Facade Embodied Carbon Reduction Strategies

1. Introduction and Background

Facades are increasingly being recognized as a major contributor to whole-building embodied carbon. In life cycle assessments that consider the superstructure, substructure, enclosure, and interiors, the enclosure can constitute 15-30% of the embodied carbon of the building. The substructure and interiors tend to be in the same range, which means that only the superstructure is a significantly larger contributor than the enclosure.

Designers have a good grasp of how to reduce the embodied carbon of structures. Because the structure is such an important driver of embodied carbon, the building industry has formulated a range of strategies to reduce the embodied carbon of the superstructure and substructure of the building. New innovations continue to reduce the global warming potential of concrete, steel, and mass timber.

On the other hand, the embodied carbon reduction potential of facades is currently largely unrealized. Embodied carbon has not typically been a metric that is discussed and weighed along with other performance metrics. When facade embodied carbon is discussed, it is often at the level of system-level comparisons. We ask questions such as how a standard UHPC rainscreen assembly compares with an equivalent standard GFRC rainscreen assembly. System-level comparisons are a good start, but we need to move beyond that, just like we did with structures. Instead of asking how a standard concrete building compare with an equivalent standard mass timber building, we now ask how and how much we can reduce the embodied carbon of a standard concrete building and a standard mass timber building, and how the optimized structures compare. Similarly, we should ask how we can reduce the embodied carbon of facades, how big the reductions are for different assemblies, and how the optimized assemblies compare.

There are currently several ways that the embodied carbon of facades can be assessed. Different tools and data sources can be used in different stages of the design. In early stages, the available information is limited, but the potential to impact the building embodied carbon is high, since options are still open. As the design progresses, the available information is more detailed and accurate, and as a result the outputs are richer.

Early in the design process, early-stage tools such as Kaleidoscope by Payette[1] can be used for a system-level comparison. The only required input is the set of assemblies that the user wants to compare. The output is a system-level comparison of the total global warming potential (GWP), which can be broken down by life cycle stage (A1-A3, A4, B2-B5, C2-C4, D) or by material (exterior finish, support system, insulation, other). Other impact indicators can also be displayed. Each of the impacts can be over different lifespans: Module A only, 60 years including Module D, and 60 years excluding Module D. Biogenic carbon can be included or excluded.

As more information becomes available, the team can use spreadsheet calculations with EPDs. The required inputs are the set of assemblies that the user wants to compare, the material quantities, and the specific products that are being considered. The outputs can include system-level comparisons, but also product-level optimizations and the whole-façade impact.

Toward the end of design, whole-building life cycle assessment tools can be used. In addition to the required inputs stated above, these tools require an understanding of the life-cycle assumptions such as transportation mode and distance and end-of-life scenario. In addition to the outputs of façade-level spreadsheet calculations the tools reward the team with the whole-building impact.

[1] https://www.payette.com/kaleidoscope/

2. Research Questions and Methods

This paper addresses some of the top questions on façade embodied carbon that are investigated during design:

1) How can we reduce the embodied carbon of opaque and glazing assemblies?

2) How do different opaque assemblies and glazing assemblies compare in terms of embodied carbon?

3) How does the window-to-wall ratio impact facade embodied carbon?

4) How do we effectively implement embodied carbon reduction strategies?

This paper draws on project experience, existing literature, and a review of environmental products declarations (EPDs) to answer each of these questions. It aims to model a thorough façade embodied carbon assessment from early stage to through specifications as a reference for future projects.

3. Results and Discussion

This section addresses how we can reduce the embodied carbon of opaque and glazing assemblies. The embodied carbon of opaque assemblies is mostly driven by the cladding material and the steel or aluminum support system. The embodied carbon of glazing assemblies is driven by the embodied carbon of the insulated glass unit (IGU) and the (often aluminum) window frame. Because cladding materials, IGUs, and aluminum are the largest contributors to embodied carbon, they offer opportunities for carbon reductions. Embodied carbon reduction strategies for steel are generally well-known because of the prevalence of steel in structural systems, so they will not be discussed in this paper.

