Lab Takes on Residential Energy Efficiency in the Southeast
In 2002, the research team in the Building Technologies Research and Integration Center (BTRIC) at DOE’s Oak Ridge National Laboratory (ORNL) began a concerted effort to improve the overall health, safety, and comfort of occupants of typical new and existing construction in the Southeast region where the lab is located. ORNL engaged builders, homeowners, researchers, and manufacturers in pursuit of this goal. The journey started in eastern Tennessee when ORNL researchers demonstrated that it is possible to build cost-effective energy-efficient affordable homes using techniques that can be implemented by the existing contractor network. The next challenge was to demonstrate cost-effective energy efficiency in larger, market-appealing homes—2,500 square feet on average. After pushing the cost-effectiveness limit, ORNL researchers and Schaad Companies launched ZEBRAlliance, a public-private partnership designed to achieve two goals. The first goal was to evaluate next-generation technologies and strategies that might generate even deeper energy savings. The second goal was to decide how best to market these technologies and strategies.
Table 1. Campbell Creek Envelope and Equipment Technologies
Table 2. Comparison of Appliance and Water Heater System Energy Savings Potential in Annual Costs (electricity rate of $0.1/kWh)
Table 3. ZEBRAlliance Envelope and Equipment Technologies
Table 4. Annual Normalized Source Energy and Cost Savings
In 2008, there was a slump in the new-housing market, and more savings could be realized by improving existing homes. Accordingly, ORNL researchers expanded their scope to include the residential retrofit market. Since then, ORNL and its industry partners have been pursuing efficiency solutions for the entire residential market, from affordable to sophisticated and from new to existing construction. ORNL and its industry partners have a long-term goal of realizing 50% energy savings across the residential sector in the Southeast.
Affordable Low-Energy Homes
In 2002, ORNL partnered with Tennessee Valley Authority (TVA) and Habitat for Humanity to pioneer affordable, very low-energy homes by designing and building five demonstration houses using advanced materials, building techniques, and energy-efficient technologies that were researched, tested, and in some cases developed by ORNL. The first four houses—each averaging around 1,000 square feet—cost about $100,000 per house to build, not including the rooftop solar-PV systems. The houses featured geothermal HVAC, heat pump water heaters (HPWHs), highly insulated airtight walls, and advanced roof and attic systems. These modest, but pleasant, all-electric bungalows were a living laboratory for researchers who studied the integrated performance of the energy-saving components. According to Rudy Shankar, TVA’s vice president of technology and innovation, “these homes were among the first participants in TVA’s Green Generation Partners program, in which the house actually generates renewable power that TVA purchases at a premium price over our retail rates.” Factoring in the PV power, the houses’ energy bills averaged $25 per month and contributed electricity back to the grid during peak-load periods. While the homes were relatively small, they showcased energy-saving building techniques that made them not only affordable to build and operate, but comfortable to live in, and were accomplished using currently available materials and technologies. These houses have served as a practical example of low-energy affordable homes throughout the region.
Measuring Relative Efficiency Performance: Campbell Creek
In another research initiative based out of Knoxville, Tennessee, ORNL partnered with TVA to support a unique testing facility. The project compared the energy performance of three research houses with nearly identical floor plans. These houses were designed to determine the energy savings of whole-house retrofits and high-performance housing, as compared to the savings of homes that complied with the 2006 International Energy Conservation Code (IECC). The three houses were the Builder House (CC1), which acted as a benchmark and represented typical new-home construction practices under IECC 2006; the Retrofit House (CC2), which was identical to CC1 but included a major whole-house energy efficiency retrofit package in its original construction; and the High-Performance House (CC3), which included solar PV and thermal systems, and was constructed to be the most efficient home that the existing market economics could tolerate. The features of all three houses are shown in Table 1.
Unlike the occupied Habitat for Humanity homes, these houses have been kept unoccupied, and national average occupant impact on energy consumption has been simulated. This simulation is based on the 2009 Building America Research Benchmark for three-bedroom homes, and ensures valid and accurate comparisons between the houses, eliminating the potential for variations in occupant behavior from one house to the next that might skew the results. Energy use, temperature, and humidity were closely monitored for all three houses. As an example of the kinds of results achieved, utility bills for these all-electric homes during one 12-month period showed that total energy consumption for CC1 was 19,883 kWh; CC2 used approximately 40% less energy than CC1; and CC3 used 48% less energy than CC1, or 66% less energy from the grid than CC1, with the balance supplied by the solar-PV system. Some of the insights gained from this research are illustrated in Table 2 (see p. 27). For example, Energy Star appliances save about $47 per year if the house has a standard electric water heater, or about $17 per year if the house has a heat pump water heater (HPWH). Overall, the integration of a HPWH into a home with Energy Star appliances was shown to generate energy cost savings of about $265 per year when compared to a home with a standard electric water heater and appliances.
In contrast, a solar-thermal water-heating system would generate savings of about $179 per year in a home with standard appliances. However, if that same home had Energy Star appliances, the savings from the solar-thermal system would be reduced to $157 per year.
