It may seem obvious that cities filled with big buildings use energy more efficiently than dispersed suburban landscapes, and that newer, taller buildings are more energy-efficient than older, squatter structures.
People widely understand that New York City, for example, ranks well on energy use per person, where housing tends toward the vertical, one boiler room can serve many units, and heat rises into the units above, rather than being lost to the sky.
But now some climate-minded architects and engineers are questioning that orthodoxy. It’s starting to look like there might even be a sweet spot for building height: the four-story courtyard.
That was the unexpected conclusion reached by researchers from Adrian Smith + Gordon Gill Architecture. They modeled housing units in nine different configurations, from a 215-story building with 2,000 residential units at one extreme and 2,000 dispersed suburban homes at the other.
The question the study asked was how efficient are ultra-tall versus low-rise residential buildings. According to their analysis, the four-story courtyard used the least energy per household.
The question matters, because buildings are directly or indirectly responsible for nearly 40 percent of all greenhouse gas emissions, according to the International Energy Agency.
That AS + GG asked these questions at all is somewhat surprising, given that the firm develops ultra-high-rise buildings, including the first kilometer-high building in the world, the Jeddah Tower, now under construction in Saudi Arabia.
“We wanted to look at 2,000 units and whether they are sustainable in as unbiased a way as possible,” said Christopher Drew, the firm’s director of sustainability. The architects published their research in a paper in the International Journal of High-Rise Buildings, and later in the book Residensity, which expanded on the paper to account for land use and underground utilities.
Other research teams also hint that an unexamined preference for tall buildings may be misplaced. Daniel Godoy-Shimizu and six colleagues at UCL Energy Institute in London analyzed the electricity and gas bills for 611 office buildings in England and Wales. They found that high-rises of 10 stories and higher used 76 percent more electricity per square foot than low-rises of five stories and under.
“The results show that height is a significant predictor of energy use, even accounting for other variables,” the researchers wrote in a paper published in the journal Building Research and Information. In fact, each additional story in a building is associated with a 2.4 percent increase in electricity use and 2.9 percent increase in fossil fuel use, more than doubling the average emissions per square foot for the tallest buildings in their survey.
Lifting people, air and water, and windows that leak heat and cold
One big reason for this is that tall buildings have elevators. In the tallest buildings, elevators account for as much as 10 percent of energy use, AS + GG found. Tall buildings also rely on fans to push and pull air through heating, cooling and ventilation systems, and pumps to lift water hundreds of feet up.
At the opposite end, single-family homes are, as expected, more carbon-intensive. Beyond their dispersed heating and cooling infrastructure, they’re exposed to the weather on all sides, whereas four-story courtyard apartment units share one to two walls each.
Elevators, fans and pumps aren’t the only culprits. Windows also reduce efficiency compared to solid walls, and tall buildings tend to have a lot of windows.
“The best-performing glass is, with almost with no exceptions, worse than the worst-performing solid opaque wall,” said Ann Edminster, a zero-energy and zero-carbon consultant who wrote a chapter on height in the book The New Carbon Architecture.
The reasons for this are fairly obvious. Windows allow unwanted heat into buildings in summer and allow it to escape in winter, forcing mechanical systems to run harder during more hours of the day, which increases carbon emissions. Typical glass windows insulate only one-fifth as well as the worst wall material, Edminster said.
A predilection for tall, glass-fronted buildings makes solving this problem a challenge for architects. Glass windows and facade can be equipped with shading devices that cut unwanted rays, but building owners “want to preserve that glass facade look, and that is a real problem,” said Charlie Curcija, a mechanical engineer at Lawrence Berkeley National Laboratory who works on energy performance and windows.
Implications for the next generation of buildings
As electricity gets cleaner and building codes are updated, newer tall buildings will become more energy-efficient. The team at AS + GG explored these possibilities in their analysis of nine types of new-build structures in a Chicago climate, taking into account land use, new utility service, and the carbon emitted to make the concrete, steel and other building materials involved.
According to this analysis, the four-story courtyard still had the lowest overall emissions after 40 years of operations. Small triplex units were the next lowest, while a 65-story high-rise took third place. But buildings higher than 100 stories compared poorly, largely due to the steel and concrete needed to keep them standing.
Other research shows that large buildings account for more emissions than their smaller counterparts.
Research done by the Zero Cities Project shows that in Seattle just 3 percent of buildings are larger than 20,000 square feet, but these are responsible for 45 percent of all building-related emissions, with similar ratios in other American cities. In Cambridge, 4 percent of buildings were responsible for 74 percent of building emissions.
But for architect and educator Ed Mazria, this concentration of the emissions offers a focus for efficiency efforts. If there are only 5,000 large buildings, “I can deal with that if I am a city,” he said. Cities should require dramatically reduced emissions from these buildings and tie these requirements to the buildings’ capital improvement cycles.
Major renovation projects can invest in changing the windows, for example, and use the resulting efficiency gains to downsize the heating and air conditioning system, saving money. Lawrence Berkeley National Laboratory is working on thin, triple-glazed windows that reach R-8 to R-10 levels of insulation. Crown Electrokinetics has developed a window insert that can respond to temperature by blocking or allowing light. This market is growing. View, Inc., Heliotrope Technologies, Kinestral Technologies and Saint-Gobain are among those operating in the smart glass sphere.
“There are [many]...ways to prevent emissions when you retrofit a tall building,” said Cathy Higgins, research director with the New Buildings Institute in Portland, Oregon. Besides the obvious — LED lights and heat-pump-based heating and cooling — she cited heat-pump water heaters and ice chilling.
Cities and states across the country are hard-pressed to find methods like these to reduce the cost of ambitious building efficiency mandates. New York City’s Local Law 97 requires its 50,000 largest buildings, almost all those over 25,000 square feet, to cut carbon emissions by 40 percent by 2030 and by 80 percent by 2050. President-elect Joe Biden’s Build Back Better plan calls for upgrading 4 million buildings.
Whether height is paramount for reducing the climate impact of buildings depends on who you ask. Curcija pointed out that elements of a building’s skin, like high-efficiency windows — or, perhaps, building-integrated photovoltaics — could make up for tall buildings’ gravity-driven inefficiencies.
Katrina Fernandez Nova at Adrian Smith + Gordon Gill architects says their research findings on different height configurations did surprise them and have “shifted our focus at the firm to how we can reduce the embodied carbon of the materials.”
Ann Edminster asks, “Is the net effect of allowing taller and taller buildings serving or destroying your efforts at carbon reduction?” She predicts that the best height for buildings ultimately will be determined to be in the six- to -12 story range.
Since we are still loading the atmosphere with vast amounts of heat-trapping greenhouse gases, the answers are urgent.