3.1 Cladding Materials

The Kaleidoscope tool by Payette allows users to compare the embodied carbon of various envelope assemblies, broken down by material. Figure 1 shows the embodied carbon of common rainscreen assemblies, using data from the Kaleidoscope in combination with data from EPDs for zinc, ACM, Ductal, FibreC, TAKTL, and Terracotta. The graph illustrates the importance of considering the embodied carbon of the whole assembly. The embodied carbon of the cladding materials (shown in green) ranks significantly differently from the embodied carbon of the assemblies. Breaking down the embodied carbon by material helps us identify reduction opportunities. For rainscreen assemblies with metallic cladding (steel, zinc, ACM), the main reduction potential lies in the cladding product itself. For assemblies with cementitious cladding (fiber cement, Ductal, FibreC, TAKTL, GFRC), the support system is the main driver, and low-carbon steel and aluminum are more impactful. The next step in the study of façade embodied carbon is to use system-level comparisons (as provided by the Kaleidoscope) as a basis for product-level optimizations. Rather than comparing the embodied carbon of one baseline assembly to another, we must assess the reduction potential of both assemblies and compare the optimized assemblies.

Figure 1: Embodied carbon (A1-A3) of different rainscreen assemblies using data from Kaleidoscope combined with data from cladding EPDs.

3.2 IGUs

In a study by Arup and Saint-Gobain Glass,^[1] the proportion of embodied carbon of the facade associated with glass ranges from 26 to 60% depending on the system. Figure 2 explores the relationship between embodied carbon and glazing configuration. Saint-Gobain has documented the embodied carbon of 15 different configurations, with varying glass types (clear glass, low-iron glass, and safety glass, shown in different shades of blue), glass thicknesses, laminations, and coatings. This graph illustrates that the embodied carbon penalty is the strongest for overall glass thickness, followed by the use of thermally toughened safety glass. The use of low-E coatings does not have a major impact on the embodied carbon. This is in line with findings by Arup and Saint-Gobain Glass, which showed that glass coatings have an good ‘return on investment’ due to their negligible impact on the embodied carbon (approximately 1 kgCO₂e/m²), and relatively high positive impact on operational carbon and occupant comfort.

Figure 2: Embodied carbon (A1-A3) of different insulated glass unit configurations by Saint-Gobain.

3.3 Curtainwall

IGUs cannot be considered in isolation, since other components such as the aluminum extrusions, backpan, and insulation contribute significantly to the overall embodied carbon of the glazing assembly. Figure 3 shows the embodied carbon of different curtainwall products based on EPDs, and of a project where the embodied carbon was calculated manually based on material quantities and EPDs. Manual calculations better represent the project-specific design than standard EPDs. However, detailed material quantities for curtain wall are usually not available until later phases when the system is already selected. Moreover, manual calculations for curtainwall embodied carbon tend to omit certain materials and components (such as gaskets) and simplify the volumetric calculations for framing, typically leading to underestimates. The graph illustrates that even for a project with a typical curtainwall design, using a standard EPD instead of a custom calculation could have resulted in an embodied carbon value of 14% less or 47% more.

A lot of projects use custom curtainwall instead of standardized curtainwall. In that case, it is recommended to work closely with the curtainwall contractor to assess the embodied carbon of the curtainwall system used on the project. Some curtainwall contractors can also assist the team in reducing the embodied carbon by proposing different system designs and identifying procurement-based optimization opportunities.

Figure 3: Embodied carbon (A1-A3) of different curtainwall products based on EPDs, and of a project where the embodied carbon was calculated manually.

