When we think about embodied carbon, slabs, beams, and footings all come to mind, but what other elements are missing and how important are they? Look inside any building with partition walls, and you’ll notice the square footage of partition wall is considerable—often considerably more than the floor area in that space. All that material adds up to the embodied carbon, and selecting the right material can make a big difference.

Rethinking Non-Structural Elements

The selection of materials for partitions has many criteria, be it envelope requirements, insulation, constructability, and even detailing simplicity. It would be easy to think of these as an afterthought, not worthy of consideration for the full building performance on the embodied carbon stage—but can we really brush these aside?

Comparing Common Wall Assemblies

Take for instance an interior partition wall that’s architecturally exposed. It needs a 2-hour fire rating, and some form of acoustic performance. Three common options for this would be wood/steel stud and drywall, CMU walls, or perhaps even cast in place concrete. Each of those assemblies will have its inherent benefits and detriments, and many opinions whether it be from engineers, architects, contractors, or clients about which is the best approach. When we think about those three wall types, we might summarize their functionality as below:

• Wood or Steel stud + drywall is a very light assembly, installed by a combination of several trades. It can be insulated within the cavities, does not impose considerable loads on the base building structure, services can run within it. It does however require more detailing; joints need to be well considered to adjacent walls or other materials. And acoustically, it does not carry an inherent mass so may not perform as well as other wall types.
• CMU walls are a more “structural” wall assembly in a conventional sense in that they are reinforced with rebar, grouted and filled to various extents to suit their height and function. CMU walls are considerably heavier than a steel stud alternative and impose more load on the base structure. CMU walls are stiff, and in seismic conditions require more careful detailing to avoid detrimental interaction with the primary structure.
• Cast in place walls generally form part of the base building structure, rather than a secondary partition element. In some cases, they are used as architectural features or walls, using movement joints to separate from the structure in a sense forcing it to become a secondary element. In some markets, however, the use of cast in place walls in suite partitions is extremely common.

Embodied Carbon: How Do Materials Stack Up?

So how do these three compare when we isolate the embodied carbon impact? As you might expect, the heavier assemblies generally carry a higher embodied carbon intensity.  This comparison doesn’t capture the secondary and tertiary impacts of a heavier assembly on the base building or supporting structure where reducing weight has knock on benefits of essentially all structural elements, but the embodied carbon premium between a stud wall, and a concrete wall of the same thickness can be upwards of 70% higher.

The non-structural systems are not limited to interior partitions. The standard building enclosure systems also have a large variability in embodied carbon.

• Glazing systems (120 kgCO₂e/m²)
• Composite aluminum panel supported on steel stud (60 kgCO₂e/m²)
• Masonry veneer panel supported on steel stud (200kgCO₂e/m²)
• Masonry veneer panel supported on concrete (270 kgCO₂e/m²)

To provide context for these choices relative to the whole building embodied carbon, we can assume the partition and enclosure areas are both equivalent to 50% of the floor area, and the primary structure embodied carbon is roughly 250 kgCO₂e/m².

Best case scenario: (steel stud interior walls, 40% glazing, 60%ACM panel) the non-structural assemblies will contribute around 62 kgCO₂e/m² to the building total, equivalent to 25% of the primary structure.

Worst case scenario: (CIP interior walls, 30% glazing + 70% masonry veneer on concrete), the non-structural assemblies will contribute 145 kgCO₂e/m² to the building total, equivalent to 60% of the primary structure.

Why are High-Carbon Systems Still Used?

Why do we see cast in place concrete facades, partition walls, and concrete features on so many projects? Some of this is rooted in historical practice. Historically we see painted concrete facades without insulation surrounding all stairs to make a robust fire enclosure, or mass concrete walls in partitions for acoustic separations.

All these historical uses have and will continue to have merit; however, the higher embodied carbon assemblies are at odds with the ambitious targets to drop overall embodied carbon on buildings. We have seen some of these historical practices slowly phased out as knowledge and industry have changed. Exterior concrete façade walls for instance appear to be falling out of favor as thermal isolation and envelope requirements are gaining sophistication for example.

In the context of buildings, we’re cognizant that we’re not reporting the full story, but we can be proactive in advocating for the right materials for the items that are not fully tracked.

Moving Forward: A Proactive Material Strategy

The biggest hurdle ahead of us, likely isn’t the knowledge or technical choice of different material but more a choice rooted in “we’ve always done it this way” or that a brute force approach can often pencil out as a reasonably cost effective one. How the industry adapts to more ambitious and lofty goals of lowering project embodied carbon is yet to be seen, but a proactive approach will likely require a hard look at not just what is most simple or common, but what is appropriate.

Or to think about it another way, if there are wall assemblies that offer a considerable saving in embodied carbon relative to others that meet the same functional requirements, what excuse are we using to use the higher intensity options?

Together, we can contribute to a more sustainable built environment. If you are interested in sustainability and would like to discuss any of the topics in this article, please get in touch with us at [email protected].

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Written by Nick Maerkl