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You probably know just how efficient the materials, appliances, and systems are that you install in the homes you build. But what about the home itself? Increasingly, owners are measuring whole-home performance to plan upgrades or improve their selling proposition, and buyers are using that information to differentiate between potential purchases.
One common way to make this assessment is through the HERS Index. A HERS rating is a measure of a home’s energy performance compared with a reference model. More than 227,000 homes in the U.S. were HERS rated in 2017, according to RESNET. That’s up 10 percent year over year and a new record. The addition of the Energy Rating Index (ERI) alternative compliance path in the 2015 International Energy Conservation Code (IECC) has helped raise the profile of HERS scores among builders and remodelers.
HERS scores can range from the low 70s for a home built today based on 2015 IECC standards, to 100 for a home built 10 years ago. Older homes are likely to score in the 130s and higher. Net zero homes, which produce as much energy as they consume, have a score of zero. The average HERS score in 2017 was 62. Moving one point up or down the linear scale is equivalent to a 1 percent change in energy performance.
Construction professionals should be aware of how their homes stack up according to this efficiency measure, as it offers a reliable starting point for further upgrades. The first step to a HERS score is assessing whole-home energy efficiency. There are two popular ways to do so: a home energy survey and general energy audit.
Knowing your options
A home energy survey is an inspection that determines an existing home’s energy performance. It considers the type, features, and age of the building envelope, appliances, and lighting. It also looks at occupant comfort, visible moisture, and any occupant health or safety issues. By examining utility usage and costs, it can determine potential areas for energy savings. A home energy survey does not result in a HERS rating. Instead, it gives recommendations for improving efficiency throughout the house, including basic changes that homeowners can make themselves, says Ryan Meres, program director at RESNET, which develops home energy efficiency standards and certifies home energy rating professionals.
The process takes about an hour and typically costs between $100 and $300, according to Meres.
“It is most common for new homes to get a HERS rating, that involves conducting a ‘Projected Rating’ based on building plans, followed by a pre-drywall and final inspection during the construction process.”
A general energy audit collects more details about the home’s energy use and does a more thorough analysis of its energy costs. Also unlike the home energy survey, the audit uses diagnostic equipment such as a blower door test, combustion analyzer, and infrared camera to create a performance profile for the home. The audit identifies the number and location of leaks throughout the building envelope and HVAC ducts, and determines how effective the current wall and ceiling insulation is. It will also identify current or potential combustion safety concerns.
The audit is completed by a certified HERS rater, taking between three and four hours and typically costing up to $800. It can include a HERS rating.
While the survey and audit are tailored to existing homes, Meres says, new homes can also undergo a similar process to achieve a HERS score.
“It is most common for new homes to get a HERS rating,” Meres says. “That involves conducting a ‘Projected Rating’ based on building plans, followed by a pre-drywall and final inspection during the construction process.”
The impact of fuel choice
HERS considers the home’s many systems – such as space and water heating, clothes drying, and cooking – as parts of a single unit, resulting in one score for the entire home. The index is fuel-neutral, which means it focuses just on the building load rather than on energy used by sources such as propane, natural gas, fuel oil or electricity to meet those loads, says Philip Fairey, a building scientist and deputy director of the Florida Solar Energy Center who helped develop calculations for the HERS Index.
To calculate a HERS rating, the home being tested is compared to a reference model that reflects its size, location, systems, and fuel type. That model is based on energy requirements that are about a decade old, and efficiency levels of products and systems on the market today have improved since then.
Because the baseline HERS performance requirements are less efficient than what is standard today, building more efficient homes results in a lower (i.e., better) rating and building less efficient homes results in a higher rating, relative to the standard. This can be significant, Fairey says: For example, standard gas furnaces on the market today have an annual fuel utilization efficiency (AFUE) of 90% whereas the federal minimum efficiency requirement is 80%.
Unless a home’s energy is generated on-site – such as power derived from photovoltaics – using one fuel source or another won’t directly impact the home’s HERS rating.
Beyond the score
Some builders and building scientists look at additional information when determining a home’s energy profile. Chief among them is source energy, which is the energy required to produce and distribute the energy used in a building. Some types of energy require more source energy than others. A standard multiplier can be used to level out the differences.
For electrical energy, the site-to-source factor is around 3.2, whereas for gas (natural gas and propane) it is lower at 1.1, says Chris McTaggart, HERS supervisor and co-principal at HERS rater and consultancy Building Efficiency Resources. That’s because electrical power tends to require more source energy than does gas.
Growth in grid-tied renewable power as well as factors like location and time of day can impact those figures, however. Those same factors influence energy cost and availability. Coupled with project-specific considerations such as climate zone, budget and product availability, the relationship between fuel source and a project’s bottom line can vary.
For example, gas furnaces are typically preferred over electric heat pumps in northern climates that experience very cold winters. In parts of the country that experience milder winters, builders often specify heat pumps with a gas furnace as backup to support occupant comfort year-round.
Check out our Propane Energy Pod Home model to learn more about achieving whole-home energy efficiency with propane based on home size and location.