Embodied carbon reduction has become one of the defining challenges of modern building construction. As the building sector accounts for nearly 40 percent of global energy-related carbon emissions, architects and engineers are increasingly turning to bio-based structural materials to lower the environmental footprint of new construction. A landmark example of this approach is San Mateo County’s County Office Building 3 (COB3), designed by Skidmore, Owings and Merrill (SOM) and currently under construction in Silicon Valley. This hybrid mass timber headquarters targets an 85 percent reduction in embodied carbon compared to conventional steel and concrete construction, setting a new benchmark for sustainable civic architecture. For building professionals seeking to understand embodied carbon measurement strategies and their application in real projects, COB3 offers valuable lessons in material specification, design optimization, and integrated passive performance.
Structural Design Strategy: Hybrid Mass Timber Systems for Maximum Carbon Reduction
The structural approach at COB3 exemplifies how engineering teams can optimize mass timber buildings for both performance and carbon efficiency. Rather than pursuing a full timber structure, SOM developed a hybrid system that strategically combines mass timber with steel and concrete where each material performs best.
Material Optimization Through Strategic Hybridization
The design team focused on three key material reduction strategies:
- Lowering the total volume of timber used in the project by optimizing member sizes and spans
- Reducing the piece count of cross-laminated timber (CLT) panels through larger panelization and simplified grid layouts
- Minimizing the use of structural steel connections and reinforcement through careful load path planning
The result is a structure that achieves an embodied carbon intensity of just 110 kgCO2e per square meter, compared to the 500 to 700 kgCO2e per square meter typical of conventional office building construction. This places COB3 well ahead of the typical 65 to 75 percent reduction range associated with standard mass timber buildings.
Mass Timber Performance Metrics
| Metric | COB3 Project Value | Conventional Building Benchmark |
|---|---|---|
| Building height | 21.9 m (72 ft) | Similar |
| Gross floor area | 19,231 m2 (207,000 sf) | Similar |
| Embodied carbon intensity | 110 kgCO2e/m2 | 500-700 kgCO2e/m2 |
| Embodied carbon reduction | 85% | Baseline |
| Typical mass timber reduction range | Exceeds by 10-20% | 65-75% |
This performance was achieved without sacrificing programmatic requirements or spatial quality. The hybrid approach allowed the project to maintain the architectural vision while pushing the carbon reduction targets beyond what a purely timber structure could deliver. Building professionals studying mass timber building codes and their application in civic projects will find COB3’s approach instructive for future low-carbon public buildings.
Low-Carbon Material Specifications and Alternative Systems
Beyond the primary structural system, the project team pursued embodied carbon reductions across every material category. This comprehensive approach reflects a growing understanding that structural carbon represents only a portion of a building’s total upfront emissions.
Eliminating Carbon-Intensive Finishes
One of the most notable decisions was replacing conventionally hung acoustical ceiling systems with alternative acoustical solutions. Standard suspended acoustical tile ceilings contribute significantly to a building’s embodied carbon through mineral fiber manufacturing, metal grid systems, and the associated distribution infrastructure. By specifying alternative strategies such as:
- Exposed mass timber deck finishes with acoustic treatments applied directly to the structure
- Acoustically absorptive wall panels and baffles positioned strategically in open-plan areas
- Integrated acoustic cloud elements that serve both sound management and spatial definition
the project eliminated thousands of kilograms of embodied carbon while maintaining the acoustic performance required for office environments.
Low-Carbon Concrete and Steel Optimization
For the elements that remain in steel and concrete, the project specified low-carbon alternatives. Concrete mix designs with reduced cement content and supplementary cementitious materials lowered the carbon footprint of foundations and cores, while structural steel elements were optimized for minimum weight through advanced analysis. The integration of zero-carbon mass timber construction principles from projects like the Catalyst Building in Spokane informed material selection and specification strategies throughout the design process.
Passive Design Strategies for Operational Energy Performance
COB3 targets LEED Platinum certification, and a significant portion of the energy strategy relies on passive design measures that reduce operational carbon alongside the embodied carbon savings from the structural system. These strategies demonstrate that low-carbon construction and high-performance operations are complementary, not competing, goals.
