The control or builder home was built to meet current building codes and earned a Home Energy Rating System score of 90, slightly better than a typical code-compliant home. It is equipped with two heat pumps, one for each floor, that have a total capacity of 4.5 tons.
The retrofit house includes energy-efficient upgrades that focus on the building envelope and mechanical equipment. These allowed the HVAC system to be reduced to one, three-ton heat pump located inside the conditioned envelope. The retrofit home earned a HERS rating of 66 — a better score than the builder’s home.
“The retrofit unit provided 35 percent measured heating energy savings from the builder home, yet offers a package of technologies that are considered to be a reasonable upgrade for many homes in the United States,” said Jeff Christian, a senior researcher in the Department of Energy’s Oak Ridge National Laboratory.
Improvements to the home include installing low-E gas-filled windows, changing all light bulbs to compact fluorescents, and replacing the ceiling insulation with spray polyurethane foam insulation on the underside of the roof deck and attic walls to make it an unvented, semi-conditioned space.
“An unvented attic is particularly helpful in climates where heating and cooling equipment is located in the attic,” said Cam Shafer, building science manager for Rhino Linings®. “Modifying the attic to create an indirectly conditioned space helps significantly reduce energy consumption and improves mechanical equipment performance.”
Rhino Linings donated the insulation for the study.
High-performance improvements to the third home made it a near zero energy home with a HERS rating of 34 and a measured space heating energy savings of 65 percent compared to the builder home.
While more extensive, some of the improvements could still be made to a deep retrofit of an existing home. They included 2.5 kW solar panels, solar hot water heater, triple-layered windows with an R-value of 7, structurally insulated sheathing and spray foam insulation in the walls, and R49 attic insulation with radiant barrier sheathing. The builder was able to downsize to one, two-ton heat pump because of the envelope improvements.
“With three houses with actual identical simulated occupancy we will have research capabilities that are world-unique,” Christian said. “And the really exciting thing is that these homes will be available for research for seven years, so we will be able to replace, test and accelerate the development of even more efficient technologies.”
So how much do energy-efficient upgrades cost? The upgrades included in the retrofit home cost $4 per square foot or about $9,800 more than the control home. Upgrades to the near zero energy home cost $21 per square foot or $51,576 more than the control home. Based simply on projected energy savings, homeowners who implement the retrofit upgrades would recoup their costs in eight and one-half years. It will take homeowners who implement the near zero energy upgrades 22 years to recoup their costs.
While the current round of results includes the heating season only, monitoring will continue during summer, and results for a full year of the homes’ operation should be available this fall.
“We’re excited to be part of the study,” Shafer said. “We’ve seen an increase in the number of homeowners taking advantage of the Federal tax credit for energy-efficient improvements and other rebate programs. Many are doing upgrades similar to those found in these test houses. Even though these are only initial results, they are still very helpful in quantifying the energy savings from these retrofit options.”
The study is sponsored by the Tennessee Valley Authority and is being conducted in cooperation with researchers from the Oak Ridge National Laboratory.