In today's episode we are asking, are tall buildings sustainable? It is often argued that because of their density, tall buildings are essential for sustainable cities to support public transport, walking, cycling and thriving communities. However, once you start looking into it, it is far more complicated than that.
In this episode, tall buildings are analysed based on their environmental, social and economic sustainability with some surprising results.
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Headspace: The Psychology of City Living
According to the New London Architecture annual Tall Buildings survey of London, in 2020 there was 525 tall building projects in the pipeline, with 89 under construction across the city. That is equivalent to about 100,000 homes.
Note: In this episode, particularly when I'm talking about the economic aspects, I am very much leaning on my experience of the UK context. In other parts of the world, different planning systems and market conditions may well make some of my points less valid. But I think on the whole, there is a lot of general principles in this episode.
One of the challenges of talking about tall buildings is that it is a very broad typology. We could be talking about a 10 storey modernist housing estate or a 50 storey high end office building. These two things are very different so we need to be careful about the language we use so we can bring a bit of nuance to the topic.
At Urban Initiatives Studio in London, where I work as an urban design consultant, one of our areas of expertise is tall buildings. We do a lot of work consulting with cities and developers about the strategic role of tall buildings in cities. Over the years we've developed a way of categorising tall buildings, which is based on their height relative to the height of buildings in the surrounding context. So for instance, if you're in an area where all the buildings are two storey homes, then a 6 storey building will appear as a tall building, because it's three times the context height. Whereas that same building in a city centre of 4 or 5 storey context height would not stand out at all. Typically once you get a building that is more than double the context height, you can start calling it a tall building.
So this is a good way of defining tall buildings from an urban design point of view because it takes into account the characteristics of the place. However, in this episode we also want to talk about energy efficiency and the carbon footprint of tall buildings. From a construction point of view, a six storey building is never tall, it's just a conventional building. Once you get to 10 storeys, buildings really start to take a different form, and require different construction techniques, so you might consider this to be tall but there's no clear definition. From there, a skyscraper is typically defined as above 40 storeys.
It's pretty confusing right? I'll make sure to flag up what sort of scale tall building I'm talking about as we go through the various topics in this episode.
Purpose of Tall Buildings
Before we get into the detail, it's worth taking a step back and considering what the role of tall buildings are in cities. This is a huge topic that is much debated but I'll try to give a succinct summary of my thoughts on this. First of all is the idea of tall buildings expressing power and importance.
Because these tall structures also tended to be located in central locations, they serve another purpose, which is navigation, wayfinding and legibility. Think about how humans found our way before google maps and even before paper maps. We relied on objects we could see and recognise to place ourselves in context. So, for instance a mountain or a river or even the stars, people would navigate by the north star. We do the same thing in cities. We look around for things we recognise to help us navigate. Tall buildings that are centrally located and immediately recognisable are the perfect wayfinding tools because you can see them from all around and you always know where you are in relation to them.
It's worth noting too that throughout history, most tall buildings were not about increasing density. A cathedral does not provide more habitable rooms or business space. All this changed with the advent of the skyscraper in the late 19th century. At this time, architects in North America started figuring out how to construct buildings of 10 storeys and higher using steel frames. This meant they could be constructed relatively quickly and cheaply and have usable floor space on every storey.
Now tall buildings have become so prevalent in some cities that they no longer fulfill the role of marking significant places or functions or assisting with wayfinding and making a city legible. They could be located in any random place. And far from being centres of religious worship or societal importance, they are often anonymous office or residential buildings. Hardly something to celebrate by taking up prominent skyline space. However, our brain still expects them to fulfill these roles. And they can if they are carefully located and designed.
Okay, there's my canned history of tall buildings. Let's move on to the question at hand. Are they sustainable?
Let's start with carbon emissions. How do calculate the carbon footprint of a building? Well we need to look at both embodied carbon, which is the emissions related to all the materials and construction of the building. And let's look at operational emissions, the energy use associated with heating, lighting, air conditioning and everything else in a building day to day.
In a Guardian article from 2020, Tim Snelson, an engineer from Arup, reports that they have calculated that a skyscraper will typically have double the carbon footprint over its lifetime compared to a 10 storey building of the same floor area. So at about 10 storeys you're still talking about a fairly conventional building in terms of its construction. Once you get up to skyscraper level though, the structure changes dramatically. Tall buildings are inherently unstable, having a small footprint compared to their height, and have to fight much more against wind. So this translates into a lot more material, steel and concrete, in the foundations and structure, and more complex construction methods compared to conventional buildings. That adds up to significantly more carbon emissions before the building even opens for business.