3.4 Aluminum

Aluminum is often used in large quantities in building envelopes, including in support systems for cladding materials, in window frames, and as a cladding material as part of a metal composite material (MCM). Aluminum is known to have a high embodied carbon and a high potential for embodied carbon reductions. Increased recycled content is typically seen as the main way to reduce embodied carbon, but a paper presented at the Facade Tectonics 2022 World Congress^[2] showed that the sourcing location of the primary ingot can be more impactful than recycled content: “[…] a 75% recycled aluminum with 25% primary ingot from India actually has a 50% higher carbon footprint compared to 25% recycled content with 75% primary ingot from a hydro source in Canada.” This section examines the embodied carbon of aluminum and its relationship to the sourcing location, recycled content, and finish, in order to make more informed recommendations on which aluminum specifications have the largest impact on embodied carbon. It must be noted that the comparison of products in this section is somewhat limited since the European and North American EPDs are generated using different product category rules (PCRs), methodologies, and databases. However, in the absence of international standardization, an imperfect analysis with this caveat in mind is deemed better than no analysis at all.

Figure 4 shows the embodied carbon (A1-A3) per kilogram of aluminum for all examined products, ranked from lowest to highest embodied carbon. Aluminum embodied carbon ranges widely, from 1.1 kgCO₂e/kg for Norwegian aluminum by Hydro with a mill finish and 95% recycled content, to 18.6 kgCO₂e/kg for Cyprus aluminum with an anodized finish and 0% recycled content.

Figure 4: Embodied carbon (A1-A3) per kilogram of aluminum for all examined products.

The previous figure shows some initial trends. Embodied carbon increases with (listed from most to least determining):

1. Sourcing location: Norway performs best, followed by Sweden, Spain and Portugal (which are almost identical), North America, Greece, Turkey, and Cyprus
2. Finish: mill finished has the lowest impact, followed by coated, anodized, and painted
3. Recycled content: generally, higher recycled content means lower embodied carbon

Figure 5 organizes the products first by sourcing location, then by finish, and finally by recycled content. The Kawneer products were excluded from this graph because their low numbers for anodized and painted aluminum and missing numbers for mill finished aluminum skewed the results. The graph solidifies the ranking identified above. For example, regardless of the finish or recycled content, aluminum has a lower embodied carbon when it comes from Norway, then Sweden, then Spain and Portugal, then North America and Greece, then Turkey, and finally Cyprus.

We cannot fully isolate the impact of each of these characteristics, as the North American products inherently have a much lower recycled content than the Norwegian products. However, understanding which characteristics of aluminum are the strongest determinants of embodied carbon prevents us from writing prescriptive specifications that do not necessarily result in a lower embodied carbon. Instead, performance-based specifications are a more effective way to reduce embodied carbon. For example, one might specify aluminum with a maximum embodied carbon (A1-A3) of 7 kgCO₂e/kg.

The magnitude in difference between the European products and North American products also suggests that local products are not necessarily better from an embodied carbon perspective. If a US project procures Hydro aluminum from Norway and transports it to site by ship and truck, the A1-A4 embodied carbon will likely still be lower than if the project procured North American aluminum (with a similar embodied carbon as the North American industry average) and trucked it to site. The A1-A4 embodied carbon is unique to each project and needs to be calculated to assess the tradeoffs.

Figure 5: Embodied carbon (A1-A3) per kilogram of aluminum, organized by sourcing location, finish, and recycled content.

[1] https://www.arup.com/perspectives/publications/research/section/carbon-footprint-of-facades-significance-of-glass

[2] https://www.facadetectonics.org/papers/embodied-carbon#purchase

4. Conclusions

Based on the results from the previous section, this section addresses how different opaque assemblies and glazing assemblies compare in terms of embodied carbon, how the window-to-wall ratio impacts facade embodied carbon, and how we can effectively implement embodied carbon reduction strategies.

