Is the greenest building the one you don’t build? The answer is a resounding “usually.” Conventional wisdom about building reuse is questioned and quantified in a much-anticipated report released today by Preservation Green Lab, part of the National Trust for Historic Preservation. Using a life-cycle assessment (LCA) approach that takes both operational and embodied impacts into account, the report compares the environmental impact of retrofitting an existing building for high performance vs. tearing down the building and replacing it with a high-performance one. It also looks at a more common real-world scenario—pitting high-performance new construction against continued use of a building that has only average energy performance.
While reuse generally has less impact, the advantages of retrofitting vary greatly depending on building type, climate, and materials used. In one notable exception to the overall results, adaptive reuse of warehouses for multifamily housing can actually have a greater environmental impact than demolition and new construction—highlighting the fact that decisions about retrofit vs. demolition will still need to be made on a case-by-case basis.
The most catastrophic effects of climate change can only be prevented in the next 20 years or so, making global warming potential one of the most pressing environmental impacts to consider. Since it can take decades for a new building to “pay back” its embodied carbon through improvements in operational efficiency (see A 2030 Challenge for Building Product Manufacturers), this study’s conclusions about carbon emissions should come as no surprise: based on climate-change considerations alone, almost every useable building in every region of the U.S. should remain standing—even if these buildings are not retrofitted to improve energy performance. Carbon payback time for the buildings studied ranged from 10 to 80 years.
While showing the clear benefits of building reuse, the report cautions that “reuse alone cannot fulfill the urgent task of reducing climate change emissions,” adding, “Reuse and retrofitting for energy efficiency together offer the most significant emissions reductions.”
Even when a retrofit is undertaken, the materials used in the retrofit must be carefully chosen. If the building footprint is expanded or if materials with high embodied energy are chosen, “the environmental benefits of reuse can be eroded or substantially eliminated.” Materials used for energy-efficiency measures can also harm human health and ecosystem quality.
“One of the results that I find interesting is just how much materials do matter,” says Jason McLennan, CEO at Cascadia Green Building Council and one of the study’s lead researchers. “If you are completely gutting a building and putting in tons of new materials, it’s beginning to act not like reuse” but instead like new construction. Buildings that are easily adaptable or those that can fulfill program requirements without added materials appear to be better choices for reuse, he says. “Existing building reuse is an incredibly important part of a strategy for energy reduction. It needs to be at the top of the list.”
The study’s methodology is unusual, combining large datasets with case studies of particular buildings. Energy-performance projections were based on national survey data, while case studies were used to determine the relative embodied impacts of retrofitting vs. new construction. While this methodology could leave the study results open to critique, the report claims this choice helped researchers accurately capture the complexity of comparing retrofits with new construction. The results, they say, actually increase the data’s reliability since embodied impact estimates were based on actual retrofit and construction projects rather than on modeling. Researchers call for further study to compare the status quo with more aspirational construction scenarios, such as net-zero-energy construction, lower-impact material choices, and increased urban density.
The Preservation Green Lab completed the study in partnership with Cascadia Green Building Council, Green Building Services, Skanska, and Quantis.
Copyright 2012 by BuildingGreen, LLC