Operational Carbon versus Embodied Carbon

Evolution of the energy codes over the last three decades has offered a significant decrease in operational carbon emissions. Only recently has more information become available to help builders and designers make decisions regarding reducing embodied carbon. Unlike operational carbon emissions which can be reduced over time with building energy efficiency retrofits, future electrification of HVAC and the addition of renewable energy, embodied carbon emissions are locked in place once the building is built.

Including embodied carbon in the suite of policies, building codes and design choices is another tool for jurisdictions to address climate impact of the built environment.

 The American Institute of Architects has identified 10 steps that can be taken to reduce embodied carbon.

1. Reuse buildings instead of constructing new ones.
2. Use low carbon concrete mixes.
3. Limit carbon intensive materials.
4. Choose lower carbon alternatives.
5. Choose carbon sequestering materials.
6. Reuse materials.
7. Use high-recycled content materials.
8. Maximize structural efficiency.
9. Use fewer finish materials.
10. Minimize waste.

LEED, the International Green Construction Code, and Living Future are helping to build market awareness of these material choices and create a market for beneficial products. In addition,  jurisdictions can have a significant impact through incorporating embodied carbon policies into and alongside codes and standards.

“Building elements such as foundations, frames and other forms of superstructure often represent the biggest contribution to embodied carbon, not least because of the large volumes of material they use. But, additionally, these elements often contain carbon intensive load bearing structural materials such as steel, concrete and masonry. Facades may also contribute significantly if they utilize large amounts of aluminium and glass, both of which have carbon intensive production processes.”

• Concrete is the most widely used construction material throughout the world and it’s responsible for 6-11% of global CO2 emissions. Using less cement in the concrete mix, whilst maintaining its structural integrity, is the most effective way to reduce the carbon footprint of concrete. Design teams can also be incentivized to use less concrete in many structures.
• For steel, the embodied carbon impact can differ based on the manufacturing process used. Most of the current global steel production is manufactured using a basic oxygen furnace (BOFs) production method. Under this method 37% of materials are recycled content. Whereas as electric arc furnaces (EAFs) can use up to 100% recycled material without compromising metallurgical properties. Furthermore, since EAFs are powered by electricity there is potential for this manufacturing process to further reduce emissions when powered by renewable energy.
• Wood naturally sequesters carbon while growing and will continue to store carbon when used in construction. Using reclaimed wood and specifying use of wood from sustainably managed forests is the best way to reduce embodied carbon emissions from wood harvesting. Where sustainably harvested wood is used as a structural alternative to steel or other more emissions intensive products it reduces embodied carbon. This has led to growing interest in mass timber framed buildings and structural products such as cross-laminated timber, that have significant potential economic and emissions benefits over more traditional steel-framed commercial buildings.
• Insulation can contribute significantly to a building’s embodied carbon or alternatively help to sequester carbon depending on the materials used. When selecting insulation, thermal operational energy considerations should be balanced with an awareness of embodied carbon impacts and targets.

This is just a subset of building materials that have low-carbon alternatives. The current challenge is accurately quantifying the impact of reducing embodied carbon. However, this will only become easier as awareness grows throughout the industry and more industry practitioners decide to focus on the embodied carbon impact of their buildings.

Determining the Embodied Carbon of Building Materials and Products

Accurately quantifying the embodied carbon or CO₂ equivalent emissions associated with a building material’s “cradle to grave” footprint is challenging, but becoming increasingly more accessible and affordable. Three tools are used to determine the embodied carbon: the Product Category Rule (PCR), Life Cycle Assessment (LCA), and Environmental Product Declaration (EPD).  

Product Category Rule.  The PCR sets up the rules or criteria by which a specific building material or product type is evaluated regarding its environmental impacts.  These criteria may include things like the market location, or rules for averaging. The lack of standardized methodology in developing PCRs has resulted in incomparable environmental claims  and has reduced the overall credibility of the LCA-based product claims for decision making.

