Life Cycle Assessment and Embodied Carbon
When we think of carbon, a charcoal-like lump of material is usually what comes to mind. But when we consider life cycle assessment and embodied carbon, we’re focused on the carbon dioxide (CO2) emissions that are the result of all the steps and stages that go into making a product—now commonly referred to as the carbon footprint.
Each product has a life cycle that begins at extraction or harvest, proceeds through transportation and manufacturing, and continues through its use and ultimate disposal. These stages are called system boundaries, and the entirety of this life cycle is understandably called cradle to grave. When assessing whole buildings, we always use cradle to grave system boundaries. But some life cycle assessments of materials only consider the initial stages of the process, then stop at the point the product has been manufactured and is ready for sale. This level of assessment is called cradle to gate.
System boundaries are further broken down into sub categories within each stage and denoted with letters and numbers. A1-A5 constitute the distinct steps of extraction, transport, and manufacturing in the product stage. B1-B7 comprise the use stage and include things like maintenance, repair, and refurbishment. C1-C4 make up the end of life stage, including demolition, transport and disposal. And stage D represents issues beyond disposal, like reuse and recycling potential. Each of these steps and stages are clearly outlined in the product life cycle assessment and are reported in a document called an Environmental Product Declaration, often EPD for short.
EPDs can exist for any type of permanently-installed building product from toilets to floor tiles and from concrete to door hardware. These documents are generated based on European EN 15804 and International ISO 14025 standards that define parameters for the assessment and calculation methodologies used and dictate how they are reported and compared. There are different flavors of EPDs as well. It might be product specific, known as Type III, or Industry Wide, based on information aggregated from numerous participating companies with similar products.
When looking at an EPD for the impact of carbon emissions, one would look at the Global Warming Potential (GWP). It’s important to note two things: GWP is made up of several different emissions of concern that are all reflected as CO2 equivalents, and GWP is not the only impact category of concern to the environment.
There are several chemical emissions that together form greenhouse gasses that blanket the Earth and trap heat in our atmosphere. Because carbon dioxide (CO2) is the one most commonly focused on, it has become the unit of measure for all of the gasses of concern. Each one – methane, chlorofluorocarbons, sulfur dioxide, nitrogen, ozone, and others – are converted into CO2 equivalents (CO2e) so that they can all be reported together. Methane, for example, has more than 80 times the effect on global warming than CO2. So the off-gassing of 5lbs of methane is equivalent to 400lbs of CO2e.
GWP is only one of the six environmental impact categories that are evaluated in a whole building life cycle assessment. Each category uses the same approach as GWP with respect to converting the gasses of concern in that category to a single metric that can be used to report them all. These six impact categories include:
Stratospheric ozone depletion which represents the loss of naturally occurring ozone in the stratosphere that shields the earth from harmful radiation. Man-made chlorofluorocarbons are very effective at breaking apart this necessary ozone, so gasses in this impact category are converted to CFC 11 equivalents.
Acidification of land and water speaks to the precursors of acid rain–which include sulfur dioxide and nitrogen oxides. These chemicals, when mixed with water in the atmosphere, create acid rain. For this category, off-gassing chemicals are converted to sulfur (SO2) equivalents.
Eutrophication focuses on the excess nutrients like nitrogen and phosphorus that are introduced into freshwater systems, typically from agricultural runoff and sewage discharge. These nutrients encourage the growth of algae, which then consume the dissolved oxygen essential for other aquatic life. Algae also produces substances that are toxic to animal life that ingest or absorb them. You may be familiar with the term algae bloom and the restrictions at lakes and shorelines that accompany that phenomenon. The impacts are calculated in nitrogen (N) equivalents.
Tropospheric ozone depletion looks at ozone from a different perspective. While ozone higher in the atmosphere in the stratosphere is desirable as a necessary shield, ozone that occurs in the breathing zone of the atmosphere is quite harmful to life. It is in fact the primary ingredient in smog. Industrial processes generate methane, carbon monoxide, and other volatile organic compounds. When these chemicals mix with sunlight and nitrous oxides, they create ozone (O3). Ozone in the troposphere can lead to chronic disease in humans and block plants’ ability to filter CO2 out of the air, further increasing the impacts of climate change. This category is measured in O3 equivalents.
Depletion of non-renewable resources represents the sixth impact category. Gasoline, coal, oil, propane and natural gas are all used for energy in manufacturing, as well as for component parts in many products. This impact category quantifies a material’s embodied energy in the constant of Megajoules (MJ). It takes roughly 8.64 MJ to power a typical household’s incandescent lightbulb for a year.
So what does all this mean for the building sector and the built environment specifically? The United Nations Environmental Programme (UNEP) evaluates the impact of the building sector around the world annually, most recently done in the 2020 Global Status Report for Buildings and Construction. The UNEP identifies two categories of GHG emissions with regards to building design and operations. The Building Construction Industry is the smaller portion accounting for 10% of global emissions. This includes the embodied carbon in the materials, transportation of products to the site, and the energy used to install the products in building construction. Operation of Buildings also known as operational energy, which accounts for about 28% of the global share of emissions, is the largest portion of facility emissions and is at an all time high. This includes energy used for heating and cooling, mechanical equipment, lighting and plug loads, etc.
Burgeoning populations mean continuing construction of more and more buildings, as well as replacement of aging building stock. According to the Carbon Leadership Forum, by 2060 the world will build the equivalent of an entire New York City every month for 40 years. Most of the carbon footprint of these new buildings will take the form of embodied carbon. This means that embodied carbon will be responsible for almost half the total new construction emissions between now and 2050. The emissions that come from energy use, i.e., operational carbon, can be reduced over time with energy efficiency measures, renovations and renewables. But embodied carbon emissions are locked in place as soon as a building is constructed.
Embodied carbon is a bit of a double whammy on the environment. There is the original output of emissions necessary to create new structures, but all of that energy and emissions required to put a building together sit latent in that structure until it is time for demolition. Every time a building is torn down, that latent environmental impact, or embodied carbon, is once again available in the global exchange of energy and emissions. Those materials can be thrown away in a landfill, effectively wasting that energy used and emissions created, or they can be recycled to prevent new products from being made to replace them.
Using a life cycle assessment to analyze the embodied carbon of materials during the design process can help identify the major contributors in the component parts of the building. For example, a building with CMU walls has significantly more carbon footprint than a steel or stick frame building because of the energy and processes required to make concrete and concrete blocks. Choosing to implement a TPO roof instead of a built-up roof can also reduce the carbon footprint of a building. Each of these decisions creates trade offs when it comes to durability, maintenance and environmental impact, so each needs to be considered carefully and balanced against project goals and sustainable outcomes.
We encourage project teams to consider opting for a life cycle analysis as a part of the scope of their design. When completed early, this analysis can provide guidance to the design team about embodied carbon and the building’s impact across all six environmental impact categories. Understanding the ecological effects of material selection will lead to better choices of both virgin and recycled materials, and more consideration of end of life strategies for deconstruction and reuse.
For teams with certification goals, the USGBC awards LEED credit for conducting a life-cycle analysis and further points for making decisions that reduce the total impact of the buildings. Consider adding this to your next certification strategy and reduce the carbon footprint of your development.