CivMin researchers are exploring ways to meet our housing needs while emitting less
Taller buildings get a bad rap. New research from U of T Engineering’s Centre for the Sustainable Built Environment (CSBE) has found that while adding height does slightly increase embodied emissions, other building and neighbourhood design factors are far more important.
The paper, recently published in Resources, Conservation and Recycling, looked at how building height affects embodied emissions for new five- to 20-storey reinforced concrete residential buildings.
“There’s a fairly loud narrative that taller buildings are bad buildings, but from a resource use perspective and an embodied carbon perspective, mathematically something seemed off,” says Professor Shoshanna Saxe (CivMin), director of the CSBE.
“Let’s say you need a certain amount of housing. If you planned to build a 20-storey building but instead you build a 10-storey one, you’re going to need to put those 10 extra floors somewhere else. That means more roads, more utilities, more elevators. Another building quickly swamps out the small increased emissions from height.”
Avery Hoffer (CivE 2T2 + PEY, MASc 2T5), lead author on the paper, says that right now, figuring out how to build more while using fewer materials is paramount.
“We’re currently facing two crises, not just in Toronto, but more broadly as a country,” says Hoffer.
“We have the housing crisis in Canada where we need 3.5 million additional units by 2030, and we also have the climate crisis where buildings account for 11% of our total embodied greenhouse gas emissions. We’re also not on track to limit global warming to 1.5°C above pre-industrial levels.”
For their study, researchers modeled 128 concrete apartment buildings, changing various key design features such as height and slab thickness. For each building version, they calculated the materials needed and the embodied emissions for the building.
When the study was complete, researchers found several design decisions have a much stronger impact on carbon emissions than height, with concrete slab size being the worst offender.
“Just the floor slabs alone can account for up to 75% of the total embodied emissions of the structure,” says Hoffer.
“What we see in Canada is that a lot of these buildings are built with slabs in the 200–225-millimetre thickness range, which is more than what we’ve found is needed. So just by changing a small amount of your floor slab, you can save very large amounts of emissions.”
While the amount of concrete is a major driver of emissions in taller buildings, the researchers found that other common design decisions can also significantly increase a structure’s carbon footprint.
“Often for ease of design and constructability, most of the floors in a building are designed to be the same,” says Professor Evan Bentz (CivMin), a CSBE investigator and co-author on the paper.
“A 15-storey building might just have two floor designs, where the top floor ends up being much stronger than it needs to be because it’s easier to just keep building the same way up.”
Saxe says that both city bylaws and norms in the industry are contributing to more polluting, expensive and resource-intensive buildings.
“In Toronto, for example, there are requirements for things like step backs that require thick concrete transfer slabs, or garbage truck turn around space, which messes with floor plates.”
“There’s also an issue of construction and design culture and what we’re willing to pay for. We pay a lot to pour concrete. We don’t pay very much for top notch design.”
There is a limit to the building heights modelled in the study. Other research that has tested buildings above 20 storeys suggests the height premium does gets more pronounced at around 50 storeys and above.
“We’re not saying we should build 120-storey buildings everywhere, but we should certainly be allowing five- to 20-storeys in a lot more places and making it much easier for those heights to be built,” says Saxe.
“We need housing and we need to build it in a sustainable way. Every time we lop a unit out of a building and put it somewhere else, that second building is more polluting than having it all in one structure.”
By Samantha Younan
This story originally published by Engineering News