May 08, 2016

Retrofit houses with highest monthly bills during cold months for energy efficient cities

Many programs encourage owners of homes and other buildings to improve their energy efficiency, sometimes offering substantial subsidies or tax incentives for doing so. Now, planners may have a way to determine where such programs can get the most return for that investment: New research shows how to identify the buildings where retrofitting for energy efficiency will have the biggest impact on a city’s overall greenhouse gas emissions.

The authors point out that 44 percent of all energy used in buildings in the United States goes toward heating and cooling, and this accounts for 20 percent of the nation’s carbon dioxide emissions. So, making a significant dent in that sector could help the country meet its commitments for curbing greenhouse gas emissions.

But not all housing is created equal, and making retrofits in some of the least efficient buildings could have a much bigger impact on emissions than fixing up buildings that already perform relatively well. Figuring out how to identify the buildings most in need of improvement, however, at a scale useful for city officials and utility companies, is not a simple task.

González explains that there are 82 different parameters that can have an effect on the overall thermal efficiency of a building, but much of these data require detailed on-site measurement and in some cases specialized equipment, making it impractical to do such assessments at the scale of entire cities. But after a careful study of representative areas in Boston and Cambridge, Massachusetts, the team found that it is possible to use just eight of these factors to reach conclusions that are almost as accurate, which makes the task much more feasible.

Just three of the factors alone can account for 85 to 90 percent of the variability between buildings, making it much easier to perform at least a first-order evaluation of a city’s housing stock.

The key to making useful assessments turned out to be a direct measure of people’s energy use during cold months: their monthly gas bills. By comparing anonymized bill information from the gas company with city-provided data on the size and volume of the buildings, and with weather data showing the outside temperatures during the study period, the team found they could make detailed predictions about which buildings would benefit the most from the retrofits, which include such steps as adding insulation, sealing leaky windows and doors, and replacing older single-pane windows with newer double-pane versions.

Selecting just 16 percent of the buildings in Cambridge for such retrofits, based on the formula they developed, would be enough to eliminate 40 percent of the city’s natural gas usage.

In this image of Cambridge, Massachusetts, the colors represent which buildings could be retrofitted to obtain different percentages of total energy savings. Converting just red and orange buildings would achieve 40 percent of the total potential energy savings in the city from efficiency improvements. West Cambridge has a dividing line that shows efficient, dense housing next to a several homes that could use a retrofit. This might be because that neighborhood was built at a different time, or because the homes there are larger. Illustration: Jameson L. Toole

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