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The window-to-wall ratio at Cortel Group's three-tower development in Vaughan, Ont., is just 43 per cent.Quadrangle

When the Cortel Group begins marketing a three-tower, 830-unit condo complex at Jane and Rutherford Road in September, buyers may notice that the exterior design is noticeably different from many recent high rises. Unlike the standard green-ish glass towers that have sprung up across the downtown in recent years, the Cortel project, designed by Quadrangle, will feature smaller windows, more exterior metal panels and a staggered array of hexagon-shaped balconies.

According to Quadrangle principal Richard Witt, a key point of differentiation comes in the form of a technical metric that plays an increasingly well-recognized role in determining the energy performance of a building.

For the yet-to-be-named Cortel project, the “window-to-wall” ratio (WWR) – sometimes also known as the “aspect ratio” – is just 43 per cent, meaning less than half of the tower’s exterior wall area are made of glass. On many all-glass towers, Mr. Witt says, that measure can be upward of 90 per cent.

It’s not difficult to understand what’s behind the explosion of glass towers: the proliferation of smaller, deeper units; investor demand for expansive views; material costs and speed. But as a growing number of condo dwellers have learned the hard way, those enticing floor-to-ceiling windows can create all sorts of heating and cooling headaches, especially if they are south or west facing.

In terms of a building’s overall energy efficiency and therefore its operating costs, glass towers are something of a disaster. While the Ontario Building Code requires individual wall components to have an r20 rating, Mr. Witt says the overall insulation levels of glass towers is far lower because heat leaks through framing assemblies around windows and the concrete floor slabs that extend out to the balconies. (The r-value is a measure of how effectively a material impedes heat loss.)

In some cases, he says, a glass tower’s actual insulation value can be as low as r2 or r3, even though it should be closer to r7, the minimum. “We’ve ended up with a building standard that doesn’t perform to the standard that was intended.”

From a climate-change perspective, these are grave flaws in a city where the high-rise building boom shows no sign of ebbing any time soon.

The Cortel development aims to avoid the heating and cooling issues that face towers that are 90-per-cent exterior glass.Quadrangle

According to most estimates, buildings account for about 40 per cent of all direct greenhouse gas emissions – a figure that reflects non-renewable energy used for heating, cooling and air circulation, as well as the carbon embedded in construction materials such as concrete, steel or gypsum.

While much of the roiling public debate about emission-reduction policy focuses on politically contentious tools such as the carbon tax or cap-and-trade systems targeting polluters, architectural design choices – such as the amount of glass used to clad large buildings – play an important role.

In jurisdictions with a strong focus on green building design and high energy costs, the WWR has become an increasingly important tool in driving reductions to built form-related emissions. Most of those jurisdictions are in northern Europe and Scandinavia. But this past spring, New York mayor Bill de Blasio launched a Green New Deal that will include a de facto ban on glass and steel skyscrapers that will come in the form of more stringent energy performance regulations, according to Vice Canada.

Here, the 2018 update to the Toronto Green Standard recommended a less than 50-per-cent window-to-wall ratio as part of a suite of energy efficiency measures, including solar shading, high-performance glazing assemblies and energy-recovery ventilation systems. (The standard also provides cash incentives for developer who can demonstrate that their buildings will be more energy efficient.)

The new thinking about glass towers reflects a growing body of building-science evidence showing that newer high-rises don’t perform as well, from a sustainability perspective, as those constructed in the 1960s and 1970s. One key reason: Those earlier high-rises had smaller windows and heavily insulated walls.

Developers are always constrained in their cladding choices by market forces, municipal planning policies and the range of available materials (basically, window walls, brick, metal panels, fibre cement panels and precast concrete). Yet, Mr. Witt says there’s an opportunity now for less glazing over all, reduced balcony exposure and better window-framing techniques that reduce heat loss.

These aren’t the only approaches that have made their way into the design arsenals of sustainability-minded architects and developers, of course. A recent issue of Architizer itemized five case studies in sustainable curtain walls for high-rises, including the use of metal scrims on the exteriors, so-called “low-iron” glass, and specialized curved and fritted glazing designed to reduce thermal transmission.

But others point out that towers built to demanding sustainability benchmarks, such as the so-called Passive House standard that has seen a surge in builder interest in Vancouver, don’t preclude the use of prefabricated curtain walls with large windows because these assemblies, typically manufactured off-site, can be built with more quality controls, meaning fewer thermal leaks.

The missing element, however, is purchaser awareness about the role that poorly installed floor-to-ceiling windows can potentially have on the livability of a unit, as well as its heating and cooling costs. “My experience is that people don’t ask the right questions,” Mr. Witt says. “We find [buyers] don’t look far enough ahead.”

Yet, as building codes become ever more stringent about heat loss and developers gradually win market acceptance with projects such as Cortel’s Vaughan complex, the general lack of consumer knowledge about metrics such as WWR may be a moot point. As Mr. Witt predicts, “The glass tower’s days are numbered.”

This article is part of an occasional series about recent advances in sustainable design and construction.