Zero Carbon Isn't Really Zero: Why Embodied Carbon in Materials Can't Be Ignored

September 1, 2009 · by Engin Ayaz and Frances Yang

In the pursuit of zero-carbon buildings by ever-reducing operational emissions and in the anticipation of a price on carbon via cap and trade, embodied carbon is likely to become one of the key metrics to address in whole-life building sustainability

Embodied carbon has been traditionally deemed optional in carbon emissions analyses for buildings because it was estimated to be of small magnitude compared with operational carbon (WRI, Build Carbon Neutral). In addition, the lengthy and demanding data gathering process needed for quantification made accounting for embodied carbon difficult since tracking materials sources from all their raw forms requires reliable data on established manufacturing processes and supply chains. However, in the pursuit of zero-carbon buildings by ever-reducing operational emissions and in the anticipation of a price on carbon via cap and trade, embodied carbon is likely to become one of the key metrics to address in whole-life building sustainability.

In this article, we will first define the differences between energy vs. carbon and embodied vs. operational. A case study will follow highlighting the increasing impact of embodied carbon in the context of operational efficiency, clean power, refurbishment, and rebuild.

Definitions

It is important to differentiate energy and carbon, which are often interchanged incorrectly. In this article, we have adopted “carbon” as an abbreviation for “carbon equivalent,” the metric for global warming potential (which mainly includes CO2 but also CH4, N2O, and other gases). The amount of carbon emissions related to energy use varies depending on fuel type: Fossil fuel-derived energy will produce high carbon emissions; on-site renewable energy may produce zero. In this way, operational energy and carbon are roughly proportional for a given fuel mix and thus often colloquially interchanged.

However, embodied energy and embodied carbon do not have a direct relationship because material processes can emit or sequester carbon as well. For instance, cement emits about half of its embodied carbon because of an inherent chemical process unrelated to energy use. In contrast, timber sequesters carbon during its growth. Thus, it is particularly important to distinguish between carbon and energy when speaking about a building’s embodied as opposed to operational impacts. A summary of the differences is given in Figure 1.

Using carbon instead of energy as the metric is advantageous since the literacy regarding carbon has increased tremendously in recent years. While Al Gore brought worldwide attention to atmospheric CO2 levels in his renowned movie An Inconvenient Truth, California passed an assembly bill (AB32) that warrants an 80 percent reduction of U.S. greenhouse gas emissions by 2050 when compared with a 1990 baseline. Conducting this comparative analysis in carbon units thus allows us to relate to these macro-scale goals and address a wide audience.

To better illustrate the highly variable relationship between embodied and operational carbon, we will take a U.S. office building as a hypothetical example.

Scenario-based Case Study

In this case study (see graphs), we will vary a range of assumptions regarding operational efficiency, type of energy supply, and the type of intervention at the building’s 30th year. This is an illustrative case study showing the order of magnitude difference between embodied versus operational carbon. The data is based on a range of projects and cannot be attributed to a specific set of buildings.

Case 1: baseline. In this baseline case, the building has an energy use intensity that is consistent with ASHRAE 2004 baseline levels, has a service life of 60 years, and has no renewables in its electricity and heating supply portfolio. It is assumed that no major refurbishment takes place throughout its lifetime other than the usual replacement of finishes (such as carpets and paint) every 10 years. As a result, embodied carbon for this building accounts for approximately 10 percent of the total building energy consumption. Just a few years after construction, the total operational carbon emissions exceed the embodied carbon emissions.

Case 2: energy efficiency. Operational energy of an office building could be reduced significantly using strategies such as reduced loads, passive solar facade design, natural ventilation, efficient HVAC systems, enhanced commissioning, and energy recovery. Considering the California Energy Commission regulations that aim at net zero-carbon commercial buildings by 2020, energy reductions of up to 50 percent are likely in the years to come. Thus, we have assumed a 50 percent operational carbon reduction, which leads to embodied carbon taking approximately 20 percent of the total whole-life carbon pie.

Case 3: clean power. The technologies used to generate and supply electricity and heating to buildings have a big impact on the operational carbon of a building. In particular, the more renewables are used both on- and off-site, the lower this emission rate becomes. In this case, we have assumed 30 percent additional renewables and thus 30 percent operational carbon reduction, which raises the percentage of embodied carbon to 35 percent compared to whole-life carbon.

This is a realistic scenario considering emerging policies and business models such as renewable portfolio standards (RPS) and power purchase agreements (PPAs). While RPS set utility targets such as supplying 33 percent of the electricity via renewables by 2020, PPAs help the building operator finance on-site photovoltaics more cost effectively compared with traditional ownership models.

