Our buildings have been accused of creating around 40 percent of the world’s carbon. The resonance by regulators, investors, and the property community has been to invest in ways to make buildings more sustainable. Electrification, better design, and more efficient equipment have helped buildings lower their energy consumption, but these measures can only go so far. For us to really hit our decarbonization goals, we have to not only upgrade our buildings but also think about how they influence the rest of our lives. When it comes to the overall impact that buildings can have on the sustainability of our lives, there are few more important considerations than density—high density, to be precise.
There’s a strong connection between density and sustainability. According to the Center for Sustainable Systems, doubling urban density reduces carbon emissions from household travel by 48 percent and reduces residential energy use by 35 percent. So, part of the solution to meeting our ultimate net zero goals will involve creating more communities featuring higher densities. More buildings on less land. It’s not a new concept. The neighborhoods of popular international travel destinations like Paris and Amsterdam are more than just quaint; they’re high on sustainability, with connected mid-rise residential buildings, narrow streets, and fewer cars. The same can be found in certain areas of New York City, which nears the top of the list of the highest-density cities in the United States.
Yes, the packed sidewalks of people walking shoulder to shoulder in brownstone-laden neighborhoods are literally the very picture of high sustainability, simply because most of the residents use public transportation and the majority of people reside in apartment buildings. The sustainable benefits of public transportation are obvious, while the advantages of apartment buildings are less talked about but just as important. “Residents in apartment buildings get the transferred heat and cooling from their next-door neighbors and from the people below them and above them,” said Cindy McLaughlin, head of product with CarbonBuilt, a company that manufactures low-carbon concrete. “When you share a wall with somebody, you’re going to get some leakage from their apartment. So often in the city in the winter, you don’t need to turn on your heat because the building, in general, is warm enough.” Infrastructure becomes more efficient, too. Water pipes, for example, serve far more people in an apartment building of 60 people than they do in a single-family home of four.
A densely populated, transit-oriented neighborhood isn’t always inherently sustainable. A New York City district peppered with detached residential skyscrapers is as low on sustainability as the sprawling suburbs, where each house has its own carbon-emitting infrastructure. “Concrete and steel are two of the most carbon-intensive materials. So when each home uses a whole bunch of it for its foundations, for the street in front of it, for the driveway, for the superstructure of the house itself, it starts getting really carbon intensive,” McLaughlin said. High-rises in big cities are nearly as high on carbon intensity as suburban residences because the deep underground infrastructure required to support the weight of such tall buildings consists of a great deal of high carbon-emitting steel and concrete. Additionally, not only do these detached skyscrapers not benefit from transferred temperatures, but they’re also exposed to the ambient heat and cold from the outdoor environment.
The sweet spot of low carbon footprint per capita, or prime sustainability, can be summed up as a neighborhood packed with mid-rise, attached residential buildings that naturally provide transferred heat and coolness, nearby commercial services, and narrow streets designed more for biking and walking than driving. “When you have high-density neighborhoods with lots of people living within close proximity to each other and to all the commercial things they need, you’re able to live a very low carbon lifestyle,” McLaughlin said.
Looking ahead, to achieve maximum sustainability, developers and city planners are beginning to turn their attention to the concept of the 15-minute city. As Carlos Moreno, a leading researcher in urban planning and a driving force behind the 15-minute city concept, explained during a TED Talk, the 15-minute city centers on the concept that cities should be designed to provide people the ability to “access work, housing, food, health, education, culture and leisure, the essence of what constitutes the urban experience,” within a 15-minute walk or bike ride.
No plan is without its downsides. A recent paper presented in People and Nature, a journal of the British Ecological Society, concluded that there is a negative correlation between population density and green spaces, where with each doubling of density in a neighborhood comes a 3 percent decrease in tree cover. It’s not a significant trade-off, but it is something planners have to take into consideration. Another potential downside to high-density neighborhoods is the stress that some can feel with a high number of people packed into a small amount of space. Ultimately, most experts believe the sustainable benefits of high density far outweigh any negative aspects.
The creation of the 15-minute city can’t happen everywhere. It centers on transit, and so car-centric locales like Los Angeles and Dallas would be challenged to create such neighborhoods. But high sustainability is still attainable without the 15-minute city. The mixed-use campus is a close cousin of the 15-minute city, and a growing number of developers are pursuing such projects across the U.S. New developments can also bump up the sustainability of construction by using products such as mass timber and ultra-low carbon concrete. These products will likely be readily available thanks to a federal government initiative to invest $2 billion in projects that use local and sustainable building materials.
Grouping multiple property types in one building offers greater sustainability. Existing high-rise office buildings, for example, can be retrofitted to incorporate residential and retail elements. From a maximum sustainability perspective, it could be argued that we shouldn’t be developing pure office structures at all. “Office buildings are inherently unsustainable when you’re looking at it from a utilization metric,” McLaughlin noted. “Even if everybody were coming into the office five days a week, you would still be using that office space for only 40 or 50 hours a week of 168 total potential hours. So, you’re going to max out at one-third utilization of that building.”
Be it the European-like neighborhood of attached mid-rise residences, mixed-use destinations, or redeveloped office towers, higher-density development translates to higher sustainability. It seems like a no-brainer, but transforming the built environment in the U.S. is going to be a challenge. More zoning laws will have to change to encourage mixed-use development. Building and transportation departments, which often have competing goals, will have to find a middle ground. It will be slow going, but if developers and property owners make high density a priority, we’ll have less embodied carbon in our built environment and a more sustainable future.