Energy Ratings

How do I get a good HERS score?

As a measure of energy efficiency, the HERS Score includes every part of a home that uses or loses significant energy. Uses include heating, cooling, hot water, laundry, and kitchen appliances. Losses come from the building envelope as heat is gained or lost through walls, floors and ceilings. The lower the HERS score the better.

The overall shape of a home can have a significant impact since the more exterior surface area there is the greater the heat gain and loss through it. A two story home tends to model better than a single story because it has less roof and foundation. Boxy is better than sprawling. Multi-family housing does better than single family due to shared walls and floors between units. Few people will alter their home designs for these reasons, but it's good to be aware of them.

The number of occupants relative to size has an impact. A small home with many bedrooms will model better than a large home with fewer bedrooms.

Insulation obviously has a large impact. Continuous insulation (like rigid board on exterior walls or blown insulation above truss chords) has a bigger impact than cavity insulation between studs and joists. Insulation quality also matters, and Grade 1 insulation is required in Massachusetts for new construction.

Window performance is measured by U-factor (an insulation rating) and SHGC (solar heat gain). In the Northeast where heating dominates energy use, the U-factor matters most and lower is better. All windows sold in the region should have a maximum of 0.30, and good performing windows will be 0.27 or lower. Some triple glazed windows can be under 0.20.

Windows have much lower insulation values than solid walls, so the more windows there are the worse the score. In hot climates the solar gain from windows plays an even bigger role, and shading from overhangs becomes important. In colder climates that solar gain can be an advantage in the winter months.

Air sealing is a critical area where many projects struggle. Very large energy losses can occur through small, unseen, or overlooked gaps in the building shell. HERS rated homes are tested with a blower door that will find these gaps, and poor test results can mean not passing energy code. Good air sealing practices start at the framing stage and continue all through the project.

Heat pumps are typically measured with HSPF2 for heating efficiency and SEER2 for cooling. Both are important in most climates, but heating efficiency matters most in the Northeast. Energy Star certified cold climate heat pumps will have an HSPF2 of 8.5 or above. Good ducted equipment can be 9.0 - 10.0, while the best single zone ductless models can now exceed 13.0 and offer significant savings over anything else.

Boiler and furnace efficiencies are measured with AFUE ratings. High efficiency natural gas and propane models can be 95% or above, while oil equipment will max out around 87%. Note that while fossil fuels are still allowed in most Massachusetts communities, it can be very difficult to achieve a passing HERS score if they are used. Significant efficiency improvements in other areas will likely be necessary to offset the negative impact fossil fuels have on the required HERS score.

Duct leakage can have a large effect on a HERS score, especially when the ducts are located in an unheated attic or basement. All ductwork needs to be properly sealed, and in Massachusetts all new construction ducts must be tested as a part of the HERS rating process even when they are inside heated space.

Many energy codes now require balanced continuous ventilation with energy recovery, which is typically accomplished with an ERV or HRV. Most models on the market will meet efficiency standards, but they also need to be properly sized to the home and its natural air infiltration rates. The exact ventilation CFM needed will depend on the final blower door test results.

Water heaters are the second largest users of energy in most homes and they have a large impact on the score. For homes using fossil fuels, tankless water heaters offer the best efficiency. For all-electric homes, a heat pump or heat pump hybrid water heater will offer far superior performance than a straight resistance model, and they typically pay for themselves in just a few years.

Kitchen appliances also figure into the HERS score. High efficiency and Energy Star certified models can be worth a full point or two, and can mean the difference between passing or failing energy code for a borderline home.

On-site power generation from solar or wind can offset some or even all of a home's energy use and lower a HERS score to zero or below. However, in Massachusetts the score used for code purposes excludes solar.