The conversation about how our buildings use energy has shifted over the past two decades.

Back then, “energy-efficient” design was evaluated primarily by energy cost savings. These were the early days of green building certification, and the building design and construction industry was just starting to process and communicate the simple return on investment for “LEED premiums” and the like. Reducing utility bills was tangible and the conversation was simple, “How long is the payback?”

With the emergence of Ed Mazria’s “2030 Challenge” in 2006, we started a mad dash to set our building projects on a trajectory to be net-zero within 25 years. The industry was prompted to shift its collective focus toward gauging efficiency not by cost, but rather by units of energy – such as kilowatt hours (kWh) or by thousand British Thermal Units (kBTU). We started gathering measured data from our buildings and figured out how a new project compared to similar existing buildings. “Energy use intensity” (or EUI) became the “MPGs for buildings.”

Today, the conversation is shifting again as the industry focuses on reducing greenhouse gas (GHG) emissions in measurable ways – by project and across portfolios. We realize that using energy matters to the extent that it contributes carbon emissions into the atmosphere. According to the U.S. Energy Information Administration (EIA), only Kentucky, Wyoming, and West Virginia create more carbon emissions per unit of energy generated than Indiana. So, energy efficiency makes a big impact in the Hoosier State.

To be clear, “carbon” in this context is usually a shorthand for carbon dioxide (CO2). Moreover, CO2 emissions are typically used as a proxy for all GHG emissions. The industry likes to use terms like “CO2 equivalent” or “equivalent carbon emissions.” You might see it expressed as CO2e.

When we gauge just how “efficient” a building is, you may increasingly receive feedback in the form of kilograms of carbon dioxide equivalent (kgCO2e). The conversation might still involve energy cost savings and kilowatt-hours, but the focus is now on low-carbon building projects.

There are five primary ways in which carbon is being assessed on building projects: operational, embodied, construction, transportation, and site-related.This is the carbon (CO2 equivalent) emissions contribution resulting from the energy required to power our buildings. Currently, a building in Indiana will contribute operational carbon whether it is powered by electricity or natural gas. Generally, natural gas emits around half of the CO2e of coal. However, if the energy sources of our electricity grid decarbonize – this will likely take decades – that would make electricity’s carbon impact negligible. This is the basis of the argument for 100 percent electrification of buildings. It is a calculated risk that our utility grid will decarbonize well within the life of new building projects. However, the race to dramatically reduce carbon emissions over the next decade makes the decision to electrify a project in Indiana nuanced. A decarbonized energy infrastructure would lead to net-zero carbon.This is the carbon emissions contribution resulting from creating, transporting, installing, and removing the materials and products that comprise our buildings. Every component of a building required energy resources to get it into a building – that energy has a carbon impact that can be measured. International rules and processes for the building design and construction industry ensure that everyone measures embodied carbon in a consistent and comparable way.

Embodied carbon accounts for about a third of all carbon that comes from buildings. However, unlike operational carbon, which accrues over the life of the building, embodied carbon occurs upfront when the project is constructed.

If you are thinking to yourself that renovation projects must save a lot of embodied carbon, you are very correct. Some materials are great “carbon sinks” that reduce carbon on balance, such as sustainably managed wood and various rapidly renewable materials. Technological breakthroughs with concrete, aluminum, steel, glass, and other high-carbon materials may open the door for low- to net-zero carbon options.

In addition to the embodied carbon to get building products and materials created and installed on a job site, there are various other activities that take place during the construction of a building project that are not directly attributable to specific components, and are therefore outside of embodied carbon calculations. These activities include tradespeople going to the job site, the oil and electricity used by construction equipment, getting equipment to and from the job site, the resources that go into staging, trailers, site preparation and control, job site water consumption – the list goes on.

The industry has not figured this out yet. However, there are some contractors that operate in Indiana – such as Pepper Construction and Turner – who are making major strides toward quantifying construction carbon impacts.

Where a building is located matters when it comes to carbon. Increasingly, net-zero carbon building certifications are calling for project teams to assess the carbon that comes from how building users get to and from the project.

My firm’s own internal research has found that carbon contributions from transportation can roughly equal the operational carbon of a moderately energy-efficient building located in Indianapolis. For a high-performance LEED-certified project, the proportion of carbon from transportation may constitute about two-thirds of the total carbon.

Sitting a building in dense, walkable, bikeable communities with access to viable alternative transportation matters. Carpooling matters. Yes, telecommuting has sizable impacts here. Project teams can help building owners lower their “transportation carbon intensity” in these ways. Moreover, like the building sector, the transportation sector is moving toward 100 percent electrification. Same gamble here, if our electricity infrastructure decarbonizes, then electricity-based transportation moves toward net-zero carbon.

Among their many ecosystem services, we know that tress are essential to the air we breathe. According to the Arbor Day Foundation, in one year, a mature tree may absorb more than 48 pounds of carbon dioxide from the atmosphere. As with embodied carbon, our industry now has tools to measure the carbon sequestration potential of trees on our project sites. How much carbon can our trees soak up?

On a recent medical office building in the suburbs of Fort Wayne, we assessed the trees across the site and found that over the life of the building, the trees would offset about a third of the building’s embodied carbon. The vegetation on our sites is a massive carbon-soaking sponge. We can quantify this carbon removal, which direct serves to move a building project closer to net-zero carbon.

Collectively, these five aspects of gauging the carbon intensity of a building project offer a more holistic assessment how much CO2e our building projects are responsible for. As a global sector, buildings account for approximately 39 percent of all GHG emissions. As our industry advances strategies and solutions that take us further down the road to net-zero carbon, we are contributing to lowering carbon emissions in one of the most impactful ways possible.

Daniel Overbey, AIA, LEED Fellow, is an Assistant Professor of Architecture at Ball State University and the Director of Sustainability at Browning Day. He is also the 2023 President of the Indiana Chapter of the American Institute of Architects.