Case 4a: refurbish. In 40 years’ time, only an estimated 30 percent of buildings will be new (Burrows, 2009). Therefore, refurbishment of existing building stock is necessary if we are to meet zero-carbon targets. While encouraged over complete demolition, replacement of mechanical systems, facades, and finishes at 30th year of operation add significantly to the embodied carbon, as demonstrated in Case 4. As a result, the embodied carbon becomes 45 percent of whole-life carbon emissions.

In this case, refurbishment leads to implementation of additional operational energy efficiency measures, which could reduce building energy consumption 10 percent at the cost of increased embodied carbon.

Case 4b: rebuild. When only considering cost and operational carbon, a building owner may conclude that rebuild offers an advantage over refurbishment. Starting with a clean slate may make it easier to adapt to changing needs of the business while leading to more significant energy savings due to availability of new technologies. However, replacing an existing building before it has seen its design life also increases the significance of its embodied carbon. In fact, reducing the building life from 60 years to 30 years in Case 4b results in embodied carbon being responsible for 50 percent of whole-life carbon emissions.

This case highlights the sensitivity of the proportion of embodied carbon to building lifespan. While the Case 1 scenario, like many comparisons, assumes a building life of around 60 years, the lifespan of a building in fact varies widely between 20 and 80 years. Demolition has historically been mostly dictated by change of use imposed on an inadaptable building (Minnesota, 2004), not by energy efficiency targets. Joining these two ideas together, a strong rationale emerges for designing for adaptability and ensuring financial incentives to refurbish existing buildings instead of demolishing them.

Besides inadaptability, natural hazards such as fire, flood, earthquakes, and hurricanes also threaten building lifespan. In fact, traditional design to code-prescribed hazard levels only assures no loss of life. Building codes do nothing to prevent loss of the material and monetary investment embodied in buildings. The rebuild effort involved can also be represented by Case 4b, urging us to design for more resilient buildings. Performance-based design and utilization of protective systems offer advancements above conventional code-prescribed construction, which can lead to solutions that do not necessarily increase embodied carbon. Even if some solutions to durability strategies -- such as structural redundancy for earthquake resilience -- increase the total embodied carbon in the building, they will likely avoid rebuild and reduce embodied carbon overall.

Conclusion

There are a number of factors that can significantly change the importance of materials to achieve whole-life carbon reduction. The above scenarios have shown that for structures taking on popular approaches to carbon reduction (lowering operational demand, sourcing cleaner energy, facade and MEP refurbishment, or rebuild), embodied carbon can account for up to 50 percent of the total carbon emissions (Smith, 2008).

Therefore, the design community’s responsibility is to acknowledge the importance of embodied carbon when considering carbon reduction strategies. In a future where legislation and technologies will emphasize radical operational reductions while the large existing building stock will necessitate major refurbishment or rebuild efforts, it is crucial to employ a whole-life carbon accounting method as displayed in the case study above. As such, we can communicate the tradeoffs between various design options and make environmentally and financially informed decisions that contribute to climate change mitigation.

References

Build Carbon Neutral. “Building Projects III: Commercial Interiors & Core and Shell,” USGBC Carbon Reduction Webinar Series, Feb 2008. Embodied CO2 of building has been cited in the range of 6 percent to 12 percent of total, embodied plus operational

Burrows, Steven, “Survival of the Most Sustainable,” DesignIntelligence. July 2009

Minnesota Demolition Survey: Phase Two Report, February, 2004. Provided through Athena Sustainable Materials Institute, www.athenasmi.org

Smith and Feldson of Simon Group estimate up to 80 percent. “Whole Life Carbon Footprinting,” The Structural Engineer, March 2008
WRI Green House Gas Protocol. Scope 3 emissions are currently optional.


Engin Ayaz is an energy and resources consultant in the San Francisco office of Arup. Ayaz’s current focus is carbon, climate change, and rating systems in the context of architecture, engineering and planning. He enjoys exploring synergies across various technical focus areas such as energy, materials, waste, water, transportation and sequestration, and developing integrated design scenarios. Previously, he worked on a sustainability research assignment across five other Arup offices.

Frances Yang is a structures and materials sustainability specialist in the San Francisco office of Arup. With combined background in architecture, civil and environmental engineering, she brings an understanding of life cycle assessment, performance-based seismic design, and building envelope integration to her structural design work. Her assignments in research and development, facade, materials, and sustainability groups have furthered her interest in how materials and structural systems contribute to whole-life sustainability performance of buildings.

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