The hidden carbon footprint of highrises

Although some experts have long called for higher buildings to accommodate for population growth in Canadian cities, towering apartment and office buildings aren’t the only solutions to increasing density, according to a recent study. Building their counterparts — mid- and low-rise buildings — can help a city grow, while keeping a significant amount of carbon out of the atmosphere.

The taller the building, the more carbon it emits at every stage of its life. A July study from npj Urban Sustainability found a 140 per cent increase in emissions from a neighbourhood full of skyscrapers compared to an area of low- and mid-rise buildings. The authors found increasing the number of medium-sized buildings could accommodate growing populations more sustainably than cities focused on single-family homes and skyscrapers because they emit less carbon.

Tall buildings have high emissions once they’re built due to complex heating and cooling systems and other operations. But they also impact the environment before their lights are even turned on.

A new study out of Halifax examined the carbon output of two developments that propose knocking down more than a dozen low-rises near the downtown core and replacing them with four highrise towers. Buildings For the Climate Crisis — A Halifax Case Study, authored by environmental scientist Peggy Cameron, found a significant amount of "embodied carbon" would arise from building the towers using cement and steel. She found renovating most of the existing low-rise buildings and constructing a nine-storey infill project would emit 40 per cent less carbon than the proposed new builds, while still increasing density.

Embodied carbon, Cameron explains, makes up all emissions produced during a structure's building phase — from the manufacturing of materials such as windows and doors to the transport of those materials, and then the energy consumed during construction. Carbon released during demolition is also taken into account.

Tom-Pierre Frappé-Sénéclauze, director of buildings and urban solutions at the Pembina Institute, agrees — accounting for embodied carbon is essential.

Steel and concrete are typically used to frame tall buildings, which are high-emission materials, whereas wood, which can sequester carbon, can be used for low- and mid-rise structures, he explained. Although there are some low-carbon concrete options, they’re not normally used.

“Typically, higher-storied buildings are going to be built with concrete and steel,” he said.

“... In most common construction practices, if they're not forced or encouraged to reduce the carbon emissions from concrete and the steel, they will use the cheap stuff. And it’s not always a question of price, it’s also what engineers are used to.”

Worldwide, the total emissions from buildings, including embodied carbon emitted during construction, made up 38 per cent of carbon emissions from energy in 2019. Embodied carbon is responsible for a quarter of those emissions. There are no official Canadian figures for embodied carbon.

That number is important, said Cameron, who explained most green building conversation is around operational carbon — emissions released once a building is finished. She sees a disconnect between the drive to increase density in Halifax and the need to build highrises. Cities like Paris maintain high-density with a low-rise environment, and she wonders why Canadian cities can’t do the same.

So, although the federal government is working to reduce emissions from operational carbon through net-zero building codes and retrofits, Cameron said a significant amount of emissions is being ignored. Leaving embodied carbon out of the calculation makes it much easier to justify tall buildings — green or not — she said.

“If we're not looking at the embodied carbon in the products that we're using, we're actually causing more harm than if we just sat around and turned the heat up in our leaky, old homes,” she said.

“A lot of people have this misperception that when we're building green buildings — and there are so many different standards that are certified out there — that we're doing the right thing for the environment. But, in fact, it's not the case.”

Cameron’s study looked at two high-rise development proposals next door to each other. She found they would release 31,000 tonnes of carbon dioxide equivalents (CO2e), or the emissions from 9,497 passenger vehicles, into the atmosphere throughout the demolition and construction process.

“That means by maintaining the existing buildings, where possible, and building mid-rise instead of highrise, the embodied carbon of the block redevelopment can reduce by more than 40 per cent (13,000 tonnes of CO2e),” reads the report.

“This amount of carbon emissions is equivalent to about 17 years of operational carbon from the existing buildings.”

Also important is changing zoning regulations to not allow for tall buildings that can’t be carbon-neutral or carbon-negative. Infill options show a realistic alternative that could see a lot less carbon enter the atmosphere, she said.

Cameron points to the Royal Institute of British Architects calling for a stop to all demolition and the City of London rejecting a project because of its estimated embodied carbon as positive moves towards recognizing how intertwined building processes and climate change are.

“I think that we have to imagine how we're going to regulate both embodied and operational carbon emissions in the construction and building industry, and then have legally binding targets, and they need to be year by year,” she said.

“And they have to start immediately.”