Despite growing concerns about the carbon emissions associated with their construction and operation, skyscrapers continue to spring up around the globe. Here, Philip Oldfield sets out seven ways to design tall buildings that are more sustainable.
Is there such a thing as a low-carbon tall building? Or are skyscrapers inherently unsustainable, the SUV of the built environment?
By rising up above their surroundings, tall buildings are exposed to more sun and wind. This could, in principle, be a good thing (think free heating and ventilation). But since so many towers are fully glazed, with little shade, instead they often experience overheating or excess heat loss, increasing operating emissions.
Tall buildings also suffer from what skyscraper architect and engineer Fazlur Khan calls the "premium for height". As we build taller, towers face higher and higher lateral forces from the wind and seismic loads. To resist these, tall buildings use more structural materials – typically carbon-intensive concrete and steel. The upshot is that taller buildings have a higher embodied carbon than mid- and low-rise blocks.
Across their lifecycle, tall buildings typically generate more emissions
Overall the evidence suggests that across their lifecycle, tall buildings typically generate more emissions than mid-rise. So, should we stop building them?
A major challenge we face over the next few decades is housing billions of people comfortably and safely while radically reducing emissions; 1.6 billion people currently live in inadequate housing, according to the UN. Mid-rise could, and should, provide the basis for much of this. But it's naive to think there's a one-size-fits-all solution for every city, and every site.
Where land is scarce, tall buildings can provide greater density, putting more people in close proximity to low-carbon public transit, and the civic infrastructure of the city. The question is: how can we design towers to have far fewer carbon emissions than the norm?
Below are seven principles to follow:
Retrofit first
Given that they are an investment of thousands of tonnes of steel and concrete, it seems senseless to demolish a tall building. We only need to look at 270 Park Avenue in Manhattan, built in 1960, retrofitted to LEED Platinum in 2012, but then demolished to howls of despair from architects, historians and environmentalists alike only nine years later so that it can be replaced by a slightly taller and shinier edifice.
A much better approach is to retrofit, reuse and reimagine existing towers, rather than raze and rebuild. The Quay Quarter Tower, by 3XN and BVN, upcycles a 1976 modernist tower block in Sydney, maintaining the core and much of the existing floor plates but entirely transforming the architecture – and increasing the floor area by 35 per cent. This approach reduced embodied carbon by around 8,000 tonnes compared to a new build.
Reject the curtain wall
Glazed curtain-walling is the go-to cladding of any skyscraper. Visually monotonous, but also environmentally criminal. You don't need to be a building physicist to understand why. Even the highest-performance triple-glazing with argon gaps, e-coatings and all the bells and whistles won't perform thermally as well as a simple insulated wall.
Of course, we need daylight and view, so some glazing is essential – but do we really need to glaze down to the floor and illuminate the tops of our feet? Future tall buildings should embrace shade and solidity in their facades, with glazing limited to perhaps no more than 40 per cent of the wall area.
We can take inspiration from the National Commercial Bank in Jeddah. Designed by Gordon Bunshaft of SOM (who ironically also helped design one of the first fully glazed towers in the world, Lever House, in New York), it has glazed inner courtyards but solid stone external walls as a response to the harsh desert sun.
Embrace Passivhaus
One of the environmental benefits of tall buildings is that they are compact, meaning they have less envelope to lose heat from compared to low-rise buildings. This characteristic lends itself to Passivhaus – a performance standard that achieves very low operating energy needs through compact forms, super-insulation, air-tightness and heat recovery.
WKK Architects' 1075 Nelson Street skyscraper planned for Vancouver will be the tallest Passivhaus in the world when complete, at 178 metres. Better still, why not save both embodied and operational emissions by retrofitting an existing tower to Passivhaus, like ERA Architects have done with the Ken Soble Tower in Hamilton?
Flush out the heat
The flipside of a compact shape is that once unwanted heat gets into a high-rise it can be more challenging to get it out again. People and equipment inside buildings give off heat, and because towers are compact and often bulky, they can be more challenging to cross-ventilate.
There are some solutions – designing high-rises with atriums, skygardens, or with permeability can create pathways for breezes to flush out unwanted heat. In Woha's The Met, in Bangkok, deep balconies provide shade from the sun, while voids cut through the building channel breezes and allow units to be cross-ventilated. Residents have reported little need for air-conditioning as a result, even in the hot tropical climate.
Build with timber
Cement, the primary ingredient of concrete, is responsible for around eight per cent of all human-made carbon emissions. Since tall buildings are big consumers of concrete, can we look to use something else?
Step forward timber. Timber structures have the benefit of lower embodied-carbon emissions than steel and concrete. They are also able to store carbon in the wood for the lifetime of the building.
White Arkitekter's 20-storey Sara Kulturhus Centre is built from cross-laminated timber (CLT) and glued laminated timber (glulam). The timber in the building stores twice as much carbon as was emitted during its construction.
Reduce first, generate second
It's much better to embrace energy efficiency and low-embodied-carbon strategies first before thinking about on-site energy generation. The TU Wien Plus-Energy Office High-Rise in Vienna is the retrofit of a 1970s office block (are you seeing a trend here?).
Through the use of a super-insulated and airtight facade, a heat-recovery system, night-flush ventilation and low-energy appliances, primary energy was reduced from 803 kilowatt-hours per square-metre per year to just 56kWh/m2/year. This radical reduction means that with photovoltaic panels on the roof and facade, the tower will generate more energy than it uses over the year.
It's tempting to add wind turbines to the top of a skyscraper – but don't do it! While it might create a bold green visual statement, it won't reduce emissions much. Wind speeds increase with height, so it seems sensible to use this to generate clean energy, but turbines also create noise, meaning their use in urban areas is far from ideal.
Forget about supertalls
Supertall buildings (those over 300 metres) and megatall buildings (those over 600 metres) need exponentially more materials for construction. More concrete, more steel. This means more embodied carbon. Too often towers of this height are created merely as icons, symbols of power and corporate wealth rather than providing essential societal needs. When was the last time you heard of affordable housing in a supertall building, for instance?
Fortunately, there are signs that we are moving away from using tall buildings as urban trophies. In China, where most supertalls have been built, the government has announced a ban on towers over 500 metres, with those over 250 metres "strictly restricted".
In our climate-change challenged world, every kilogram of material we use is precious – so let's not waste them on an inane race for height. Low-carbon tall buildings are possible – but we have to put environment before elevation.
Philip Oldfield is Head of School of the Built Environment, UNSW Sydney. He is the author of the Sustainable Tall Building: A Design Primer (2019).