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- Feature Story
New homes today are roughly twice as efficient as older homes in the United States on a per square foot basis. Building America played a critical role in making that happen. Yay us! I mean all of us. Builders, HERS raters, trades, manufacturers, code officials, utilities, efficiency advocates, Building America, and other government programs like ENERGY STAR®. We all played important roles in radically improving the expected energy performance of modern homes. We should be very proud.
But the elephant—or should we say herd of elephants—in the room is all the rest of the homes. The flip side of our success with new construction is that well over 100 million American homes can use twice as much energy (on a per square foot basis), compared with the state of the art homes. That could be a staggering amount of wasted energy. If we could cut in half our energy use intensity (EUI) in existing homes, we could save a tremendous amount of energy—over 10 QUADrillion British thermal units (Btu) to be more precise. That’s over 10% of all the energy used in the entire United States. It won’t happen quickly, that’s for sure. But every long journey starts with the first step.
So in 2010, Building America started to focus about half of its resources on improving the energy efficiency of this huge population of existing homes. What did we learn about existing homes since then? Of course we learned a lot about how hard it is to improve all those homes. But we also added a great deal to the body of building science knowledge that we will need if we’re want to take home performance mainstream and save all that energy. Here are a few highlights of what we have learned so far.
Building America identified and analyzed three categories of home retrofits since 2010. We call the first type “shallow retrofits.” These retrofits involve investing a few hundred dollars in typically do-it-yourself projects that can provide energy savings around 10%. Shallow retrofit projects include simple tasks like replacing conventional lightbulbs with light-emitting diode (LED) lights. The Florida Solar Energy Center (FSEC) Building America team looked in depth at this type of retrofit in the report Phased Retrofits in Existing Homes in Florida Phase I: Shallow and Deep Retrofits. They also showed that shallow retrofits can easily be completed by home performance experts during home energy assessments and can be a gateway to deeper retrofits that are not good do-it-yourself projects.
"Home performance contracting" or "weatherization" projects take a more holistic approach to assessing home energy saving upgrade opportunities. They require more significant investment, and usually result in around 20%-30% energy savings. Home Performance with ENERGY STAR, utility rebate programs, and low-income weatherization programs promote this type of upgrade, which typically involves a professional who assesses the home and works with the homeowner to develop a scope of work for upgrading equipment, adding attic air sealing and insulation, and other cost-effective upgrades. These projects often improve comfort and reduce health risks in addition to the energy savings. Building America teams looked at several aspects of these mid-range upgrade projects, such as how to optimize assessments (see the report Field Assessment of Energy Audit Tools for Retrofit Programs), ventilation, and combustion safety testing. Find more detail about the individual technologies below.
“Deep energy retrofits” (DERs) usually require the largest investment by the homeowner. DERs also use a whole house system approach and require a home performance contractor or engineer, but in DERs the professional is usually hired to achieve radical reductions in energy use, targeting 50% savings or more. Building America projects have validated and documented energy savings over 40% in a number of cases, as documented in the reports Systems Evaluation at the Cool Energy House and Sunnyvale Marine Climate Deep Retrofit. Lawrence Berkeley National Laboratory studied over 100 DERs in this Building America report A Meta-Analysis of Single-Family Deep Energy Retrofit Performance in the U.S., including up to 70% savings and a few net zero energy projects (with the addition of renewable power systems). One Building America researcher achieved net zero energy in his own house over the course of many years, as documented in the report Achieving Very High Efficiency and Net Zero Energy in an Existing Home in a Hot-Humid Climate: Long-Term Utility and Monitoring Data.
In addition to better understanding the approaches needed to reach a wide range of energy saving targets and homeowner budgets, Building America also dove deep into technology-specific research in existing homes, trying out different innovative approaches to energy savings and retrofit cost reductions and, equally as important, helping learn how to retrofit homes safely. Here are a few highlights from this work:
- Foundations: Building America research explored innovative ways to transform wet, cold basements into more comfortable, habitable spaces and reduce home energy losses from the basement. Installing insulation on the exterior of a buried foundation wall is a daunting task, but one Building America team has been exploring ways to avoid full-scale excavation by using a more “surgical” high pressure water system to break apart the soil in a narrow trench around a foundation. Once there's room, rigid insulation and liquid expanding foam insulation can be added. Read about it in the report Excavationless Exterior Foundation Insulation Field Study.
- Exterior Wall Insulation: Rigid exterior insulating sheathing provides increased thermal resistance and can increase air tightness. Building America research identified the best methods for sheathing and cladding with exterior insulation to improve envelope performance and also address wind and gravity loads. We’re continuing to develop water management details for integrating exterior insulation strategies with other building envelope elements and connections. Just one of several Building America project reports on exterior insulation systems for existing homes is the report Initial and Long-Term Movement of Cladding Installed Over Exterior Rigid Insulation.
- Attic Air Sealing: Field studies have shown that airtightness in a building is equal to or more important than overall thermal resistance (R-value) in determining the performance of the building envelope, and that air leakage between the house and the roof or attic is a critical air leakage pathway. Building America has developed guidance for air sealing attics to provide high cost savings, and continues to collect supporting data on this topic, including methods to minimize moisture problems in attics. A good one is the Measure Guideline: Guide to Attic Air Sealing.
- Ducts in Attics: Duct work in unconditioned attics can cause a large amount of heat loss and gain. Building America research has developed several effective solutions for addressing this problem. Several approaches bring the ducts inside conditioned space by moving the insulation to the roof deck, sealing the holes, and conditioning the attic. In addition, duct system improvements alone can result in upwards of 5%-20% total energy savings. One innovative approach is highlighted in the Building America report by Steven Winter Associates, Reducing Thermal Losses and Gains With Buried and Encapsulated Ducts in Hot-Humid Climates.
- Existing Equipment: Building America field studies show that more than half of installed air conditioning systems are operating with significant defects, and proper maintenance of cooling systems can reduce their energy use by up to 50%. Simple tune-ups to existing furnaces and ducts can also result in a 23% increase in system efficiency. Read the report Improving Gas Furnace Performance: A Field and Laboratory Study at End of Life to learn more.
- Combustion Safety Simplified Test Protocol: Home performance professionals perform combustion safety testing on natural gas appliances before they perform energy-efficiency upgrades on homes, and homes that fail these tests are often rejected for upgrades. Building America supported the development of a simplified test protocol for combustion safety that saves time and more accurately identifies unsafe application installations that require remediation. Read the report Combustion Safety Simplified Test Protocol Field Study to learn more.
We’ve made great progress. But we’ve only touched the surface. Building America teams continue to drive innovation forward in a variety of home improvement technology areas. Learn more about what we’re currently working on.
And if you have ideas about research needs in existing homes that you think Building America needs to tackle, tell me about it. My email door is always open: email@example.com.
-Eric Werling, Building America Program Director