Embodied Carbon Studies at C&H

At Coldham and Hartman, we believe that it’s our responsibility to ensure that our designs are responsive to the persistent challenges posed by climate change. Our firm was founded on this. When Bruce Coldham opened shop back in 1989, he did so in response to a changing resource landscape and in anticipation of the threats that humans posed to the global climate. Facing the reality that buildings use far more energy than necessary, ecologically-minded builders, building scientists, and architects such as ourselves, have gotten the art of energy efficiency down to a science. We know how to design a building that uses less energy than it produces, a Net Zero Energy building. C&H has had the opportunity to do this repeatedly for clients throughout the Northeast.

But, as we know, a Net Zero design addresses only part of the equation. A traditional Net Zero building only accounts for what is called operational energy, or the energy that is used to operate the building – the electricity for heating, cooling, plug loads and so forth. When we complete a Net Zero Energy building, we measure its operational energy, which is pretty straightforward – often as simple as looking at the energy bills. Are they positive or negative?

Another key piece of the energy equation is embodied energy, or all the energy that is expended getting the materials produced and the building built. As the design and construction industry has gotten better at minimizing operational energy, we have begun to look to strategies to minimize the embodied energy in buildings. Through a process called Life-Cycle Assessment (LCA), we’ve been able to quantify the environmental impact of materials from extraction, through construction and use, all the way to disposal or ideally reuse. Many firms are now interested in what is called embodied carbon. Embodied carbon is the total greenhouse gases emitted over a material’s life – from extraction, refining, transportation, maintenance, and disposal, minus any carbon dioxide it might have sequestered and stored, as in the case of wood, grasses, or other plants that absorb CO2 from the atmosphere. Embodied carbon is typically normalized to a kilogram of carbon dioxide, abbreviated as kgCO2e (e for equivalent).

Embodied carbon is important for a couple reasons. Primarily, by preserving carbon sequestering materials, such as wood, in a building, the carbon stored in its fibers is prevented from being released back into the atmosphere by decomposition or burning. Additionally, by limiting the amount of carbon released in constructing a building, we are able to reduce our contribution to climate change when it really matters – now. The more quickly we can draw down our greenhouse gas emissions, the more likely we are to avoid irrevocable climate catastrophe.

So, we know our buildings are performing at Net Zero operational energy, but we were curious how they measured up in terms of embodied energy. We took one of our built projects as a case study and started where anyone would start: the beginning. It’s a 3,800 square foot, 4-bedroom farmhouse in Southern New England. Metal roof. Solar panels and all electric systems provide an annual net operational energy of 0 kilowatt hours. To figure out the embodied carbon of this design, we used a carbon counting tool called Tally®, designed by an offshoot of the Philadelphia-based architecture firm Kieran Timberlake. We also ran the project through BEAM by Builders for Climate Action and had some friends check our math. There are a number of products and databases out there, but we used Tally® because it fully integrates with our design software so we could easily translate our design into materials that Tally® could assess for their embodied carbon. BEAM was released as we finished up the analysis, so we decided to give it a try for comparison. We ran an analysis on the house as-designed and then compared it to various alternative scenarios, each of which would produce comparable building performance in terms of operational energy.

What we found was, as-built, the house stores more carbon in its materials than were released in their production. In other words, the design was net-negative embodied carbon. In fact, the amount of carbon stored in the building’s materials, some 6,600 kgCO2e (figure 1), is comparable to taking a car off the road for almost a year and a half (figure 2). Then the curiosity kicked in. We tested a number of scenarios that altered various building components, one at a time, to see how they would affect the building’s overall embodied carbon profile.

What if, for example, instead of using cellulose insulation, we had used a typical spray foam – a common ingredient in high-performance building? Sure, we know that foams are petroleum based and the blowing agents in spray foam are harmful greenhouse gasses…but just how harmful? From our analysis, we found that replacing cellulose with spray foam of comparable insulative value not only flips the house into carbon-positive territory but brings the house’s net-carbon impact up to 116,500 kgCO2e! This is equivalent to driving a standard passenger vehicle for over 25 years (figure 2)![1] With newer, low global warming potential (GWP) blowing agents, our house would have a carbon impact of 36,200 kgCO2e. As we can see, the blowing agents account for a significant portion of the spray foam’s carbon impact, but even with the less harmful ones, it would be a challenge to achieve Net Zero carbon.

Fiber cement siding, a combination of Portland cement, fly ash and wood pulp, is praised for its ability to replicate traditional wood-siding patterns but with far greater durability. We found that by replacing the wood clapboard siding in our model with 5/16” fiber cement siding, we added 40,000 kgCO2e to our project! That’s 20 tons of carbon emitted (or avoided) just by switching one material. The net embodied carbon in this scenario is equivalent to operating an average American, non-Net-Zero home for four years (Figure 3).

Mineral wool is often heralded as a durable, rot- and fire-resistant, foam-free insulation alternative. But we also know that its production requires melting rocks and spinning them through what is essentially a giant cotton-candy machine – a particularly energy-intensive process. But just how energy-intensive? Well, if we were to use rigid mineral wool boards around our slab and foundation walls, instead of expanded polystyrene foam (EPS), that would actually increase the embodied carbon of the project by 2,500 kgCO2e, a significant increase for a considerably small volume of insulation.

