Vertical living may not be the most environmentally friendly way of reducing greenhouse gas emissions in the built environment, according to a new study by researchers from the University of Colorado, Edinburgh Napier University, and the University of Cambridge. The study, published in the journal npj Urban Sustainability, suggests that low-rise, high-density cities are more environmentally friendly than their high-rise, high-density counterparts, though both are still more sustainable than low-rise, low-density sprawling suburbs.

The researchers arrived at their conclusion through an analysis of an urban environment’s life cycle GHG emissions (LCGEs), which include both embodied and operational emissions. By feeding 5,000 urban environment simulations into an algorithm, with varying height and densities, researchers concluded that taller urban environments significantly increase LCGEs (+154%), while low-density urban environments significantly increase land use (+142%).

The team determined that dense environments of approximately 6 to 10 stories in height offer the most environmentally friendly balance, emitting approximately 365 tons of carbon dioxide per person less than high-density high-rise alternatives. By including greenhouse emissions created during construction, such as the manufacturing and transportation of materials, the researchers determined that the increased land needed to construct 10-story buildings is offset by the savings in materials needed to construct tall buildings, which often require disproportionately large foundations and structural support.

A key finding by the team is that, across all configurations of density and height, the LCGE increases as tallness increases, independent of the amount of land required to house the population. In contrast, the density of buildings has little impact on LCGE; meaning that for all scenarios, low and high-density typologies resulted in similar LCGE results, regardless of height. The findings suggest that low-rise, high-density cities such as Paris, with predominately 5-6 story buildings, offer a more environmentally-friendly model for urban development than high-rise, high-density cities such as New York City or Hong Kong.

“We’ve always been looking at this problem from a building perspective,” says Francesco Pomponi, the study’s lead author and a professor at Edinburgh Napier University. Speaking to Fast Company, Pomponi notes that “if you look at the building perspective and you analyze the footprint, of course a tall building is better. The high-rise building houses more people. But when you start looking at the bigger picture, you realize you cannot put two high-rise buildings as close as you can two low-rise buildings. To build tall, you need heavier structures, chunkier foundations and also, for a lot of good reasons like privacy, ventilation, and daylighting, high-rise buildings need to be further apart.”

The researchers see their findings as an important factor in the design of future urban environments in Africa and China, which are expected to undergo rapid urbanization throughout this century. In developing their findings, the team hopes to include other interdisciplinary considerations to guide city planning, such as occupant comfort, the urban heat island effect, competing land use, carbon sequestration of green spaces, urban policies, resource consumption, and how urban environments affect crime.

“Each building should not be identical to the next with a very fixed and prescribed height,” Pomponi told Fast Company. “It’s more about having an upper threshold that, unless you’ve got a really, really good reason, it should not be exceeded. This will ensure that over time we get a diverse-built environment that doesn’t trigger the environmental costs of building tall.”