Facade Design for Solar Control
The building envelope was designed specifically for the Silicon Valley climate, with differentiated strategies for each orientation:
- North and south facades: Glass enclosures are recessed and shaded by structural overhangs, reducing direct solar gain while maintaining daylight access
- East and west facades: A series of vertical fins protect windows from low-angle morning and afternoon sun, which is the most difficult solar exposure to control
These measures minimize heat gain during cooling seasons, reduce the need for artificial cooling energy, and optimize natural daylighting throughout occupied spaces. The projected energy savings from passive design alone amount to 609,876 kWh annually, equivalent to nearly 2 million Btu.
Energy Performance Summary
| Passive Strategy | Application | Benefit |
|---|---|---|
| Recessed glazing | North and south facades | Reduced solar gain with daylight retention |
| Vertical shading fins | East and west facades | Low-angle sun control |
| Bioretention planters | Site-wide stormwater management | 100% onsite runoff treatment |
| Exposed mass timber structure | Interior finish strategy | Reduced material layering and thermal mass |
The integration of these passive strategies with the mass timber structure creates a building that performs efficiently from both an operational and embodied carbon perspective. This holistic approach mirrors trends seen in leading timber office building design projects across North America and Europe.
Water Conservation and Site Ecology
Rather than diverting stormwater runoff to municipal treatment facilities, the project incorporates onsite bioretention planters that absorb and treat 100 percent of the site’s runoff. This approach amounts to annual water savings of 523,395 gallons, reducing both the operational water demand and the embodied carbon associated with municipal water infrastructure. The bioretention system also supports site ecology by filtering pollutants and providing habitat for local plant species.
Prefabrication and Construction Process Efficiency
The embodied carbon benefits of mass timber extend beyond the material itself to the construction process. Prefabricated timber components significantly reduce construction site emissions compared to conventional building methods, contributing to the project’s overall carbon performance.
Construction Carbon Reductions Through Prefabrication
Factory-fabricated CLT panels and glulam beams arrive on site ready for assembly, eliminating several carbon-intensive activities associated with traditional construction:
- Reduced truck deliveries: Fewer material trips compared to cast-in-place concrete or stick-framed construction
- Eliminated formwork and shoring: Prefabricated components are self-supporting during erection, removing the need for temporary structures
- Shorter construction schedule: Faster enclosure allows earlier start of interior finishes and reduces total construction duration
- Lower equipment emissions: Lighter components require smaller cranes and less heavy equipment operation on site
- Minimized construction waste: Factory cutting produces precise components with minimal offcuts, and waste timber is often recyclable into engineered wood products
Quality Assurance and Precision
The prefabrication approach also delivers quality benefits that contribute to long-term building performance. CNC-machined connections ensure tight tolerances and accurate load transfer between timber elements. The precision of factory fabrication reduces the likelihood of thermal bridges and air leakage at structural connections, improving the building envelope’s thermal performance. For construction teams familiar with conventional methods, the transition to prefabricated mass timber requires adjustments in sequencing and coordination, but the carbon and schedule benefits make it an increasingly attractive option for public building projects.
Lessons for Building Professionals
San Mateo County’s COB3 project demonstrates that ambitious embodied carbon targets are achievable with current technologies and design approaches. The key takeaways for building professionals include:
- Hybrid structural systems can outperform fully timber or fully steel structures on carbon metrics when each material is deployed where it adds the most value
- Low-carbon material specification requires examining every building component, from primary structure to finishes and acoustical treatments
- Passive design strategies and material carbon reduction are synergistic, not separate, objectives in LEED Platinum and net-zero carbon projects
- Prefabricated timber components reduce both embodied carbon and construction-phase emissions while improving quality and schedule performance
The 85 percent embodied carbon reduction achieved at COB3, measured at 110 kgCO2e per square meter, sets a new performance standard that future civic and commercial buildings can realistically target. As more jurisdictions adopt embodied carbon limits in building codes, the strategies demonstrated in this project will become essential knowledge for architects, engineers, and construction professionals alike.