Destruction of old house by excavator. Bucket of excavator breaks concrete structure.
DedMityay/Adobe Stock Destruction of old house by excavator. Bucket of excavator breaks concrete structure.

“Finishing ends construction, weathering constructs finishes.” The opening statement in On Weathering: The Life of Buildings in Time (MIT Press, 1993) by Mohsen Mostafavi and David Leatherbarrow addresses the inevitability of decay and mortality in buildings. “This assertion would seem to defy one of the most ancient commonplaces of architecture: Buildings persist in time. Yet they do not,” they write.

Deterioration represents not only a material change but also an associated carbon cost. Discussions of embodied carbon in buildings today focus on the early stages of the material life cycle. The CO2 emissions generated by raw material harvesting, product manufacture, and construction indicate the initial carbon footprint associated with a new building. And yet, embodied carbon connects to a building’s entire life cycle—including its maintenance and end of life. In other words, we must account for the emissions that result from renovation and deconstruction just as we do for manufacture and construction.

Predicting the CO2 emissions of the future replacement of building components is generally feasible based on existing knowledge about material longevity, although adherence to a specific replacement schedule is not guaranteed. More challenging is presaging the timing of a building’s demise, given the fact that many structures are demolished for reasons other than deterioration. In both cases, researchers are forming a clearer picture of how maintenance and end-of-life scenarios influence the carbon footprint of materials, with corresponding recommendations for emissions reduction strategies.

For example, researchers at Australia’s Curtin University and the Ohio State University recently published a study quantifying the carbon footprint associated with steel corrosion. The authors point out that while information on corrosion’s economic cost has existed for more than half a century, little attention has been paid to its emissions-related impact. However, this carbon cost is worth quantifying, given that the global economic impact alone is estimated at more than 3% of gross domestic product worldwide. This study specifically estimates the CO2 footprint of new steel required to replace degraded material that is currently in service. Based on current steel industry emissions, the researchers attribute between 4.1% and 9.1% of total CO2 emissions to corrosion. Given this not-insignificant quantity, the authors recommend adopting a total energy and CO2 (TECO2) rating, which would include this replacement cost in material estimates. This comprehensive measure would likely motivate the increased use of corrosion-resistant alloys that, although initially more expensive, would result in a lower overall TECO2 rating.

According to a 2016 Preservation Green Lab report, 'it takes 10 to 80 years for a new building that is 30% more efficient than an average-performing existing building to overcome, through efficient operations, the negative climate change impacts related to the construction process.'

A popular axiom in sustainable design circles comes from architect and former president of The American Institute of Architects Carl Elefante, FAIA, who said, “The greenest building is the one that is already built.” But how much greener is it, exactly? Portland-based nonprofit Restore Oregon hired research firm ECONorthwest to run the numbers. The charge: to measure the environmental performance of renovating an existing structure versus razing it and building a new one. The ECONorthwest study employed the term “avoided impacts” to describe this approach, referencing a 2016 United Nations report by architect Michael Adlerstein, FAIA: “This embodied energy/carbon is considered the ‘avoided impact’ that is not spent if a renovation is undertaken.” The researchers based their assumptions on the fact that a larger structure would likely replace the existing one (a conservative estimate in a rapidly densifying location like Portland). According to their calculations, the team found that constructing a new 3,000-square-foot residential building would generate 126 metric tons more CO2 emissions than maintaining an existing 1,500-square-foot structure it would replace. But what if the new construction is more energy efficient? According to a 2016 Preservation Green Lab report, “it takes 10 to 80 years for a new building that is 30% more efficient than an average-performing existing building to overcome, through efficient operations, the negative climate change impacts related to the construction process.” Thus, energy efficiency can require a lengthy payback period. Based on this assessment, Restore Oregon advocates for better preservation incentives and protections for existing structures.

When buildings are eventually demolished, material recycling and reuse represent additional possibilities for carbon footprint reductions. Recently, a team of engineers from Nantong University in China endeavored to measure the potential CO2 savings based on construction, demolition, and renovation waste (CDRW) in Jiangsu Province, north of Shanghai. The researchers note that CDRW comprises more than 40% of total municipal waste in China and that recycling rates vary extensively. The team utilized two established calculation methods—waste generation rate calculation (WGRC) and nonlinear autoregressive artificial neural network (NARANN)—to estimate the potential carbon reduction represented by recycling seven primary materials: steel, concrete, wood, bricks, ceramics, glass, and mortar. According to the authors, the results indicate a possible increase from 3.94 Mt CO2e (metric tons of carbon dioxide equivalent) to 58.65 Mt CO2e with proper levels of recycling—particularly in the case of steel and concrete. In addition, the team estimates that more than 245 Mt of CDRW will be generated in Jiangsu Province by 2030, with the potential to reduce 82 Mt CO2e from this waste with adequate recycling methods.

As awareness of embodied carbon extends beyond new construction to encompass the entire life cycle of buildings, the architectural design and specification process will likely evolve accordingly. Similar to life cycle costing, in which the argument has been to look beyond first-cost toward lifetime cost, materials will be viewed with regard to their lifetime embodied carbon as well as their initial CO2e. This perspective shift will result in the consideration of longer-lasting materials to reduce maintenance-related impacts. But since lifespan is not predictable, given that many structures are demolished based on reasons other than technical obsolescence, multiple scenarios must be considered, including premature repurposing. For this reason, design for disassembly may finally receive the emphasis it deserves as a strategy to facilitate material reuse at the end of a building’s useful life. After all, finishing may symbolically end construction, but construction—just like weathering—is never really finished.

The views and conclusions from this author are not necessarily those of ARCHITECT magazine or of The American Institute of Architects.

Read more: The latest from columnist Blaine Brownell, FAIA, includes a look at pollution from microplastics, the year's buzziest material technology trends, and the hydrogen economy.