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Operating vs Embodied Carbon in the Built Environment:
The Difference and Why It Matters

The tremendous work to reduce operational carbon from buildings in recent years can be seen in the rise of net-zero energy buildings. But without a comparable focus on reducing embodied carbon in construction materials, that work will not lead to the hoped-for mitigation of climate change.

The built environment is responsible for more greenhouse gas emissions (GHGs) than any other sector of the economy — more than transportation, agriculture and industry. This situation needn’t be so, as we have the means to greatly reduce or eliminate operational carbon from the building sector. We are making substantial progress in developing methods to reduce the embodied carbon of construction.

What is operational carbon in buildings?

Operational carbon refers to the total from all energy sources used to keep our buildings warm, cool, ventilated, lighted and powered. Typical energy sources for this purpose are electricity and natural gas, with occasional contributions from fuel oil, propane and wood. The “carbon” part of operational carbon is a stand-in for all the GHGs released from these many energy sources and is the total of all GHGs released by them. You will occasionally see the abbreviation “CO2e” which signifies carbon dioxide equivalent. When CO2e is calculated, GHGs that have more greenhouse potential than CO2 are over-weighted in proportion to their impact (methane, for example).

What is embodied carbon in buildings?

Embodied carbon is the total emissions of GHG from all energy sources used to mine, log, harvest, extract, process, manufacture and transport to the construction site; and assemble the thousands of materials that go into a typical building. As with operational carbon, we measure this using CO2e. In our previous article, we mentioned that operational carbon is approximately 28 percent of global and US GHG, while embodied carbon is another 11 percent.

Building codes and carbon

The regulatory environment of the last two decades, as applied to design and construction through building codes, has become ever more stringent. For example, the International Building Code (IBC) used in the US has required a 50 percent decrease in energy use in new buildings since 2000. That decrease seems poised to continue and may reach net-zero energy — a condition in which a building or property produces as much renewable energy as all the energy it consumes in the course of a typical year — by 2035 or so.

While the IBC is leading to greater energy efficiency for new buildings and is reducing GHG emissions from new construction, it ignores two important factors: First, existing buildings are only required to be upgraded when undergoing substantial renovation; it is possible for inefficient older buildings to remain in that condition indefinitely. We’ll examine this issue in greater depth in a future article. The second gap in the building code, used by designers and contractors, is that it completely ignores embodied carbon.

Efforts to reduce embodied carbon in construction

Architects and engineers have recently begun to develop tools to track and calculate the embodied carbon of their designs. Several different software platforms are now commercially available and are not prohibitively expensive. Using these platforms, a clear picture of embodied carbon is emerging. A few key findings include:

  • The structure of a building is responsible for approximately 50 percent of a building’s total embodied carbon;

  • The exterior envelope of a building is responsible for another 30 percent;

  • The interior of a building is responsible for the remaining 20 percent; and

  • Concrete is the single most impactful material used in construction. The global concrete industry — if it were its own country — would be the world’s third-largest GHG emitter, after China and the United States.

Architects have recently established voluntary targets for embodied carbon reductions, through the Architecture 2030 program. Those targets are an immediate reduction of 40 percent, then 65 percent reduction by 2030, and zero emissions from materials by 2040. Achieving these results will not be easy and will require may innovations in the design process and materials technology.

A tremendous amount of hard work has gone into reducing operational carbon from building operations in the last several years. Net-zero energy buildings are the visible symbols of that effort. Without a new and comparable focus on reducing the embodied carbon of construction materials, that work will not lead to the hoped-for reduction in climate change that GHG emissions cause.

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