4.1 How do different opaque assemblies and glazing assemblies compare in terms of embodied carbon?

When comparing façade assemblies in terms of embodied carbon, it is important to consider all the main façade components. For rainscreen assemblies, the ranking from lowest to highest embodied carbon changes significantly when the support system is considered in addition to the cladding materials. Generally, opaque assemblies with wood, metallic (steel or zinc) or stone veneer (limestone, thin brick, and granite) cladding perform well. ACM performs worse and is in the same range as fiber cement, phenolic resin, Ductal, and FibreC. TAKTL performs yet a bit worse, and terracotta and GFRC are at the top of the embodied carbon range. Terracotta assemblies tend to have high embodied carbon because of the high temperatures required to manufacture terracotta. GFRC panels have a low embodied carbon in comparison to other cladding materials, but they are typically supported by large hollow steel profiles which are carbon intensive. The embodied carbon of rainscreen assemblies should not be considered to be deterministic but as a baseline from which reduction opportunities can be explored.

For glazing assemblies, the embodied carbon penalty is the strongest for overall glass thickness, followed by the use of thermally toughened safety glass. The use of low-E coatings does not have a major impact on the embodied carbon. Curtainwall assemblies vary significantly depending on the manufacturer and can vary even more if the project employs a non-standard system. In general, standard curtainwall tends to have an embodied carbon higher than rainscreen assemblies on a per area basis.

The embodied carbon of façade assemblies is project specific and depends on a range of parameters including module size and structural design loads. This paper hopes to encourage project teams to measure and manage façade embodied carbon to help the building industry develop a better understanding of the opportunities and challenges.

4.2 How does the window-to-wall ratio impact facade embodied carbon?

The comparison of opaque and glazing assemblies addresses how the window-to-wall ratio impacts facade embodied carbon: if the selected glazing assembly has a higher embodied carbon than the selected opaque assembly, a higher window-to-wall ratio increases the overall façade embodied carbon. If the selected glazing assembly has a lower embodied carbon than the selected opaque assembly, a higher window-to-wall ratio decreases the overall façade embodied carbon.

4.3 How do we effectively implement embodied carbon reduction strategies?

To reduce the embodied carbon of facades, design professionals need to measure and manage embodied carbon from concept through construction. During the project kickoff, the team should target a whole-building embodied carbon intensity or reduction based on project ambitions and local opportunities. During early design, the team should strategize how to reduce façade embodied carbon to achieve the targets and calculate the anticipated reductions. Throughout design, calculations will transition from early-stage tools with limited inputs and high-level outputs to more sophisticated analyses that require detailed inputs and generate richer outputs. Toward the end of design, the team should specify and procure materials to implement the identified strategies. Performance-based specifications are more effective than prescriptive specifications because they are less restrictive and acknowledge that there are different pathways to achieve embodied carbon reductions. During construction administration, it is essential to follow through and ensure that the specified materials are not substituted with higher carbon alternatives and that schedule changes do not result in decisions that significantly increase the embodied carbon, such as flying in materials from overseas.

Reducing the embodied carbon of facade materials can cost more and impact aesthetics more than reducing the embodied carbon of concrete or steel. In addition, the lack of product-specific environmental product declarations for products like aluminum extrusions makes it difficult to demonstrate reductions and take credit for them as part of green building certifications. Nonetheless, because facades are a major part of whole-building embodied carbon, we need to find ways to overcome these implementation hurdles.

Arup and Saint Gobain. n.d. “Carbon footprint of façades: significance of glass.” Accessed December 4, 2023. https://www.arup.com/perspectives/publications/research/section/carbon-footprint-of-facades-significance-of-glass

Bougher, Tom and Braunstein, Richard. 2022. “The Carbon Footprint of Aluminum Fenestration.” Accessed December 4, 2023. https://www.facadetectonics.org/papers/embodied-carbon#purchase

Payette. n.d. “Kaleidoscope: Embodied Carbon Design Tool.” Accessed December 4, 2023. https://www.payette.com/kaleidoscope/