The Carbon Sweet Spot

Report Summary: With increasing emphasis on decarbonization, building owners and developers face myriad choices as they try to balance cost and value and meet social and corporate-level carbon emissions reduction goals. Emissions in the building sector primarily fall into categories of “operational” and “embodied” carbon emissions; combined, they account for nearly 40 percent of global carbon dioxide released into the atmosphere.

This report presents three hypothetical analyses of buildings in different geographic and regulatory contexts: London, New York, and Singapore. These examples illustrate how real estate decision-makers can navigate the tradeoffs and opportunities that arise when pursuing reductions in both embodied and operational carbon emissions (known together as life-cycle emissions).

The tradeoffs are particularly pronounced in decisions related to the building envelope. Building enclosure components are long-lasting—with a service life typically exceeding 25 years—and among the costliest of all building systems. They also have an outsized impact on operational carbon emissions and energy consumption over the life of a building, both regulating heating and cooling loads and enabling the advanced mechanical systems required for efficiency and electrification.

Based on the results of the three project analyses and discussions with leading developers and industry experts, this report highlights the critical design decisions that impact the building facade and offers frameworks for considering total carbon emissions over the life of a building investment. Ultimately, it suggests a process by which decision-makers can identify the whole life-cycle carbon “sweet spots” for their buildings.

Three Buildings, Three Contexts

The three buildings examined for this report offer a glimpse into a variety of architectural design decisions, climate contexts, grid carbon intensities, and use types.

• One Crown Place in Hackney, London, was chosen to explore the effects of glass area and wall insulation on a residential building in a mild climate where electricity for heating and cooling is already low in carbon emissions.
• The One Vanderbilt office tower in New York City provided an opportunity to examine the effect of aluminum frame area and double or triple glazing in a climate with moderately cold winters and hot summers with a high-carbon-intensity electrical grid that is slated to rapidly decarbonize.
• The 18 Robinson building is mixed-use high-rise, situated in the hot equatorial climate of Singapore. It served as the basis for investigating the life-cycle carbon impact of fixed shading elements in a cooling climate where building materials are typically sourced from significant distances.

Example of hypothetical analysis done on the One Crown Place project in London.

Example Tradeoffs

Examples of some of these tradeoffs include the following:

• Thermal Performance versus Embodied Carbon: Increasing energy efficiency in operational aspects (heating, cooling, lighting) sometimes requires using more material, or material with higher embodied carbon content. For instance, increasing wall or roof insulation thickness may have a higher upfront embodied carbon footprint, but may yield lower net carbon emissions over the life cycle of the building due to energy efficiency benefits. Compliance with local codes or building performance standards, which may set requirements for energy efficiency or operational carbon emissions, also factors into this decision.
• Technological Advancements: Investments in advanced, energy-efficient technologies such as triple- or quad-pane window glazing may reduce operational carbon. But the additional glass layers require much more material, resulting in a higher embodied carbon footprint. Newer or unique technologies might also involve manufacturing processes with higher embodied carbon or be unavailable locally, increasing transportation-related carbon costs.
• Low-Carbon Material Choices: When designing a high-performance building envelope, opting for materials with lower product stage emissions (A1 to A3) or with a longer service life will reduce the tradeoff between embodied carbon investment and operational carbon savings. Sometimes these material choices can increase upfront costs or involve changes to a traditional design/build process; however, cost-comparable options are frequently available, and architects and general contractors are often able to incorporate the materials with proper planning.
• Renovation and Retrofitting: Reusing existing structures can significantly reduce embodied carbon by avoiding the embodied carbon associated with the building’s structural components such as concrete and steel. However, renovating older buildings may require investments in new, more efficient heating, ventilation, and air conditioning (HVAC) systems and structural upgrades to extend the assets’ service life and to meet code and operational energy requirements.
• Window-to-Wall Ratios: WWR is widely known to have a large impact on operational emissions, due to the lower thermal performance of glazing assemblies compared with wall assemblies. Thus, in most cases WWRs lower than 40 percent are recommended. However, depending on the materials and quantities in both assemblies, the embodied emissions of the wall could be significantly higher than those of the windows, as is the case in most curtain wall systems. In such cases, a lower WWR does not necessarily correspond to lower total life-cycle emissions.

A holistic understanding of these tradeoffs requires a comprehensive analysis of the specific context, project goals, and local conditions, and is a prerequisite for making informed decisions that align with project objectives. Striking a balance between embodied and operational carbon emissions is a crucial aspect of realizing the most building value with the least environmental impact.

Key Takeaways

The analysis determined that as developers consider carbon impacts for their buildings, strategies often highlighted for carbon reduction (such as triple-pane windows or external shades) must be carefully designed to optimize total life-cycle carbon. In particular, the following lessons gleaned from the specific buildings analyzed offer examples of ways to find the carbon “sweet spot”:

• Carbon analyses should consider both operational and embodied carbon for maximum impact. Understanding operational and embodied carbon tradeoffs can improve building performance and deliver the best value-to-cost ratio.
• The amount of glazing on a building's facade significantly influences both embodied and operational carbon emissions. Strategically planning the placement of glazing areas to minimize their extent and optimize design for low carbon emissions is crucial.
• Triple glazing should be carefully assessed. It may contribute to more embodied emissions than it saves in operational emissions.
• Increasing wall insulation tends to make only a modest difference on total carbon emissions when starting with standard code minimums.
• Smaller curtain wall module widths can increase total carbon emissions. Larger modules can reduce these impacts.
• Shading devices may increase total carbon emissions but significantly reduce peak loads. When using exterior shading, it should be strategically designed to optimize reduction in operational emissions while using less material.
• Understanding the impact of local fuel sources and decarbonization policy is key to navigating carbon tradeoffs. Local trajectories for grid cleanliness and building performance rules can significantly influence operational carbon emissions over a building’s service life and affect how much a developer should invest in embodied carbon to achieve operational savings.
• Reducing the carbon impact of materials by building for longevity, choosing recyclable materials, and considering building reuse can make the most of a material’s operational carbon savings. A building that is flexible to future uses and built to last will maximize its investment in embodied carbon by increasing life-cycle value.

Read the full report for more detail!