Cutting your home's energy use by more than two-thirds of what it presently uses is increasingly a topic of discussion, and a goal, in the home energy performance industry. While everyone from contractors to building researchers have been attempting and studying the so-called "deep energy retrofit" since the 1970s, right now the building industry is searching harder than ever for the right combination of strategies, technologies, and approaches to achieving deep energy cuts at a reasonable cost.
Currently, weatherization programs save 10 to 15 percent of a home's energy use, and some utility programs can save 20 to 25 percent. But to make a dent in the greenhouse gas emissions of the residential sector and make housing more sustainable requires much larger reductions.
"A problem we need to solve," says Iain Walker, leader of a Lawrence Berkeley National Laboratory (Berkeley Lab) research team that evaluated a group of deep energy retrofits in northern California, "is determining how we measure the success of these projects. Do we measure energy, carbon, or cost savings? Do we consider percent savings per house, per person, or per square foot? Do we use site energy or source energy in these comparisons? Is performance based upon a reduction in energy use, a comparison to a reference design, or an absolute post-retrofit energy target?" Walker is a scientist in Berkeley Lab's Environmental Energy Technologies Division.
The complexity of these projects, and the variety of energy performance measures that a homeowner can take to reach these goals, is such that a reduction in energy use at the home in question does not necessarily translate to a similarly deep reduction in energy use at the source of energy generation, or to a deep reduction in greenhouse gas emissions. "There isn't even a consensus definition on what constitutes a deep energy retrofit," says Walker. "They can range from 50 to 90 percent."
Another problem is that deep energy retrofits are relatively untried and expensive. Only a few homeowners today commit to such a large home renovation project focused solely on energy bill savings, particularly given the risks they perceive with doing anything out of the ordinary with their homes. Are there particular energy-efficiency measures or packages of measures that do a better job of improving a home's energy performance at lowest possible cost?
Today there are about 20 million home remodels a year—usually to expand the home's size, to replace worn-out interiors and poorly functioning heating and cooling equipment, or to meet aesthetic goals. Homeowners spend more than $150 billion per year on these remodels, and every year more than a million homeowners spend more than $100,000 on their homes. The majority of remodeling is not done to achieve high energy performance. The home performance industry's challenge is to figure out how to motivate homeowners to spend some of this investment on energy and comfort upgrades.
How was the study set up?
Berkeley Lab's Walker, Jeremy Fisher, and Brennan Less located eleven deep energy retrofit projects around northern California that were initiated by their owners and convinced the owners to allow Walker's team to monitor the energy use of these homes in great detail. Walker's team had no input into how the projects were done—the owners and their contractors had made all the decisions on their own.
The researchers installed wireless energy monitoring equipment that provided energy use data once a minute for every electrical and gas energy end-use in these homes—space heating, air conditioning, water heating, lighting, appliance and equipment plug loads, and miscellaneous loads. The team also measured human comfort parameters such as temperature, humidity, and indoor air quality to determine whether the energy measures had an effect on the home's livability. The data stream was available to both researchers and homeowners, who were curious to see how well their home was performing.
What was done to the homes?
No single home project was like any other. One home had been small and was significantly enlarged. Another was a tract house whose owners decided to correct mistakes and failures to meet building codes during the original construction. Another home started as a shack near the coast and was built up into a larger, modernized house.
Yet another home started life as two houses on adjacent lots, with a passageway added to connect the two. Two homes were moved across town to adjacent lots as part of an affordable co-housing scheme. The owner of another decided to stage gradual improvements to the house over a decade. Another home was historically significant, so its exterior and most of the interior could not be modified—so the owners decided to create a kind of house within a house.
The costs of the energy upgrade portions of these projects—not the total remodeling cost—ranged from $10,000 to $57,000, averaging $30,000.
Many of the energy performance upgrades to these houses fell into one of three basic categories: an extensive rebuild of the home to meet the Passive House standard or Net Zero Energy goals; an upgrade of an older home to meet energy building codes; or energy-aware occupants upgrading a relatively modern house to a higher energy-efficiency level.
The extensive rebuild of one home to Passive House-level included blowing in insulation (R-30/38 in the walls and R-68 in the ceiling), adding rigid foam insulation around the foundation, and installing R-8 triple-pane windows, a heat pump and solar hot water heater, a gas-powered tankless hot water heater, compact fluorescent (CFL)- and light-emitting diode (LED)-based lighting, and 2 kilowatts (kW) of solar photovoltaic (PV) panels. Post-retrofit, this house used 75 percent less energy than the average California home.
