air leakage than detached housing as the shared walls between units may not be airtight. The model IECC currently addresses this by allowing for higher ACH values for attached housing and smaller detached housing with higher ACH figures. 

Efficiency Increase: 2013-2023

This chart shows the average HERS rating in the United States from 2013-2023.

Efficiency In Older Homes

According to a 2008 study by the United States Department of Energy, a typical existing home in the U.S. would have a HERS® index rating of roughly 130. With a 2006 reference home scoring 100, these homes are 30% more inefficient than a reference home in 2006. Many of these homes were built before energy codes and modern materials like insulation and high-efficiency furnaces. 

In the U.S. today, roughly 82.77% of the nation’s housing stock was built before 2006, indicating a substantial portion of the nation’s homes are less efficient than a standard home built in 2006. Of the pre-2006 housing stock,  older homes will be more likely to have a HERS 130 and homes approaching a construction year of 2006 will be more likely to have a rating closer to a HERS® of 100.  

Efficiency By Climate Zone

Data from RESNET illustrates how new home efficiency has increased in each climate zone in the past decade. 

Note: Climate zones with fewer than 100 ratings in 2023 or that are no longer in use have been removed.

Efficiency By State

This list shows states with more than 4,000 homes rated in 2023. 

StateHERSNumber of Ratings
1. New York484,296
2. Minnesota497,727
3. Nevada5010,229
3. Massachusetts509,351
5. Arizona5230,389
States with more than 4,000 ratings in 2023.

See the full list.


HERS® is a registered trademark of RESNET
RESNET reports average HERS® Index figures, not median.

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