Energy savings in a home are derived from the home’s systems-level energy requirements (or loads) and the efficiency of the equipment used to satisfy those loads. The preceding examples illustrate the value of real-world testing of side-by-side research houses under simulated occupancy conditions. Similar analyses can be conducted to assess the value of various space-conditioning systems and building envelope strategies.
Low-Energy, Sophisticated, Market-Appealing Homes
In a separate venture, ORNL and Schaad Companies founded ZEBRAlliance in August of 2008. ZEBRAlliance is a public-private partnership whose goal is to maximize the cost-effective energy efficiency of buildings. In addition to the founding members, TVA,
BarberMcMurry Architects, and more than 30 ORNL industry partners became members of the ZEBRAlliance. As part of the first ZEBRAlliance project, four energy-efficient test houses were designed and built in the Crossroads at Wolf Creek subdivision in Oak Ridge, Tennessee. From the perspective of Jennifer Banner, CEO of Schaad Companies, “the market slowed down, and strategically, it was a great time for us to learn a new product and establish ourselves as this region’s place to go for an energy-efficient home.” From the perspective of ORNL, the research and data provided by the homes gives valuable insights into best practices for home construction and energy improvements to existing homes. And disseminating this information through journal, trade, and magazine publications, through the ZEBRAlliance web site, and through the media enables the entire regional industry to improve its product. Overall, the houses proved to be 55–60% more efficient than traditional new construction. Many different experiments were conducted in the ZEBRAlliance houses, several of which are described below.
The four ZEBRAlliance houses each demonstrated a different advanced envelope strategy for improving the effective insulating value of nonwindow areas and overall envelope airtightness. One house used structural insulated panels (SIPs). These prefabricated 4 x 8 modules consist of an inner core of insulating foam sandwiched between two layers of oriented strand board. With SIPs, the uninsulated 25% area associated with standard 2 x 4 framing is almost entirely eliminated, because studs are needed only where the modules are joined.
The second house used optimum-value engineering (OVE) advanced framing. With this technique, the framing contractor uses 2 x 6 studs, 24 inches on center (OC), instead of the standard 2 x 4 studs, 16 inches OC. OVE involves lining up rafters with studs, and other attention-to-detail techniques, so that the structural code is met. With OVE the uninsulated 25% area associated with standard 2 x 4 framing is reduced by half.
The third house used a double-wall framing system. Here there are 2 x 4 studs, 24 inches OC, back-to-back with a second framing layer that is identical to the first except that the studs are offset. A membrane separates the inner and outer wall cavities, and both cavities are insulated. With a double-wall system, the uninsulated 25% area associated with standard 2 x 4 framing is almost entirely eliminated, because the inner and outer studs are offset. The house also uses phase change material (PCM)-enhanced cellulose insulation in the attic.
The fourth house used an exterior insulation and finish system (EIFS). With this technique, expanded polystyrene foam insulation board is wrapped around the outside of a traditional 2 x 4, 16 inches OC frame. With EIFS, the uninsulated 25% area associated with standard 2 x 4 framing is almost entirely eliminated because of the continuous outside insulation.
Table 3 on p. 28 summarizes the four different envelope energy-saving packages tested in the ZEBRAlliance houses, all of which achieved highly effective insulating levels and low air leakage. “Many of the techniques that we’ve learned here are now part of our standard operating procedure,” says Banner. “The OVE advanced framing we have already instituted as a practice in all of our production homes. It’s very easy to teach when you have a great framing crew like we do, and very cost-effective. Moving the ductwork and air handler into the conditioned space is another of the many things that are now part of our standard practice.”
Ground Source Heat Pumps
In 2012, ClimateMaster, the nation’s largest manufacturer of water source heat pumps, announced limited production release of the Trilogy 40 Q-Mode ground source integrated heat pump (GSHP). This new product is the result of a multiyear research collaboration between ORNL and ClimateMaster to develop the technology and field-test several versions of preproduction prototypes in the ZEBRAlliance houses. In addition to heating and cooling, the Trilogy 40 Q-Mode provides 100% of the home’s domestic hot water on demand, at very high heat-pumping efficiencies. According to field tests and analysis with calibrated models, the Trilogy 40 Q-Mode saves about 60% of annual energy use and cost for space conditioning and water heating in residential applications when compared to new minimum-efficiency conventional systems. It saves about 30% compared with state-of-the-art two-capacity GSHPs with desuperheaters.
The high installed cost of GSHPs is a significant barrier to achieving large national energy savings with this technology. This cost premium over conventional space-conditioning and water-heating systems is caused by the need to drill boreholes or excavate trenches, install vertical or horizontal ground heat exchangers, and backfill the excavations. Since the length of the bore or excavation needed is determined by the home’s space-conditioning and water-heating loads, ORNL investigated whether the excavations required to construct the extremely energy-efficient ZEBRAlliance homes were sufficient to accommodate the entire length of the ground heat exchanger. ORNL analysis verified that the overcut around two of the four basement walls, plus use of the two utility trenches for power and water supply, would accommodate up to 60% of the required ground heat exchanger for a 3,700 ft2 house with walkout basement. Models developed and calibrated during the research suggest that if the construction excavations for the nonwalkout basement wall and the
below-basement floor had also been used, the entire ground heat exchanger could have been installed in construction excavations.
