Designing with the Carbon Budget: Stone's Throw and Flow
In this blog series, we'll deep dive into specific projects, outlining how we're tracking both the predicted operational and embodied carbon emissions as the project progresses, which in turn, will result in more sensitive and resilient designs that emit less carbon.
In our previous Designing with the Carbon Budget post, we explored Fog’s Edge, one of the first projects designed using our Carbon Budget initiative. That case study helped establish the foundation for how we track and reduce both operational and embodied carbon across every phase of design. As we continue to refine our process, subsequent projects have provided valuable insights —allowing us to test new strategies, compare material systems, and better understand the carbon implications of design decisions.
This post highlights how two ongoing projects, Stone’s Throw and Flow, integrate lessons from our early efforts and shape our evolving approach to low-carbon design. Our Carbon Dashboard measures each project’s predicted emissions—operational and embodied—throughout each design phase. Using tools like Tally, we can test strategies, make informed design decisions, and track progress toward our carbon reduction goals of 100 metric tons (mT) of carbon per home and 20 kg CO₂e/sf.
Key Takeaways
Across all projects, three strategies tend to stand out as the most impactful for reducing embodied carbon:
Reduce Building Floor Area Less building area means less material, which directly translates to lower embodied carbon. While the scope of this reduction depends heavily on client needs and program requirements, every square foot saved has measurable carbon reduction benefits.
Use More Wood, and Less Concrete & Steel Mass timber and mass plywood systems not only store carbon but also replace high-impact materials like concrete and steel. In Stone’s Throw, switching to a mass plywood framing system drastically reduced structural steel requirements. In any project, it’s important to engage with the structural engineer early and spend time meaningfully designing an efficient foundation system to limit concrete use.
Use Recycled or Salvaged Materials Reusing materials avoids the emissions associated with manufacturing new ones. Though feasibility varies, the team continues to test and evaluate reclaimed material options on multiple projects.
Understanding the Carbon Life Cycle
Our Carbon Budget process primarily addresses the emissions from material extraction, manufacturing, and transportation. However, it’s important to recognize that carbon impacts extend through each building's lifecycle - and even afterwards (such as reuse or recycling potential). Tracking and understanding these stages help us prioritize design decisions with the greatest long-term impact.
Using Tally as a Design Tool
Tally remains a critical part of our process, allowing project teams to quantify embodied carbon, test design options, and identify high-impact materials. It’s particularly valuable for comparing strategies, such as material substitutions or structural system options.
Our team uses Tally for comparative testing across multiple projects to better understand how specific material choices impact embodied carbon. The tool’s ability to pinpoint major impact areas allows our team to focus reduction efforts where they matter most. For instance, on a recently completed project, we found that fly ash mix yielded meaningfully better carbon reduction results than slag (slag generally is higher in embodied carbon than fly ash). Similarly, on Fog’s Edge, we tested blown-in cellulose (made from recycled paper products and treated with fire retardants) versus Knauf Ecobatt fiberglass batt insulation (made w/recycled glass & formaldehyde-free) and found that Ecobatt carries roughly one-third of the embodied carbon of cellulose. These kinds of test cases demonstrate how Tally supports our team in making data-driven material decisions beyond the three primary strategies outlined in our key takeaways.
Stone’s Throw
Stone’s Throw is a new, one-story with partial basement, 5,408sf single-family residence with a 926sf detached garage. This project demonstrates the carbon-saving potential of mass timber structures and detailed early-phase analysis.
Early Estimates: Initial embodied carbon was ~196 mT.
Design Development: By the end of design development, the project was tracking at 185 mT, and the latest Tally report measured 192 mT, which is a close correlation to the original estimate.
Material Shifts: Stone’s Throw uses mass plywood panels (MPP), an engineered wood product, as its structural roof framing—a system that significantly reduces the need for structural steel. Combined with wood’s natural carbon-sequestration properties, this approach has been shown to yield substantially lower associated embodied carbon emissions in buildings of similar type and scale. When comparing its Tally report with that of our recently completed project, Stone Villa, we identified the following improvements:
Reduction in overall steel impact from 19% to 6%
Reduction in concrete impact from 46% to 36%
Reduction in overall embodied carbon from 44.1kg/sf to 27.9kg/sf
Flow
Flow is a new, two-story, 5,729sf single-family residence with an 874sf attached garage. This project demonstrates the carbon-saving potential of vetting exterior assembly specifications and their carbon footprint at each phase of a project.
Early Estimates: 196 mT
Design Development: 161 mT
This represents an 18% reduction from the baseline embodied carbon assumptions. These reductions were possible by minimizing concrete foundations and steel structure and utilizing a 70% SCM concrete mix.
We did see opportunities for further reduction by removing exterior type X gypsum sheathing.
Embodied Carbon Metrics (current): 175 mT
The increase in total embodied carbon is attributed mainly to the addition of a concrete topping slab, despite reductions elsewhere.
Flow's Hypothetical Carbon Dashboard: 148 mT
Our team created a hypothetical dashboard to demonstrate what would need to change with Flow to reach our goal. Assuming the home is all-electric and operating net zero, the challenge lies in reducing the 175 mT of embodied carbon to 148 mT.
Our recommendations for meeting the goal:
Reconsider the proposed concrete topping slab for wood flooring throughout
Reconsider the exterior siding material, switching from cement plaster to something reclaimed or with lower embodied carbon like reclaimed metal panels or wood siding.
Through projects like Stone’s Throw and Flow, our Carbon Budget initiative continues to evolve from a tracking tool into a design driver. Each project deepens our understanding of how size, structure, and material choices influence a building’s carbon footprint - and how small design decisions can lead to meaningful emissions reductions Concurrently, we are improving our understanding of the modeling process to refine the data input & material selections to accurately reflect the design intent and subsequently the carbon analysis that it generates. While it’s not an exact science, applying these tools consistently across all our projects allows us to measure relative progress and learn from each iteration.
The two-pronged approach of growing our knowledge base to make sustainable design & material choices paired with accurate deployment of tools like Tally keeps us accountable and on track to work towards our ambitious carbon budget initiatives across all projects in the firm.