A study from University College London found that the operational energy demand of tall buildings is also much higher than mid-rise buildings. This is particularly true of office buildings. In the study they compared hundreds of buildings across London and found a clear trend where, on average, an office building of 20 storeys will have 2 and a half times the electricity consumption per square metre compared to a 6 storey building. It's suggested that tall buildings likely require more heating and cooling because they are naturally exposed to cooler air, higher wind speeds and more direct sunlight. The need for vertical transportation like elevators also contributed to greater energy use. When they compared residential buildings they found a less stark difference but there was still a clear relationship between more height and greater energy consumption.
Now consider that buildings and the construction industry is estimated to account for 39% of global emissions. So we need to take measures across the board to reduce emissions from embodied carbon, construction and the operation of buildings. From this perspective, it makes sense to avoid tall buildings whenever we can.
I want to continue on the environmental theme for the moment and bring up the issue of ecology, and specifically the problem of bird deaths in cities. Glass collisions are responsible for billions of bird deaths every year. It is second largest human-made hazard to birds after habitat loss. Tall buildings are the main culprits because they extend further into bird flight paths and tend to use a lot of glass. At night, lights on tall buildings confuse birds, who use stars and the moon to navigate.
The final environmental consideration I want to bring up is the idea of adaptability and what happens to tall buildings if they become obsolete. Look at any well preserved historic building in a major city and you will find that it has probably been through multiple uses and internal arrangements over the years. Think of victorian terraces and European courtyard style buildings. If they are too large for one home, you can subdivide them into flats. If they are too small you can knock through some walls and combine two buildings together. If you need an office space, they can be adapted to that from residential use. These typologies are infinitely adaptable, which is one of the reasons they have survived hundreds of years and are still relevant in cities today.
Tall buildings, however, are a very unique and specific building form that often cannot be easily adapted to other uses if necessary. For example, modern office buildings tend to to have high ceilings and very deep footprints. This makes them difficult to adapt to residential use.
Why is this important? Well, cities are changing all the time and especially in the post-covid-19 world we may see cities being used quite differently. This means that at some point we could see certain tall buildings lying empty as their original use is no longer necessary or profitable. If our only option then is to demolish them and build something different then we are wasting huge amounts of energy, materials and emissions. From a sustainability perspective, we want to be reusing and adapting existing buildings as much as possible. Especially if the buildings are only a few years or decades old.
You might say, but what about density? Cities need to be dense to be sustainable. Density from tall buildings allows us to build upwards rather than outwards, avoid suburban sprawl and support sustainable transport modes.
Really the question here is can you have enough density without tall buildings or are they necessary for this vision of cities?
This is a difficult question to answer in an abstract sense. It really depends on context. Of course if you have a situation where there are only a small number of constrained development sites available and you absolutely have to increase densities for some reason, then yes, tall buildings might be the only typological solution to that.
However, usually this isn't the case. Generally speaking, mid-rise development at a human scale, no higher than 6 storeys, can easily deliver over 100 homes per hectare on substantial sites, which is a high density compared to typical suburban area. In fact, here in the UK, areas of traditional terraced housing are often above 100 homes per hectare even though they are only 2 or 3 storeys in height. And researchers have identified that on many sites, courtyard style blocks, which you find all over Europe, can often achieve the same density or greater compared to tall tower-style buildings. Density is more about efficiency of space rather than height and can take many forms. And tall buildings by their nature are actually quite inefficient with space. They require large reception spaces, elevators and internal servicing spaces that means the amount of space you can actually use for living or working in gets smaller in comparison to the structure as a whole.
What is also important is achieving density over a wide area as this can play a role in supporting a range of businesses and transport options, and encourage low carbon lifestyles and successful local communities. In contrast, a single lonely tall building in a sea of low density sprawl is just injecting a superdensity into a small area but if you calculate the density for the entire neighbourhood, it would still be pretty low. Often this one tall building will not be enough to support good public transport links or a local high street. Many tall buildings outside of major cities have huge multi-storey car parks attached to them, because their residents are tied in car dependency, which puts huge pressure on the local road network and adds to congestion.
Use and Economics
I've been talking about density in a slightly abstract sense so let's now think about what tall buildings can actually deliver. Not just are they residential or office buildings, but are they actually delivering what communities need?