Heat Pump Water Heaters
HPWHs were another emerging technology field-tested at the ZEBRAlliance homes. HPWHs are transformational technology in water heating, because their efficiency is more than double the efficiency of standard electric storage water heaters. However, because HPWHs use heat from the surrounding air to heat the water, they could increase the space-heating load when they are located in the conditioned space, which would reduce their net energy savings. On the other hand, since HPWHs cool and dehumidify the surrounding air, they also have the potential to reduce both sensible and latent space-cooling loads, which would increase their net energy savings. The balance of HPWH energy savings and the impact on space-conditioning loads varies with the climate, the quality of the building envelope, the efficiency level of the HVAC equipment, and the location of the HPWH within the home. For a mixed-humid climate application, the EIFS house was used to evaluate the effect of the HPWH on heating and cooling loads and the associated net energy savings. Researchers found the HPWH energy savings (5.9 kWh per day) to be about 13 times greater than the space-conditioning take-back effect (0.4–0.5 kWh per day) for a very energy-efficient home in that climate, confirming the HPWH’s substantial energy savings.
Retrofitting Existing Homes
In an effort to advance the residential retrofit market in the Southeast, ORNL provided retrofit guidance on 14 homes. Five of the homes were retrofitted in eastern Tennessee. The remaining 9 homes were retrofitted in Atlanta, Georgia, through a partnership with Southface Energy Institute. The energy retrofit measures installed at the homes included sealing HVAC ducts; reducing air infiltration; bringing the attic and crawl space into the conditioned space; and replacing HVAC units, water heaters, lighting, and appliances. Temperature and relative humidity sensors and energy monitors were installed in the homes to validate energy savings and comfort improvements. On average, the retrofitted homes reduced source energy consumption by approximately 30%.
Table 4 shows the annual normalized source energy savings and utility cost savings that resulted from the retrofits for 8 of the 14 homes. All but 2 homes shown in the table have a yearly source energy savings of 30% or greater. Homeowners who spent more than $30,000 on retrofits consistently saved 30% or more in source energy, regardless of preretrofit energy intensity (annual energy consumption per unit of finished floor area).
Homeowners said that their investment in energy efficiency measures produced other benefits as well. For example, the owners of the house identified as Yellow Jacket in the study placed a high priority on comfort; their primary motivation for completing a retrofit was to increase the comfort of the furnished room over the garage (FROG). Prior to the retrofit, the average temperature in the FROG between 2 pm and 8 pm was 84.8°F, and the temperature in rooms on the second floor of the home varied by more than 4°F. To address this problem, the attic was sealed and the roof deck was insulated with 6 inches of open-cell spray foam. This made the attic a semiconditioned space and reduced the heat gain into the second-floor rooms from the attic. In addition, the thermal and pressure boundary was reinforced between the garage and the living space by applying 3 inches of open-cell foam in the wall that separated them, and by adding 5 inches of open-cell foam to the garage ceiling. After the retrofit, the temperature in the FROG remained stable, and it also remained within approximately 1°F of the temperature in rooms on the second floor.
From the perspective of energy savings alone, the Yellow Jacket retrofit was not cost-effective, because the utility savings over a ten-year payback period were much smaller than the initial investment. However, the homeowners believed that their retrofit was worth the money because it significantly improved the thermal conditions of the home. Prior to the retrofit, the FROG was so uncomfortable that the homeowners hardly ever used it. After the retrofit, the FROG maintained a constant comfortable temperature, and the homeowners gained 350 square feet of living space.
Similar comfort improvements were made in some of the other homes. One homeowner said that “living in Atlanta and not being muggy” was a “brand new” experience following the energy improvements. Susan Kidd, director of sustainability at Agnes Scott College, where five homes were retrofitted, adds, “The residents of the homes have already seen great results from this partnership. Simply put, utility bills are down and comfort levels are way up.”
Get more information on ORNL’s residential energy efficiency research and findings.
More information and detailed publications on the ZEBRAlliance.
The Power of Partnership
ORNL research focuses on demonstrating advanced building components and design and building techniques that can be implemented by the average American contractor to improve the affordability of energy efficiency measures in new and existing homes. “I would encourage any business that can interact in any way with the scientific realm to do so,” says Jennifer Banner. “It is absolutely an incredible interface, to take the very cutting-edge, pure science and bring it out into the world in a meaningful way, to commercialize what the scientists are doing in the laboratory.”
ORNL efforts to advance residential energy efficiency in the Southeast have realized substantial value to date. Energy savings from emerging technologies and innovative building techniques have been identified, validated, and proven ready for mass commercialization. With support from DOE, TVA, and other sponsors, ORNL will continue to build on its expertise, findings, and industry partnerships to further its goal of realizing 50% energy savings across the residential sector in the Southeast.
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