In the battle against climate change, green buildings have become the concrete-and-glass hero we need. But consider this: approximately half of all commercial buildings in the U.S. are more than 40 years old. Energy retrofits—which employ strategic interventions like new HVAC systems, solar panels and sun canopies—can conceivably cut energy use in older buildings by half, are more affordable than new construction and are a viable alternative for significantly reducing carbon emissions.
While every retrofit is unique, do certain factors influence whether it will be successful?
New research from Assistant Professor Ming Hu and David Milner (B.S. Architecture ’20) indicates a trend: compact buildings in a mild climate that are renovated within a specific construction cost range have a higher likelihood of achieving a net-zero energy goal. These findings, the researchers say, could help stakeholders better consider a building’s characteristics when creating a building energy retrofit plan. Their research appears this month in Buildings Research & Information.
“There is currently a lot of information about the benefits of energy retrofits, but very little information about how to do it successfully,” says Hu. “Our primary goal with this research was to fill that gap.”
While previous studies have focused on net-zero energy savings in residential projects, Hu and Milner’s study is one of the first to examine small- and medium-sized commercial buildings, which make up roughly 50% of all buildings in the United States.
“Deep-energy retrofits have proven to be financially feasible and technologically practical, but current research predicts only 5% of the existing stock will actually meet deep energy reduction goals,” says Milner. “That’s not enough if we want to reduce our energy consumption. We wanted to offer a comprehensive guide for what measures have proven to work and what haven’t.”
Using the building performance data of 34 small- to medium-sized commercial retrofit projects in the United States, Hu and Milner applied a regression and logistic model to analyze five impact categories—physical, technical, environmental, economic and policy—and 11 variables, such as building façade material, lighting and building size. These variables were then correlated with each of the buildings’ Zero Energy Performance Index Score to determine if certain variables were more likely to affect energy-saving performance.
While the results indicated that building size and age aren’t major influencers of a successful energy retrofit, a building’s compactness—the ratio between the overall surface area and building space, such as number of floors—was found to be the most influential factor in whether a retrofit is successful in achieving a net-zero energy goal. The more compact a retrofit building project is, the likelier it is to be energy efficient.
“When agencies and stakeholders consider retrofitting existing building stock, those with a higher compact ratio should take priority,” says Milner.
The analysis also showed that a higher price tag is not necessarily associated with improved energy performance, dispelling the myth that net-zero retrofitting design is cost-prohibitive. In fact, the team points to a cost-benefit “sweet spot” in retrofit construction: projects between $200 per square foot and $450 per square foot proved to have a higher chance of achieving net-zero energy savings. Of the three most expensive retrofits in the study, the construction elements that actually contributed to the net-zero goal, such as solar panels or window replacement, only accounted for 15% of the total construction cost.
“Net-zero retrofits have developed a reputation for being elitist or too expensive when, in actuality, that’s not the case,” said Hu.
While buildings situated in areas with mild climate conditions showed more likelihood of success, the research stressed that climate is not the sole determinant of whether a building can achieve net-zero energy savings. When combined with other interventions, buildings in harsher climates fared well in their index scores.
According to Hu and Milner, another major obstacle to more widespread adoption of deep energy retrofitting is the discrepancy between simulated results and actual performance. Using actual, real-world building performance data, the team hopes future research and practice will turn to data analytics to better understand best tactics for energy renovations. These initial findings, Hu says, can help stakeholders design policies to support retrofitting efforts and understand where retrofits might make sense.
“If society hopes to reduce energy consumption and carbon emissions, they won’t be able to do it by just constructing new energy-efficient buildings,” said Hu. “Retrofitting existing buildings must be a bigger part of that effort.”