We were also curious what the affect would be of increasing our drywall thickness from 1/2” to 5/8”. A gut sense tells us that an eighth of an inch of drywall throughout a 3,800 sf house would have only marginal effect on the total embodied carbon of the project. And that appears to be correct. The simple addition of that 1/8”, throughout the house amounts to adding a little over 800 kgCO2e to the project. While these emissions can be mitigated by specifying thinner, lighter-weight, and lower carbon types of drywall where appropriate, this change yields a relatively small impact compared to some of our other scenarios. New drywall options are now available that would push the emitted carbon even lower.

Lastly, we experimented with increasing the fly ash content in the building’s concrete foundation. Fly ash is a byproduct in coal-fired power plants and, when incorporated into a concrete mix, allows for the reduction of Portland cement – another particularly energy-intensive material.[2] By increasing the percentage of fly ash in our concrete mix, we were able to drop our net embodied carbon by an additional 5,000 kgCO2e bringing our total net impact to -11,800 kgCO2e.

There is a growing body of research into embodied carbon and the effect of the building industry on the climate. Building materials manufacturers are releasing declarations of their products’ carbon impacts over their lifespan, tools such as the Embodied Carbon in Construction Calculator (EC3) and GaBi compile this impact information on an ongoing basis, Kaleidoscope helps visualize carbon impacts of common building materials, BEAM, EPIC, and Architecture 2030’s CARE Tool provide stand-alone web-based analysis tools, and software such as Tally®, and One Click LCA help integrate this data into the design process. Notably, in the design and building community, research into the practicality and necessity of incorporating embodied carbon into the design process has been championed by Chris Magwood, Ace McArleton and Jacob Racusin. Their keynote address from the 2019 NESEA BuildingEnergy Boston conference provides invaluable information on how to think about embodied carbon in design.

The methods of accounting for embodied carbon are not without their caveats though. Variability in manufacturing and extraction methods, transportation, maintenance needs, and disposal or reuse make accurate carbon accounting difficult. Even the commutes of the tradespeople working on a job impact the calculation. As we gain knowledge of the nuances of our decisions, the choices can seem difficult.

Wood, for example, typically gets carbon credit, counting against carbon emissions. However, the sustainability of forest management and processing methods can vary greatly and with them, the net carbon impact of wood. By some accounts, using less wood has a greater global carbon impact, as the carbon sequestration and storage is carried out in the forest rather than in the building. But this must be balanced against the impact of carbon-intensive structural alternatives such as concrete and steel. When building with wood, using FSC-certified or, better yet, repurposed wood is a good step toward minimizing the embodied carbon impact of wood in buildings. Developments in hemp-based building products, including structural blocks, insulation, and cladding offer a rapidly-renewable alternative to wood and other, more carbon-intensive building materials.

Computer error plus human blindspots also factored into our learning. Notably, one of our spreadsheets had a clerical error as it translated data between two programs. The result was that the software thought we had dozens of layers of paint on all sides of the drywall which bumped up the overall drywall carbon. It was not until we showed this to a colleague who had a better gut sense of what the numbers should be that we found the error. It was a minor error, but revealed that any analysis requires proofreading by experienced people.

As we work to build more synergistically with the natural world, these tools offer us the opportunity to make more informed decisions about how to reduce our buildings’ overall climate impact. As architects, it’s in our nature and, imminently our responsibility, to work to navigate this changing landscape of information and design frameworks for the future of our buildings and the Earth as a whole.

As our culture shifts towards increasing climate mitigating and resilient design choices, we will continue to pursue this knowledge and commit to make low-carbon practices our standard practice for our clients and colleagues.


Modeling and Analysis by Nate Lumen

Preliminary Modeling by Ashley Ng

Contributions by Jesse Selman, AIA, CPHC


[1] US EPA, OAR. “Greenhouse Gases Equivalencies Calculator – Calculations and References.” Data and Tools. US EPA, August 10, 2015. https://www.epa.gov/energy/greenhouse-gases-equivalencies-calculator-calculations-and-references.

[2] Nath, Pradip, Prabir K. Sarker, and Wahidul K. Biswas. 2018. “Effect of Fly Ash on the Service Life, Carbon Footprint and Embodied Energy of High Strength Concrete in the Marine Environment.” Energy & Buildings 158 (January): 1694–1702. doi:10.1016/j.enbuild.2017.12.011.

Note: Because of a calculation error, an earlier version of this article misrepresented the carbon impact of changes to drywall thickness. The correct embodied carbon impact of changing 1/2″ drywall to 5/8″ drywall in this project is an additional 821 kgCO2e, not 4,082 kgCO2e. 

Further Reading:

Practical Embodied Carbon Advice from BuildingGreen

Carbon Accounting Report by the International Institute for Sustainable Development (IISD)

Chris Magwood’s Work on Embodied Carbon

Book: The New Carbon Architecture: Building to Cool the Climate by Bruce King ISBN: 9780865718685


Resources & Tools:

Project Drawdown

Carbon Leadership Forum

“Carbon Drawdown Now!” – 2019 NESEA BuildingEnergy Boston Keynote Session (Video)



Architecture 2030’s CARE Tool