Artist rendering of a building constructed with mass timber and columns of well-spaced out windows.
Artist rendering by CreateTO

For Greener Buildings, We Need to Rethink How We Construct Them

To meet its pledge to be carbon neutral by 2050, Canada needs to cut emissions from the construction industry. Here’s how

Concrete and steel are used in almost every building in Canada. These materials are relatively inexpensive and incredibly strong, but they have a hidden cost: a lot of carbon dioxide is emitted in producing them. About 13 per cent of the country’s annual carbon emissions are generated by the construction industry.

If Canada intends to meet its UN pledge to be carbon neutral by 2050, it will have to find a way to reduce the carbon emissions embodied in construction materials.

Using more organic substitutes, such as mass timber, is one answer. But Kelly Alvarez Doran, an adjunct professor at the Daniels Faculty of Architecture, Landscape, and Design, says Toronto needs to make several other changes as well, such as eliminating underground parking, ditching “stepbacks” and reducing average window-to-wall ratios.

Where construction materials are sourced is also crucial. Materials such as brick and aluminum require a lot of energy to produce; how this is generated has a major impact on a building’s embodied carbon. Doran recalls assessing a Toronto building that was clad in grey brick imported from Nebraska. Transporting the material here increased the embodied carbon by a few percentage points. But the state’s reliance on coal had a far larger impact, generating many times more carbon per brick than if the material had been made in Ontario, which relies mostly on nuclear and hydroelectric power.

Doran notes that to minimize a building’s lifetime environmental impact, architects must keep the impact of both embodied and operational carbon in mind. Triple-paned windows generate more carbon dioxide during construction, for example, but reduce emissions from heating and cooling over their lifespan.

In 2022, Doran and a former student, Juliette Cook, established Ha/f, a design consultancy that helps architectural firms with these complex assessments. The team is also collaborating with the City of Toronto on guidelines for reducing embodied carbon in the local construction industry. For Doran, the work has a deeply meaningful reward: “It’s how I address my anxiety about climate change.”

A Closer Look

Digital illustration of a building with numbered parts. The walls of a section of the building are missing to show the interior of the units as well as part of the underground parking. People and furniture are visible in the units and ground floor and cars occupy the underground parking.

About The Author

Author image: Scott Anderson

Scott Anderson

Editor, University of Toronto Magazine

Each of the numbers in the illustration above match the cutaways below. Illustrations by Chris Philpot.

Cutaway digital illustration numbered 1, showing the tiered balconies of units of a building, descending like steps
Stepbacks: Many of Toronto’s urban design guidelines call for stepbacks, which create a tiered effect. Doran is working with the city to investigate the inefficiencies they cause – for both construction and the environment. Stepbacks require more reinforced concrete (adding to the embodied carbon), leak more thermal energy (adding to operational carbon), and can contribute to the housing crisis, since they ultimately reduce the number of units a lot can accommodate.
Cutaway digital illustration numbered 2, showing the inside profile of several units of a building from the side as well as their tiered balconies. There are furniture and people inside.
Construction materials: Simpler building forms that eliminate stepbacks allow architects to substitute mass timber for concrete, further reducing embodied carbon. Wood can cost more, but Doran says mass timber allows for faster construction, noting that the interest savings on construction loans due to a shorter completion time can fully offset the higher material cost of wood.
Cutaway digital illustration numbered 3, showing the interior of a section of the ground floor of a building, with thick red pillars in the centre and at each corner, supporting the floors above
Transfer slabs: These heavy-duty concrete slabs help transfer the massive load of the upper floors to lower support posts and pillars. As much as a metre thick, they can be the most time-consuming part of construction, requiring 15 per cent of the total concrete. Simplifying design and eliminating underground parking reduces the need for these slabs.
Cutaway digital illustration numbered 4, showing the interior of a section of the ground floor of a building, with people milling about, and two underground parking levels, occupied by cars
Basements and underground parking: In mid- and high-rise buildings, underground parking can account for 20 to 50 per cent of the project’s embodied carbon, due to the extra concrete and other materials required. The City of Toronto has dropped its requirement for developers to provide a minimum number of parking spaces with each project, but most other Ontario municipalities still insist on it.
Cutaway digital illustration numbered 5, showing floor-to-ceiling windows interspersed with wall space along the side of a building
Window-to-wall ratio: Shrinking the proportion of windows to opaque walls helps reduce embodied and operational carbon. A wall of glazed windows with aluminum framing contains about three times the embodied carbon of the same-sized brick wall.

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