Renovating an existing building is almost always more environmentally beneficial than demolishing an existing structure and building a more energy-efficient one, according to a new report by the National Trust for Historic Preservation’s Preservation Green Lab, a Seattle-based initiative that seeks to explore the value that older buildings bring to their communities. The report asserts that renovating an existing structure is more environmentally beneficial because it takes between 10 and 80 years for the benefits of a new energy-efficient building to compensate for the carbon emissions incurred during its construction.

The report, “The Greenest Building: Quantifying the Environmental Value of Building Reuse,” utilizes a life-cycle analysis (LCA) methodology to compare the environmental effects of renovation and those of demolition and new construction on six building typologies in Atlanta, Chicago, Phoenix, and Portland, Ore. over a 75-year period.

For all six typologies—single-family homes, multifamily complexes, commercial offices, “urban village” mixed-use structures, and elementary schools—the report found that it is nearly always greener in terms of ecosystem quality, human health, resources depletion, and climate change, to let an existing structure stand.

For instance, retrofitting 1 percent of Portland’s offices and single-family homes would, over the next decade, reduce the city’s carbon emissions by 15 percent from its current levels. The only exception was the conversion of a warehouse to a multifamily residence, as this required more new materials than other reuse scenarios. The study showed a newly constructed building to have fewer effects on the ecosystem and human health in all four cities than the warehouse-to-multifamily conversion.

Retrofits also have the potential to be a greater boon to the economy than new construction projects. According to the report, historic rehabilitations have created 2 million jobs over the past 32 years, and have generated $90 billion of private-sector investment. In addition, a recent study by a research team from Amherst College in Massachusetts showed that retrofits create 50 percent more jobs than new construction projects.

Patrice Frey, the National Trust for Historic Preservation’s director of sustainability, points out that new construction projects tend to spend more money on materials, whereas retrofits spend more on labor, and thereby create more jobs. “Those are jobs that can’t be outsourced,” she says, “whereas materials can.”

The biggest question left by the report is how dense urban populations might be accomodated in older, retrofitted buildings that are considerably shorter in height than new construction projects. As Slate’s Matthew Yglesias puts it, “how tall does the new structure need to be before a new build is greener than retrofit + build elsewhere?”

Frey stresses that Preservation Green Lab’s study is “an apples-to-apples comparison” of retrofits and new construction projects, and that on the topic of density, more research needs to be done. Yet she thinks that the retrofitting of older, more historic structures and the need for smart density are compatible ideas. She points to Seattle, where developers have added stories to existing structures “in a sensitive way.” “We need to think about density more holistically,” she says. “We need to think about creating quality, human-scale communities and character-rich spaces where people want to live.”

A PDF of the full report, which was completed in partnership with the Cascadia Green Building Council, Green Building Services, Skanska, and Quanits, and with support from The Summit Foundation, can be downloaded here.