The Goldilocks Density Delivers the Lowest Life Cycle Carbon Emissions

A new study with a mouthful of a title, "Decoupling density from tallness in analysing the life cycle greenhouse gas emissions of cities," confirms much of what we have been writing on Treehugger for years—that tall buildings aren't all they're cracked up to be when it comes to sustainability." confirms much of what we have been writing on Treehugger for years—that tall buildings aren't all they're cracked up to be when it comes to sustainability.

Just a few of the posts we've written on this topic include Operating and Embodied Energy Increases With Building Height and We Don't All Have to Live in High Rises to Get Dense Cities and It's Time to Dump the Tired Argument That Density and Height Are Green and Sustainable. But hey, we're just Treehugger—and occasionally the Guardian, where I wrote this piece on cities needing Goldilocks housing density that's "not too high or low, but just right."

The study, written by Francesco Pomponi, Ruth Saint, Jay H. Arehart, Niaz Gharavi, and Bernardino D’Amico, addresses "a growing belief that building taller and denser is better. However, urban environmental design often neglects life cycle [greenhouse gas] emissions." The researchers took into account the embodied carbon from constructing the building, as well as the operating emissions. Their definition:

"Embodied energy and CO2e emissions are the hidden, 'behind-the-scenes' energy and emissions that are used or generated during the extraction and production of raw materials, the manufacture of the building components, the construction and deconstruction of the building, and the transportation between each phase."

The researchers note that "there has been a growing belief that building taller and denser is better, under the idea that tall buildings make optimal use of space, reduce operational energy use and energy for transportation, and enable more people to be accommodated per square metre of land."

But they confirm previous research and discussion on Treehugger, where we noted that as buildings get taller and skinnier, they get less efficient, with a higher proportion of the space lost to stairs and elevator cores, with heavier construction to support more floors. They also found that lower buildings do not necessarily house fewer people.

"As buildings grow taller they need to be built further apart—for structural reasons, urban policies and regulations, and to preserve reasonable standards of daylight, privacy, and natural ventilation. Furthermore, for a fixed amount of internal volume (e.g. expressed in terms of floor area times the inter-storey height), an increase in the building's tallness corresponds to an increase of the building slenderness and hence to a reduction of its compactness, which is detrimental to space optimality."

The study includes four basic urban typologies:

• a—High Density High Rise (HDHR), perhaps Hong Kong
• b—Low Density High Rise (LDHR), perhaps New York
• c—High Density Low Rise (LDLR), perhaps Paris
• d—Low Density Low Rise (LDLR), every other North American city

They then calculated the Life Cycle GHG Emissions (LCGE) for each building type and density, using a 60-year estimated lifecycle.

The results are clear. High Density Low Rise (HDLR) has less than half the Life Cycle GHG Emissions (LCGE) per capita of High Density High Rise (HDHR) buildings, which are worse even than Low Density Low Rise (LDLR). On the basis of the buildings alone, high-rise towers are worse than houses, although the study did not take transportation into account, which has much lower impact per capita at high density than at low. In the end, the study confirms what we have been saying for years:

"When considering LCGE, which encompasses both embodied and operational GHG emissions, the results provide further insight to dispel the growing belief that taller and denser is better."

The lessons of this study are pretty clear. The spiky density that you get in many North American cities, where certain limited areas are zoned for high-rise residential and everything else is very low density detached houses, is actually the worst of all possible worlds. The best form of housing from a life cycle carbon point of view would be mid-rise, what Daniel Parolek called the Missing Middle, and which I called the Goldilocks Density—not too high, not too low, but just right.

This is why Paris is so dense. The buildings are not tall, but there is not much space between them.

Another great example of this is Montreal's Plateau district, where the residential buildings reach almost 100% efficiency with the circulation—those steep and scary stairs—kept outside.

The study also notes that there are other benefits to not building tall towers. This is an attribute of the Goldilocks Density theory. It goes beyond the simple question of density; it's not just about numbers.

"Sustainability is a three-legged stool comprising the economy, the environment, and society: to be truly sustainable, all three must be in equilibrium. Therefore, interdisciplinary considerations that need to be addressed when progressing this work include, for instance, occupant comfort; the urban heat island effect; competing land use; the carbon sequestration effect of green spaces; urban policies; resource consumption; how the urban environment affects crime, etc. Cities are the central hub of modern society and to address these multi-faceted issues a highly multidisciplinary approach seems the only appropriate way forward."

Or as I wrote in an archived post on Treehugger and also in the Guardian:

"There is no question that high urban densities are important, but the question is how high, and in what form. There is what I have called the Goldilocks Density: dense enough to support vibrant main streets with retail and services for local needs, but not too high that people can't take the stairs in a pinch. Dense enough to support bike and transit infrastructure, but not so dense to need subways and huge underground parking garages. Dense enough to build a sense of community, but not so dense as to have everyone slip into anonymity."

There are many reasons to love the streets of Paris or Barcelona or Vienna or much of New York City. But this study also confirms that the low-rise, high-density building form that you see in these cities also has the lowest life cycle greenhouse gas emissions per capita of any building type by a wide margin.

It's not just confirmation bias; this is an important study that challenges the way we zone our cities and the way we build them.