The Impact Embodied Carbon Can Have on a Home's Environmental Footprint

Has your state or local jurisdiction recently implemented a policy that sets targets for decreasing energy use and greenhouse gas emissions? Looking at emission reduction strategies across all industries will be imperative for mitigating the effects of a changing climate. Within the building sector, there is often an emphasis on minimizing operational emissions by implementing energy efficiency measures — demonstrated through voluntary green building programs such as ENERGY STAR for Homes or the National Green Building Standard® (NGBS).

You might be wondering how to comply with newly set climate goals. Here are a few factors that influence overall greenhouse gas emissions for buildings.

Most buildings need electricity to operate, and unless the home is powered by solar photovoltaics (PV), a local wind farm or another source of renewable energy, the building likely is getting its energy from a fossil fuel source. As such, many programs — for instance, Zero Energy Ready Homes — focus on reducing a structure's energy load from the get-go and strive toward net zero energy.

But what about carbon dioxide emissions associated with the raw material extraction, manufacturing and transportation of building supplies? This concept is known as embodied carbon. Although production for some materials requires energy- and carbon-intensive processes, there are also resources that can act as carbon sinks — in other words, they can store carbon and remove carbon dioxide from the atmosphere. Examples include plant-based materials such as fibers made from hemp, bamboo or waste straw. One way to track this element is by conducting lifecycle assessments and recording total net carbon emissions for each material via Environmental Product Declarations (EPDs).

One study by Trent University graduate student Chris Magwood compared the net carbon emissions of the same building using different materials in each modeled scenario. The results were drastically different depending on the products used:

• The highest emission scenario used clay tiles, steel joists, high-VOC carpets, steel framing, cement brick for cladding, and high embodied carbon concrete for the slab.
• For the same structure built to the same level of performance (to local code), the net carbon emissions were lowered by 144 tonnes by using trusses and asphalt shingles, drywall and mineral wool, wood framing and drywall, fiber cement for the cladding, and an average concrete with mineral wool for the slab.

In the lowest upfront embodied carbon model, the building ends up storing carbon (with no net emissions) using materials such as Forest Stewardship Council (FSC) certified cedar shake for the roof and compressed straw panels with an upcycled drywall.

Theoretical studies can help advance the industry by exploring various materials solutions, but practical application depends on product availability, the site's climate, material pricing and much more. Exploring embodied carbon alongside strategies to reduce operational greenhouse gas emissions, however, helps to view the environmental footprint of buildings more holistically. Adding insulation could greatly reduce the amount of energy needed to power the house, but the type of insulation used might have a high global warming potential, thus increasing the building's net emissions. Careful consideration of what products are used is critical and directly affects a building's impact on the environment.

For more information about NAHB's sustainable and green building programs, contact Program Manager Anna Stern. To stay current on the high-performance residential building sector with tips on water efficiency, energy efficiency, indoor air quality, and other building science strategies, follow NAHB's Sustainability and Green Building team on Twitter.