Many large cities across the world are currently facing housing shortages, resulting in high rents and long waiting lists for social housing. One of the arguments that is often used here in the UK in favour of tall buildings is the fact that they can deliver a huge number of new homes in one development, relatively quickly. Planners love this because these just get added to a spreadsheet and they can say they are making good progress towards their housing targets.
But is this really the case? In world cities like London and New York, exquisite landmark tall buildings are often developed in prime locations, not to provide decent homes for normal people, but as luxury investment vehicles for wealthy internationals. A 2017 report from International Transparency UK analysed 14 luxury residential developments in London, comprising just over 2 thousand homes and found:
"almost 80 per cent of properties were bought by overseas investors. 40 per cent of properties sold over the 14 developments were to individuals from high corruption risk jurisdictions, or to companies based in secrecy havens".
To understand the scale of this issue, consider this: The Guardian newspaper reported in 2016 that London had 54 thousand flats priced at over 1 million pounds, many of which in tall buildings. All this in the middle of a housing crisis where ordinary Londoners struggle to afford a mortgage far outside the city.
Clearly this problem isn't just about tall buildings. Investors buys homes of all shapes and sizes. But the fact that tall buildings have mush bigger carbon footprints and a much greater impact on the city in terms of skyline, overshadowing, creating a poor microclimate you would expect them to give something back to society beyond being full of expensive flats that are empty most of the time because their owners live elsewhere. In these situations, tall buildings become very difficult to justify from a sustainability point of view.
Even when we are not talking about the super high end of the market, tall buildings are still more expensive to construct than conventional buildings for the same reasons that they are more carbon intensive, and this means that the end product, the apartments or office space, must sell at a premium. This means the viability of tall buildings is very narrow - it's a lot of up front cost and you have to be sure you will get your money back. That's why tall buildings tend to proliferate during an economic boom and they disappear during economic downturns. In fact, it was not uncommon after the 2008 financial crisis to see the skeletons of unfinished tall buildings towering over cities, as the credit available for funding them disappeared during construction.
So there's a real question of whether tall buildings are financially sustainable when they are so sensitive to market conditions, and in some locations with lower market values, they may never be viable. It's difficult to rely on them if they are a core part of our future vision for a place.
However, when they are viable, tall buildings can play a key role in regeneration. On large urban regeneration sites, the kind that must be developed in phases over many years, often developers will plan a tall building or buildings in the very first phase. They do this for 2 reasons:
Firstly, the revenue from selling off units in the tall building can then be used as seed funding for the next phases of development, which may be of a lower scale. This makes financial sense, and may make it more likely that the whole project will get built out successfully.
Secondly, tall buildings are often used as signals of economic strength and vitality. They can help to attract investment because they convey confidence in the market and the project. This comes back to my short history of tall buildings at the beginning of the episode. We are culturally hardwired to perceive tall structures as having importance and prestige, so this can help to improve the image of the part of the city that is undergoing regeneration. It's a kind of marketing tactic.
Two great examples of this is the Kings Cross regeneration in London and Porta Nuova in Milan. Both very large sites that lay vacant for a long time in parts of the cities that were perceived as being unattractive and rough. Making use of some beautifully designed landmark tall buildings help to dramatically change the character of the areas and signal that change was coming.
So I do think tall buildings can play a role in the viability equation, and also in creating an exciting new sense of place.
Let's move on now and think about the social dimension of tall buildings. In most situations, in most cities, tall buildings will not be sold to foreign millionaires. They will be more accessible. They may command a premium because of the prestige of living high up with a view but they will generally be bought and rented by ordinary people or businesses. So in this situation, can we justify tall buildings? Here is where we get into the social element of sustainability. For cities to be truly sustainable, they also have to be great places to live where people can have a good quality of life. Sometimes people use the phrase livability to describe this concept.
So, do tall buildings create good living environments. In part, this is a question of good design. We could point to exemplary tall buildings that demonstrate how we can provide an excellent quality of life in a high rise form.
But let's be honest, those examples are very much in the minority. So if we are asking the question of whether tall buildings create livable environments, our assessment shouldn't be skewed by either the expertly designed, high end luxury apartments or the disastrous post-war housing estates that are associated with crime and deprivation. Let's look at the average, ordinary tall building you would find anywhere. Nothing special, but also not particularly offensive.
Luckily there is quite a body of research on the impact living in tall buildings can have on our physical and mental health. I am pulling information here from an interesting book on the psychology of city living called Headspace by Dr Paul Keedwell and a research article from 2017 from titled The Sustainability of Tall Building Development, a Conceptual Framework. Links are in the description.
