Author: Mark Webster; Contributors: Adam Slivers, Matthew Comber
Reusing existing buildings instead of constructing new buildings has many potential benefits, including:
- Reducing the use of new materials and their associated environmental impacts by extending the lifetime of materials that have already been manufactured and are in use.
- Protecting undeveloped land by reducing the pressure to construct new buildings and infrastructure on it.
- Reducing construction and demolition waste.
- Maintaining the cultural heritage of our existing building stock.
But even so, might constructing new buildings offer life-cycle environmental benefits compared to reusing existing buildings? What if new buildings are more energy efficient than the alternative reused buildings?
Let’s look at three studies, summarized in Table 1, that use life-cycle assessment methodology to compare building reuse to new construction. These studies show much depends on assumptions regarding operational energy use and lifespan.
Empty Homes Agency
The British Empty Homes Agency, a charity working to fill unoccupied homes, compared the global warming impacts of three refurbished homes to three new homes over a 50-year time period [EHA 2008]. All six case studies are actual projects, so the embodied impact estimates are based on actual bills of material. The authors selected the 50-year time period as the assumed time between major refurbishments.
The average CO2 emissions over 50 years are 155 kg/ft2 of floor area for the new construction and 150 kg/ft2 for the rehabilitation projects. If the authors had selected a 75-year time-frame, the results would have been similar (211 vs. 219 kg/ft2). While the reused homes use more energy per square foot per year (an average of 2.76 vs. 2.23 kg/sf/yr), the embodied impacts are so much lower that at the end of the 50-year or even the 75-year time-frames the differences in total emissions are small.
Athena Sustainable Materials Institute
The Athena Sustainable Materials Institute (ASMI) conducted a study for Parks Canada of four actual rehabilitation projects [ASMI 2009]. ASMI compared the energy use and carbon emissions for the four buildings to the impacts of demolishing the buildings and constructing similar new buildings. Unfortunately the authors did not attempt to quantify the embodied impacts associated with renovation of the existing buildings.
For the base case, which assumes the new buildings meet the Canadian energy code, the average CO2 emissions for the new and rehabilitated buildings over a 50-year period are almost identical: 251 kg/ft2 for the rehabilitated buildings vs. 253 kg/ft2 for the new buildings. Only the existing Winnipeg building had better energy performance than the assumed new replacement building; the assumed new buildings had lower energy use in the remaining three cases.
Preservation Green Lab
The Preservation Green Lab (PGL) of the National Trust for Historic Preservation conducted the largest and most thoroughly documented study comparing reuse and new construction. The Greenest Building: Quantifying the Environmental Value of Building Reuse examines the life-cycle impacts of seven reuse vs. new construction scenarios in four different cities [PGL 2011]. The study’s findings are exceptionally well documented and available for public scrutiny on the organization’s website.
The PGL assumed for the new construction scenarios that the existing building was demolished and includes the demolition impacts in the LCA results. The PGL used actual renovation projects to estimate the embodied impacts of renovation activities. The study quantifies 17 environmental impacts, including climate change.
For its base case, PGL assumed that both the reuse and new construction options have equal energy consumption. To evaluate the possibility that the new construction options may be more energy efficient than the reuse options, PGL also compared the reuse vs. new cases assuming 30% less energy use in the new buildings. While PGL used a 75-year time-frame for its base case, it also looked at time-frames ranging from 1 to 100 years.
Looking at just the commercial office building and single-family residence analyses in Chicago and Portland:
- If the reuse and new options have equal energy consumption, the 50-year CO2 impacts are 12% to 17% less for the reuse option.
- If the new options are 30% more energy efficient than the reuse options, the 50-year CO2 impacts are 1% to 12% less than the new options. In other words, even assuming significantly better energy performance for the new options, the differences in climate change impact over 50 years are relatively small. Over a 75-year time-frame, the 30% more energy-efficient new buildings emit 5% to 16% less carbon than the less efficient reused buildings.
Discussion and Conclusions
The three studies were conducted or commissioned by entities that generally support building reuse. Nevertheless, the studies appear to be well planned and conducted. The usefulness of the Athena study is limited by its neglect of the embodied impacts of the existing building renovations. Many existing buildings will require significant upgrades to make them as energy efficient as new construction, and the embodied impacts of these upgrades should be considered. The PGL study found that the materials-related carbon emissions for the rehabilitated office building, for example, accounted for 12% of the total life-cycle emissions over 50 years, compared to 22% for the new construction.
The studies generally support the conclusion that over a 50-year life cycle, the environmental impacts of reuse are less than the impacts of new construction when use-phase energy consumption is similar in the two options (Figure 1). Even when the new construction options have significantly better energy efficiency, the differences in life-cycle impacts are not that large in many cases, and fall within the expected range of uncertainty associated with life-cycle assessment studies.
Because the outcome of new vs. reuse comparisons depends upon so many variables, readers are encouraged to make use of readily available LCA tools and utility data to make their own project-specific assessments. The Athena Environmental Impact Estimator and the U.S. Energy Information Administration (EIA) are good starting points.
Of importance for climate change is the timing of the carbon emissions (Figure 2). New construction generally releases more carbon emissions than rehabilitation at the start of the life-cycle. Even in cases where the new construction uses less energy, it can take many decades for the new buildings to “pay back” that investment and equalize their emissions with the reuse alternatives. If the new and reuse options have similar energy efficiency, that pay-back may never happen. Those construction-related near-term emissions have a more serious impact on climate change than the emissions that occur farther in the future. Q3 also discusses this.
Beyond the Property Line
None of the studies attempt to quantify reuse benefits that occur beyond the building footprint. Existing buildings are more likely to be in existing neighborhoods with existing infrastructure, such as water, sewer, gas, and electricity. New buildings are often constructed on greenfield sites where these utilities must be constructed to service the buildings. Furthermore, I expect that existing buildings are more likely to be served by public transportation than new buildings, and more likely to be in denser neighborhoods. Therefore the daily users of these reused buildings may not have to travel as far to reach them and may have more alternative transportation options. A valuable research project would be to quantify these non-building impacts associated with the reuse vs. new construction alternatives. I expect the results would strengthen the argument that reuse is often environmentally preferable.
Athena Sustainable Materials Institute (2009). A Life Cycle Assessment Study of Embodied Effects for Existing Historic Buildings, prepared for Parks Canada in association with Morrison Hershfield Limited (http://www.athenasmi.org/wp-content/uploads/2012/01/Athena_LCA_for_Existing_Historic_Buildings.pdf).
Empty Homes Agency (2008). New Tricks with Old Bricks: How Reusing Old Buildings Can Cut Carbon Emissions (http://www.emptyhomes.com/empty-homes-publications-and-toolkits/empty-homes-publications/).
Preservation Green Lab (2011). The Greenest Building: Quantifying the Environmental Value of Building Reuse (http://www.preservationnation.org/information-center/sustainable-communities/green-lab/lca/The_Greenest_Building_lowres.pdf).