What is a HERS score? An organization of mortgage lenders was formed in the 80’s to study the value of home energy efficiency. Their efforts became RESNET in 1995. They were hoping to add value to homes that would save the owner utility bills. In 2002 the first HERS index was introduced. It developed standards based on a complicated formula to quantify energy efficiency and estimate value in terms of walls and foundations, insulation, air leakage, heating and cooling, installed appliances and lighting. These standards were used to qualify homes for Energy mortgages, a reduced interest rate based on the home’s energy efficiency. The equation used in this process is figured by software after the home information has been entered.
Homes are rated against the 2006 energy building code standards. A house built to that standard would rate 100. Most existing homes were built 30% less efficient than that for a score of 130. Energy Star, a federal program to help homes become more energy efficient developed standards that typically resulted in a score of 70 or 30% more efficient. More particulars about how a HERS index is determined and applied to an Energy Mortgage.
The LEED qualifications require a meeting at the beginning of the project to determine the scope and points that are being pursued. I had hoped for a HERS rating of about 40 which was the low end of LEED for Homes typical scores. My evaluator told me that was unlikely without adding energy production. At the time I did not want to consider solar panels. Most of our roof faces north so it seemed an unlikely plan. The prediction based on early modeling was a score in the 60’s. That would have been good for a LEED home. If we had only met Energy Star requirements our home would have scored about 77.
Some homes are so energy efficient they generate more clean energy than they draw from the grid. A negative rating represents the additional clean energy the home is putting back into the grid, based on the scale of 100 for what a standard new home uses. Each negative point (-1, -2, -3, etc.) represents 1% of what a normal new home would use being returned to the grid. A score of -43 indicates that our home has the capacity to return 43% of that energy back to the grid. Some of that is by not needing it at all.
I was surprised that the Net Zero rating was -43. That is quite a bit lower than zero and the lowest score example I could find online was -58 in 2017. The house is in Florida in Port Charlotte designed and built by Anthony Fiore Construction.
It was rated Net Zero Ready by Energy Star which is a different rating from HERS that calculates the home before energy production is included. There are several different rating and certification programs to prove buildings are energy efficient. LEED is one of the most famous and a large part of the point system depends on the HERS score. Passive House does not use HERS but its own modeling equations. It is a more stringent standard for the home’s thermal envelope but does not include as many healthy living indicators as LEED does. Some other standards are in this graphic.
One of the reasons I was surprised is that an important test for energy efficiency is the amount of air leakage into the house from various small holes and cracks in the building envelope. We worked very hard to seal everything from the studs out. But we still had a large leakage issue from areas of the house that I had not been able to check under blower door pressure. So the Air Changes per Hour at 50 pascals of air pressure (ACH50) were higher than desired for a LEED Platinum home.
I looked for data that would explain the low HERS score with higher air leakage. I found a 2019 report from Ekotrope one of the major vendors for HERS software. They studied the characteristics of HERS rated homes using their software. Our ACH/50 was a little under 5. ACH/50 is the CFM/50 divided by the home’s volume.
The lowest HERS scores had ACH50 ratings below 5. But some homes around the 5 rating were below net zero. Adherence to the other factors such as building core, appliances, and lighting helped make up for the higher ACH50 score. I also wondered how much the solar roof added to our final score.
Although it is clear solar was helpful, a score of -40 even with over 15KW of solar is an outlier. That means that the solar helped but the other factors were just as important.