Interestingly it seems that tall buildings can affect people differently depending on their personality. So for instance, people who would be described as neurotic, meaning they are more prone to negative emotional states like depression and anxiety, unsurprisingly tend to be experience more anxiety living in high rise homes rather than lower down. This may be because there is a perceived greater risk of being victim to disasters like fires or earthquakes. Living high up can also create a sensation of being trapped and disconnected from other people.
In fact, this feeling of disconnectedness may well be borne out in reality as residents of tall buildings tend to know fewer neighbours and have less community ties compared to residents of low rise development, or even just people who live lower down in tall buildings. Research suggests that this is particularly challenging for extroverted people, who crave social contact. Whereas introverted people may be less bothered by the relative isolation.
Quoting from the book Headspace, the author says
"We know that having a social network buffers against stress and negative moods, so it would not be surprising if the relative lack of relationships higher up were to cause us to feel less content. It is also an interesting insight that people who live on higher floors are less likely to exercise. Although they have access to a lift, it is as if the slightly greater time and effort required to get to street level is enough to tip the balance toward inaction. This then deprives them of yet another proven buffer against stress, anxiety and depression – vigorous physical exertion."
Often it is argued that tall buildings are not suitable for families. And indeed, at least in the UK, most tall building developments are largely made up of one bed and studio apartments, being squarely aimed at young professionals and couples. And there may be good reason for this.
Urban psychologists have suggested that living in tall buildings can be detrimental for children's development. The argument is that it's important for young children to experience ever increasing amounts of freedom to go outside the home, experience the real world (the neighbourhood, a park, corner shop), and then return to the safety of home. This experience is only possible in low rise urban forms where parents can keep an eye on the street and be physically close to the outside. As the author says, "This interplay between dependence and autonomy that earns a child a sense of competence is missing in high rise environments".
High rise developments also tend to lack adequate open space and access to nature, elements very important for both childhood development and the physical and mental wellbeing of adults.
A research report by London School of Economics titled Residents Experience of High Density Housing in London, found that one of the key elements that made a successful high density development was the inclusion of an integrated high quality green space for residents. When these are well designed, they are well used by residents. However, when they are hard-surfaced windswept and heavily overlooked, they tend to be deserted. The report also mentions that roof gardens are generally not used very much by residents, partly because many residents don't know they are allowed to use they - they don't feel like a public space, and partly because of restrictions of use. You can't have a barbecue, kick a football around or cycle a bike on a roof terrace.
Okay, so it's looking like there are a lot of challenges for creating good living environments for the residents of tall buildings. Of course some of these issues can be mitigated by design and that is an important lesson to take from this.
How about the effect of tall buildings on the wider city? I could speak at length about the urban design implications of tall buildings, how they affect our perception of cities, affect microclimate, and the common issues around dead frontages and lack of public life around tall buildings. But to avoid straying too far from the topic at hand, which is sustainability, I will leave you with one final thought.
Many of you listening will be familiar with the Urban Heat Island effect, which is where urban areas heat up more than the surrounding countryside, which can lead to deadly heat waves and much more energy use through air conditioning.
Well, the height of buildings also has an affect on the urban heat island. The increased heat isn't just felt at ground level, heat rises after all. And the height of the heat island is generally about 3-5 times the average building height. So high rise cities have a more extreme version of the heat island affect and have more trouble cooling off during heat waves. This has knock on effect on our mental and physical health as well as emissions due to air conditioning. If you want to learn more about this topic, I covered it in detail in episode 4 of this podcast.
Okay, so that is basically everything I wanted to cover. I know there is a lot of negativity in this episode. I think it's important that we are honest about these things if we are to move our thinking beyond simply "density is good. More density". The shape of our built environment is really important.
Despite all of this, I am actually not anti-tall building. I think they have a role to place in cities just as they always have. But they need to be exceptional and they need to fulfilling a purpose that necessitates height, rather than being tall for the sake of it. We shouldn't allow tall buildings to become the norm.
There is one final question I have, which is what is the role of tall buildings in the post-covid world? People are realising that they can work remotely and don't need to be crammed into small flats near city centres. Surveys tell us that people place a much higher value on having their own open space and being close to public parks when they are spending more time at home. Businesses are also realising that they don't need to pay expensive rents in landmark office towers if half their staff are working from home at any time. It will be very interesting to see how tall buildings fit into all this. I guess only time will tell.