Turning our Attention to Embodied Carbon in both New and Existing Buildings

By Scott R Armstrong When I started in the green building industry, I recall a conversation with a developer that went something like this: (me) “What’s your construction waste diversion strategy?” (client) “BFI.” (citing a then-popular waste hauler).

Driven by the climate imperative, our industry has changed: energy efficiency, energy use intensity, thermal energy demand, carbon emissions are now top of mind. And it’s through this same imperative that we are now realizing how important construction materials are to the overall carbon impact of new buildings.

New buildings are perceived as being inherently more energy efficient and responsible for emitting less operational carbon than existing buildings. While this may be true in many cases, the past decade of operational efficiency gains has focused almost exclusively on the implementation of newer and more efficient mechanical solutions rather than enduring enclosure-first solutions or more integrated passive and active design strategies.

Programs like Passive House, Toronto Green Standard, and the Zero Carbon Building Standard are changing this mentality – with requirements like TEDI designed to obligate attention to enclosure and ventilation load. This focus, though broader, potentially still does not fully account for the implications of embodied carbon.

With a diminishing timeline for climate action, the 2020s must be the decade of action and assessing material choices using life-cycle emissions is vitally important. A report on global embodied carbon indicates that building materials account for 11% of carbon emissions in Canada 1 . Further, embodied carbon likely represents 50% of a code-compliant building’s total carbon emissions over a 30-year horizon 2 . Thus, selecting low embodied carbon materials today influences greatly a building’s emissions profile during this critical period.

On a recent project, a thermally-efficient and air tight building enclosure with optimal passive heating and daylighting helped enable simpler, more efficient mechanical and electrical systems. Since these systems are not typically included in embodied carbon accounting, further study could focus on whether ‘bonus’ embodied carbon reductions are obtainable by using such systems.

In some instances, a photovoltaic system could be sized such that it exports more energy than needed by the building, potentially achieving credit for reducing peak grid emissions.

A recent project demonstrated that a focus on low embodied carbon versions of the insulating and structural materials reduced the upfront/embodied carbon to the point where it was possible to show a net positive performance over a 30-year timeline. The practical combination of integrated energy system design, renewable generation, and material selection allowed for a new kind of low-carbon performance optimization.

Existing Buildings Turning our attention to embodied carbon means that we cannot ignore the emissions represented by existing buildings. Cumulative emissions from global concrete production “from 1928 to 2016 were 39.3 ± 2.4 GtCO 2 , 66 % of which have occurred since 1990” 3 . In other words, buildings constructed in the past 20 to 30 years represent a significant contribution to CO 2 emissions currently contributing to climate change.

Toronto’s Tower Renewal project involves approximately 1,200 apartment buildings, 8-storeys or taller, housing approximately 500,000 people. These buildings typically comprise a concrete structure and a mix of concrete and/ or masonry cladding. The form (shape, height, and windowto-wall ratio) is well suited to deep energy retrofits, perhaps approaching EnerPhit-level performance.

While retrofits represent additional embodied carbon (particularly from cladding replacement, insulation upgrades, or structural improvements), the use of low-embodied carbon or carbon-storing materials would limit the effect and lessen the life-cycle emission burden. Importantly, deep energy retrofits coupled with on-site renewable energy would significantly reduce ongoing operational carbon emissions with the potential to ‘pay back’ the new embodied carbon investment. Think about it: a climate positive building that is deleting its past contribution to the climate emergency!

Scott Armstrong is a Project Principal, Building Sciences at WSP.

1 Global Alliance for Buildings and Construction, 2019 Global Status Report for Buildings and Construction (Nairobi : UN Environment, 2019). 2 Opportunities for CO 2 Capture and Storage in Buildings, Magwood, C. October 2019. 3 Global CO2 emissions from cement production, Andrew, Robbie M., Published: 26 January 2018.