In another case, bringing an older home up to code involved similar measures, such as blown-in insulation (R-19 wall and R-43 ceilings) and double-pane argon low-e windows (R-3). The occupants added a condensing gas furnace, gas tankless hot water heater, CFL and LED lights, and 2.5 kW of solar PV power. These measures saved more than 70 percent of site energy use (more than 90 percent of source energy use), even with the addition of 1,000 square feet of conditioned space in the basement.
In a third case, performing upgrades in a modern home, contractors replaced poorly installed batt insulation, adding new insulation to R-13 (wall) and R-40 (ceiling) levels. They sealed and insulated ducts and added a condensing gas furnace, an evaporative cooler, an insulated 40-gallon gas hot water heater, CFL and LED lighting, smart power strips, and ENERGY STAR appliances. These measures saved 55 percent of site energy use and 57 percent of source energy use, with no addition of solar PV.
Full descriptions of each retrofit project will soon be available in the Berkeley Lab report by the project team.
How much energy was saved?
The research team concluded that overall, the deep energy retrofits were a success—the site energy reductions ranged from 31 to 74 percent. But, source energy reduction—the translation of site energy savings to energy saved at the source of power generation—varied widely. One house actually used 12 percent more energy at the source because it switched from gas to electric heating. The largest source energy savings was 96 percent, and the average saving over 11 houses was 43 percent. Reductions of carbon dioxide emissions-equivalent ranged from 19 to 80 percent, averaging 54 percent.
Metrics used to evaluate the energy retrofit can result in a distorted sense of a retrofit's success. For example, measuring the energy use per square foot tends to reward larger houses that use more energy than smaller houses for the same number of occupants.
Also, houses with a lot of occupants can look like energy hogs, even if those homes showed a large net energy use decrease after a retrofit.
Another example of a metric's misleading impression results from whether a house is all-electric or a mix of gas and electric use. Two houses in the study performed at about the same on a percent site-energy savings basis. One house switched from gas to electric heating during its retrofit while the other stayed with its gas and electric energy mix. The result: the all-electric house's percent source energy savings was around 5 percent, while the other house's savings weighed in at 90 percent.
"When pursuing a deep energy retrofit, we recommend choosing fuels carefully and doing so only after evaluating energy performance on a source-energy basis," says Walker.
How was comfort and indoor air quality affected?
Another finding of the study is that using our standardized notions of comfort produces misleading estimates of energy use—in several homes occupants chose widely varying indoor temperatures.
Also, in many homes, indoor air quality may be compromised: "None of the homes in the study complied with the national minimum standard for ventilation, ASHRAE 62.2," says Walker. The team's measurements of fine particles in the indoor air spiked during cooking. "Most of the standard compliance issues related to a lack of proper kitchen venting," he explains. "Good house ventilation can reduce the concentration of these particles in the air, but what these and other homes really need is a good range hood that the occupants use reliably."
Walker's team is looking for funding to follow this first study with new research that would identify homes with common problems by the way houses are built and by their geographical region. The goal would be to recommend a package of retrofit measures that would solve these problems. For example, in northern California, a common set of problems appears to be leaky, uninsulated walls and floors, poorly insulated attics, single-pane windows, 25- to 50-year-old heating technology, leaky uninsulated ducts, wasted hot water, incandescent lighting, and poor or nonexistent ventilation.
Identifying a package of standard changes for these homes would help contractors reduce the cost of deep retrofits—repeating the same steps in the same types of houses leads to savings in labor, material costs, training, and consulting. Such a strategy could reap large benefits and encourage the "mass production" of home energy performance improvements.
"These relatively simple and straightforward approaches reduce perceived risk because they use commonly available construction practices, control costs, and use readily available technologies—so we can do this right away," says Walker. "Climate scientists say we can't wait anymore to cut carbon emissions."
This research was funded by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy.
What a Standardized Deep Energy Retrofit Package for California Might Look Like:
- Air-seal the ducts and building envelope (to meet new construction requirements).
- Insulate walls to R-13 and attics to R-38.
- Replace windows with energy-efficient models.
- A sealed combustion furnace.
- A new gas tank hot water heater.
- CFL and LED lighting.
- Smart power strips to reduce energy use of always-on appliances.
- Replace appliances with ENERGY STAR models.
- Improve kitchen and bath exhaust and whole-house ventilation, to meet the ASHRAE 62.2 standard.