Life Cycle Assessment. Once a PCR has been developed an LCA is performed and a report of the results is produced.   An LCA is a science-based holistic methodology that seeks to answer the question of “how sustainable is a product, building, or process?” so that a project team may identify the “environmental hotspots” of a building with respect to greenhouse gas emissions and environmental impact, and make informed design decisions that minimize the environmental impact of the building.   An LCA seeks to provide transparency about the environmental impact of a building project over its lifetime from the extraction of materials for manufacturing, to transportation, construction, operations, and building’s end-of-life. The process of performing an LCA involves a lot of data gathering, computer modeling and documentation.   The software used to model the results pulls raw material data from one of a few existing databases.  It should be noted that there are differences in the databases and software that will produce different results for the same product.  LCAs are  detailed and often contain proprietary product formulation or manufacturing information; they are rarely released publicly.

Environment Product Declaration. The EPD is provided by a product manufacturer or industry.  A third-party will take the LCA and summarize the important information in the EPD.  Depending on how the LCA was conducted, an EPD may provide averaged industry wide information, averaged product family information, or specific product information. EPD development is standardized by ISO 14025, Environmental Labels and Declarations, Type III Environmental Declarations, Principles and Procedures. Before an EPD can be produced, a Product Category Rule (PCR) needs to be created. EPDs do not include this proprietary information and are typically available on public websites.

Comparisons of EPD data, like carbon dioxide (CO₂) emissions are only reliable when they have used the same PCR and LCA software and assumptions.  Changes in versions of software and PCRs can have a significant impact on the results. Evaluating both the embodied and operational carbon impacts of a building project should be viewed as a holistic process. There may be times when project goals conflict with one another and compromises will need to be made. For instance, there are a variety of other criteria that must also be considered during project development such as durability, fire safety, structural safety and moisture management.  It is imperative that policy makers and users understand the limitations of available data.

Advanced Codes with a Path Forward

Both Toronto and Vancouver have developed code structures that are strategically moving towards net zero. These are performance-based standards, meaning that they are based purely on energy modeling results rather than imposing specific requirements for equipment or envelope assemblies.

Whereas most North American performance-based energy codes are focused only on overall energy use, Toronto (and Vancouver) codes require compliance with three metrics: TEUI, TEDI and GHGI.

• TEUI – Total Energy Use Intensity – overall annual energy use, in kWh per unit of floor area
• TEDI – Thermal Energy Demand Intensity – annual space heating energy, in kWh per unit of floor area.
• GHGI – GreenHouse Gas Intensity – annual carbon emissions per unit of floor area.

Version 3 of the Toronto Green Standard (TGS) is Toronto’s 2019 step towards a 2030 zero-energy code. Developers of the most common building types can choose either the standard Tier 1 code or else a Tier 2 that’s a 20 – 30% improvement and provides a hefty partial refund of development fees. (15% of projects in the last code version chose the Tier 2 path.)  The current Tier 2 will then become Tier 1 for the next code cycle, and in fact Tiers 3 and 4 have also been defined to flesh out the remaining steps to Toronto’s 2030 zero energy standard

In addition, the TGS includes several prescriptive requirements which are important for ensuring long-term real-world energy performance but can’t be evaluated by using energy modeling. These include

• Solar readiness
• District energy connection readiness
• Air tightness testing
• Building commissioning
• Sub-metering
• Building labeling and disclosure

While the Toronto system is among the most comprehensive and well-vetted of energy code pathways, it is based entirely on energy modeling, which is notoriously over-optimistic when predicting the energy use of highly efficient buildings. Also, it only applies to a handful of specific building types: office, multifamily, retail and education, since high process load buildings (lab, hospital, restaurant, etc.) don’t lend themselves to hard energy use targets. On the other hand, such performance codes reward efficient design choices for building form, orientation and other aspects that would be difficult to pin down in a prescriptive code.