Sustainability in Cities: High Rise vs Low Rise Buildings
Close your eyes and picture a sustainable city. You are probably looking at a green city with urban rooftops and green walls, more pedestrians and cyclists than cars, and perhaps buildings with solar rooftops generating their own energy. Now, focus a bit more on the constellation of these buildings and think about what you see: how dense is the city and how tall are the buildings?
Do you see a city like New York with a line of skyscrapers drawing the horizon or do you see a Parisian city with its mid-rise Hausmannian buildings?
While it was long believed that high-rise buildings were the more sustainable option, as they require less land use and result in more efficient consumption, new research from a team at Edinburgh Napier University challenges this view by considering the impact of density and building height on sustainability in cities.
Density is key.
Denser urban areas make for more sustainable areas. Dense cities are typically more walkable and more accessible by public transport, with each area being better connected to other areas of the city. The flagship example of a dense urban neighborhood is Eixample in Barcelona. This bird’s eye view will easily show you why:
Here’s some background on this neighborhood: In the 1850s, Barcelona was going through unprecedented population growth, and the city was on the brink of collapse with mortality rates twice as high as other European cities, such as Paris or London, due to poor urban living conditions. The life expectancy in Barcelona dropped to 36 for the richer and 23 for the working class. Confined between medieval walls, housing the booming population became critical. In comes Ildefons Cerda, a Catalan engineer who put forward a radical expansion plan consisting of expanding the city to large grid-like districts outside of the city walls.
The opposite of a dense city is a phenomenon known as urban sprawl, which urban planners try to avoid at all costs. Urban sprawl happens when cities grow haphazardly away from their centers over previously undeveloped land in low-density patterns, into suburbs.
Urban sprawling is problematic for many reasons. With an expanding urban area inevitably comes the issue of connectivity: the city is not walkable and less accessible via public transport, hence the population is incentivized to use private cars instead. This increases fragmentation and the per-capita costs of providing key public services such as water, electricity, public transport, or waste management substantially increase. For these reasons, density is key.
Amplifying density with high-rise buildings.
When thinking about your sustainable city, you may have imagined a skyline filled with impressive high-rises. However, when it comes to sustainability, the height of a building does not dictate its superiority.
A higher embodied carbon footprint.
High-rises require heavier structures and chunkier foundations, which often means that significantly more materials and resources like concrete or reinforced steel are needed, which come with a high carbon footprint. By itself, concrete is responsible for a whopping 8% of global CO2 emissions. As a result, the embodied carbon — the carbon which is emitted before the building becomes operational — of high-rises is much higher than that of low-rises. Looking at the building structure only, the carbon footprint of high-rises is on average around 250kgCO2/m2 compared to 180kgCO2/m2 for low/mid-rise buildings. For the building as a whole, this translates into an embodied carbon which is 40% higher for high-rises than for low/mid-rise buildings for the same area.
Higher emissions in the operational phase.
Operating a high-rise building is very energy-intensive. Typically, the main energy consumption of low/mid-rise buildings comes from six main loads: heating, cooling, lighting, plug loads, fans, and water heating. On top of that, high-rise buildings have three additional loads from elevators, water pumps, and heat rejection. For the building as a whole, this translates into embodied carbon which is twice as high for high-rises than for low/mid-rise buildings.
Density x Height
To compare the environmental impact of different types of urban constellations, it makes sense to adopt a lifecycle approach and look at the LCGE (or lifecycle greenhouse emissions) which measures the GHG emissions of the full lifecycle of buildings from the construction phase to the operational phase.
To facilitate the comparison, let’s look at four types of urban constellations:
- High-rise, high-density, think New York
- Low-rise, high-density, think Paris
- High-rise, low-density, think US cities such as Las Vegas
- Low-rise, low-density, think suburban areas
Keeping the population size constant and looking at the LCGE, what we see is that urban areas consisting mainly of low-rise buildings have a far lower environmental impact than those consisting mainly of higher-rise buildings, as we’ll dive into after.
Of the four types, the worse constellation is a high-rise and high-density urban area. It incurs an increase in the LCGE ranging from 27 to 77% compared to other types of constellations. High-density and low-rise constellations seem to be the best option delivering 2.8 times less carbon with only 19% less population housed on average than the high-rise, high-density comparable. This relationship is illustrated in the graph below.
And more interestingly, comparing two urban areas of similar population, moving from a low-rise constellation to a high-rise constellation incurs a 140% increase in the LCGE.
Looking at this staggering difference in LCGE between these four constellations, our conclusion is that the environmental impact of high-rises is so substantial that it overcompensates on the density factor. In other words, density matters, yes, but nowhere close as much as the height of the buildings.
Now, you might be thinking, but shouldn’t high risers allow for more density? After all, more floors = more people per building, right?
Busting the density myth.
Well, it’s a bit more complicated than that. Did you know that Paris has a higher population density than New York? Its density amounts to 20,515 inhabitants per square kilometer, while New York’s is only 11,314 inhabitants per square kilometer.
Let’s broaden this scope and look at where the 50 densest cities are located.
Some of the densest cities like Tokyo or Shanghai are also those with many skyscrapers but so are those with mid-rise buildings like Madrid or Barcelona. Indeed, there is no direct correlation between the height of buildings and the density of cities.
You might be thinking… HOW?!
For that, let’s take a closer look at the floor to area ratio, which looks at the relationship between the total amount of usable floor area of a building compared to the total land area on which it is built. Looking at the graph below, we can better understand why two buildings of different height could have the same floor to area ratio:
High-rises need bigger structures, and due to privacy, public policy, or daylight access reasons, they often cannot occupy the whole land area, which can be the case for mid-rise buildings. Hence, whilst high-rise buildings take up less land surface, in terms of floor to area ratio, high rises are less efficient than compact city blocks.
Summing it up
In conclusion, when it comes to sustainability in cities, density is key. While high-rise buildings may seem like an efficient use of space, their larger environmental impact outweighs the benefits of their height. Low-rise buildings, when designed with a high floor to area ratio, can achieve similar levels of density as high-rise buildings without the same carbon footprint. By prioritizing density and sustainability in urban planning, we can create cities that are not only more environmentally friendly but also more livable and accessible for everyone.
