Putting high-rises on a low-carbon diet

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They are the unsung workhorses of all high-rises: the slabs of poured concrete that separate one storey from the next, and one unit from its neighbour. Everyone who has watched a tower rise from street level will have seen them – hard grey rectangles, held in place by columns and lattices of steel.

While reinforced concrete is an ubiquitous ingredient in city-building, it is also a major culprit in the climate crisis. According to recent research conducted at the University of Toronto, only about 20 to 25 per cent of a building’s lifetime emissions come from heating and air conditioning and other operational sources; approximately half comes from “embodied” carbon – the carbon used to make and transport building materials such as concrete, steel, aluminum and glass.

“We just use too much concrete,” says architect Sam Dufaux, a principal at Toronto-based SvN Architects + Planners. “We have to think of it as something we should use sparingly.”

In fact, in recent years, a growing number of architects and builders have been looking for ways to reduce embodied carbon by using less concrete without sacrificing its promise of strength and durability. Toronto city council this week backed a motion to add embodied carbon assessments to the Toronto Green Standard.

Oben Flats, a six-storey, 48-unit project in Toronto’s Leslieville, is one example. According to partner and Superkül founder Andre D’Elia, the design team decided to use so-called “hollow core,” a prefabricated product that uses 50 per cent as much concrete as a typical floor. The reason: each slab contains tube-like voids, which reduce weight without compromising strength.

Some builders and structural engineers have turned to alternative products, such as “Bubbledeck,” a proprietary product that uses voids created by recycled plastic balls embedded in the concrete to reduce weight and volume of slabs. Another technique involves the use of hybrid slabs – timber with concrete reinforcement.

“We’re currently working with a group called Infina to develop a similar system suitable to thinner slabs,” says David Bowick, a principal at Blackwell Structural Engineers.

The U of T team conducted a life-cycle analysis on a number of Toronto buildings, including Oben. “The [project] scored very well in terms of the low embodied carbon,” says Mr. D’Elia, adding that another important factor was the fact that the project doesn’t have underground parking, which is a major source of embodied carbon.

Architect and U of T adjunct professor Kelly Alvarez Doran, who has spearheaded much of the research, says municipalities could halve the embodied carbon in buildings, but doing so will require governments to act. A policy paper published last year by U of T, City of Toronto, The Atmospheric Fund and Mantle Developments calls for the regulation of embodied carbon as well as the development of standardized ways of measuring these emissions.

Toronto council’s decision to eliminate parking minimums last fall is an important step. Mr. Doran adds that products such as hollow core slabs brings cascading benefits. The reduced weight, he explains, means less loading on the foundations, which means they can be built with less concrete. “It’s just a substantial reduction, because the concrete is going to last just as long and you’ve reduced the embodied carbon associated with the structure by around two-thirds.”

For developers, the decision to go with tall-timber construction is clearly a design decision, but the more nuanced work of finding ways to use less concrete tends to fall to structural engineers. Most of the designers interviewed for this story pointed to Mr. Bowick as a pioneer, but John Peterson, a senior associate architecture in SvN’s sustainability practice points out that most structural engineers tend to be resistant to using these techniques. Often, he says, “they’re not even aware of it.”

There are, however, substantive concerns about reducing concrete use. Most high-rises, for example, use concrete shear walls to separate units, even though they could rely more on columns, which actually provide more flexibility for apartment floor plates. But in large buildings with many occupants, acoustics is an issue, and so the use of other materials for walls instead of concrete remains controversial. “In residential construction,” says Mr. Dufaux, “nobody wants to be the first one. Concrete has great acoustic properties that a lot of developers are reluctant to move away from.”

Mr. D’Elia, at Superkül, offers one other approach his firm has used for mitigating concrete use. In the Oben Flats in Leslieville and a handful of other projects, the firm has installed above-grade vehicle stacking systems, accessible off rear laneways, which store cars. “It’s all automatic,” he says. “That helps with all the underground concrete.”

The point is that when designers and developers are able to reduce the amount of carbon their buildings consume at the front end, through the use of energy and fossil fuel intensive materials like concrete, aluminum or spray foam insulation, the lower their lifetime emissions will be. “To have an impact in our built environment that mitigates our impact on the climate crisis,” Mr. Peterson of SvN says, “we have to start looking at these kinds of innovative solutions that help us reduce the amount of embodied carbon we put into our buildings.”

Adds Mr. Dufaux: “We are building so much right now with only carbon intensive materials, and that’s what needs to change.”

The Single-Egress Staircase

Recent moves by the City of Toronto and other municipalities to permit the development of multiplexes and low-rise apartment buildings in house neighbourhoods will have a clear carbon benefit: such projects intensify low-density areas, which means (at least in theory) less car use, more walking and cycling, and improved utilization of existing municipal infrastructure, like transit, water and sewer mains. Such projects, banned for years, create 15-minute cities.

Yet, an obscure but very durable building code requirement stands in the way of this kind of intensification: under existing rules, such projects must have two sets of stairwells, a design detail that tends to prevent the construction of apartment buildings with so-called single-loaded corridors (i.e., those with units opening only from one side).

Conrad Speckert, an intern architect at LGA, wants to reinvent the walk-up with the single-egress stairwell using mandatory sprinkler systems and updates to fire codes written, in many places, in the early 1900s, when most buildings were made from wood. He says Canada’s national building code limits the height of buildings with single-egress stairwells to two storeys, making it one of the most restrictive in the world.

“The goal of the code change is to make missing-middle buildings easier to build and to unlock smaller sites,” he says. “In so many ways, the code change is not a reduction in safety.”

His advocacy grew from his master of architecture studies at McGill University, and it has become part of a wider conversation about what will be required, financially and from a regulatory point of view, to enable small scale builders to get into the missing middle market.

Pointing to building codes in Seattle that allow for single-egress buildings up to six storeys, Mr. Speckert points out that such changes create a range of benefits, such as better use of space within the building, improved apartment layouts and the potential for cross-ventilation as a means of cooling – something that is not available in apartment buildings with two sets of stairwells and so-called double-loaded corridors (i.e., with units opening off both sides).

In terms of sustainability, he points out that the most obvious win with new rules on stairwells is that they’ll help prevent sprawl by providing more homes within built-up areas.

As he puts it, “You’re incentivizing in-fill housing.”