A large part of the world’s population has experienced a dramatic heatwave in the summer of 2018. We have felt the impact of high temperatures and low rainfall, and we have seen dry rivers, empty reservoirs, and wildfires from all over the world on the TV news.
Global warming has already doubled the risk of extreme weather events. Heatwaves that are at least as dangerous as the recent one could happen every other year by the 2040s.
Climate change brings major risks
A 2015 report “Climate Change: a risk assessment” reviewed in depth the risks to the global economy and society of global warming. These include:
- Heat stress for the population
- Threats to crop yields
- Water stress and drought
- River flooding
- Threats to coastal cities and infrastructure from rising sea levels
These risks will threaten the global food market and national and international security.
These are big challenges for society, but I want to focus in on the specific topic of buildings. They face major threats from climate change, and we are not doing enough to prepare for the heatwaves of the future.
Shelter is one of the critical requirements right there at the bottom of Maslow’s hierarchy of needs. Meeting our basic physiological demands and providing a place of safety.
That is what our built environment should do. Protect us, and enable society to operate.
The problem is, we are becoming an urban species. In 2008 for the first time in human history, more than half the global population lived in cities and towns. By 2050, 68% of the world’s population will be urban.
Cities are not prepared for climate change
Urbanisation brings many advantages, but there are also nasty side-effects. Densely populated areas made of hard materials like tarmac, stone, and concrete, heat up rapidly and retain the heat overnight. At the same time, machines and people are throwing out waste heat, increasing temperatures.
This is the ‘urban heat island’ effect. Urban areas can be several degrees warmer than the surrounding countryside both day and night. That can be irritating enough in an average summer. In a heatwave, it can be life-threatening.
There is an upper limit to the temperatures that humans can withstand, especially when the humidity is high. The limit of combined temperature and humidity is a wet-bulb temperature of 35C. At this point, a fit person sitting still in the shade cannot shed heat fast enough to survive. These conditions are expected in parts of South-East Asia before the end of the century.
Temperature and humidity well below this critical point are enough to make it much harder to perform the tasks of daily life and will harm vulnerable people.
Overheating is a real challenge for the world’s cities as global warming continues.
“In the 2003 heatwave, an additional 70,000 people across Europe died because their buildings could not protect them”
We do not have figures for the summer of 2018 yet, but the death toll is likely to be high.
Our plans for adapting to climate change must include making our buildings resilient to heatwaves. For new buildings, governments can require that protection from overheating is part of their construction codes. However, it is not yet happening in most parts of the world. A bigger problem is what we do with the existing building stock. In the UK, 80% of the homes we will be living in by 2050 have already been built, and we will need to refurbish 26 million properties.
The UK has an unusually old housing stock and low replacement rate, but most developed nations have a large number of buildings that need upgrading.
Governments around the world recognise the need to change the way we design and operate buildings, but the main focus has been on improving energy efficiency and reducing carbon emissions to mitigate climate change. Acting to cut emissions and limit climate change is a good thing, but the very features that reduce energy use, improved insulation and increased air-tightness, can make overheating worse. We need to build the risks of higher temperatures into the design thinking.
Air-conditioning is not the answer
Is there a quick technical fix to the problem? Why not just increase the use of air-conditioning? After all, it is an easy upgrade to inert a stand-alone air-con unit in a convenient opening like a window.
The problem is that air-conditioning operates by removing heat from inside a building and dumping it elsewhere. In the case of add-on air-con units, straight into the street. This raises ambient temperatures, particularly at night, and increases heat stress. The more people in a city use air-conditioning, the higher the external temperature and the more people want to use air-conditioning. It is a game that cities cannot win.
Although I have seen a serious suggestion from a professional organisation that we should now be increasing air-conditioning capacity in all new buildings and refurbishments to cope with rising temperatures, I cannot believe that this will be a solution. Cities would have to act to stop the uncontrolled addition of air-conditioning units otherwise buildings will sprout them like acne, and the streets will become unusable.
So if air-conditioning is not the answer, what is?
Many approaches can be used, some suitable for new build only and some applicable to new build and refurbishment. The EPA has published a useful compendium of strategies for reducing overheating in cities.
The strategies include:
- Trees and vegetation – these help to create a cooler micro-climate, shading the ground to stop heating and evaporating water to cool the surroundings. Parks, trees and other green spaces will be essential for future cities, not a ‘nice-to-have’.
- Green roofs – can have a major impact on reducing overheating in buildings. The vegetation and growing medium shades and insulates the roof. Comparative studies have shown that a green roof can lower the temperature at the roof surface by 11C-25C compared to a bare surface. Evaporation of water from plants also cools the roof.
- Cool roofs – roofs made of highly reflective and emissive materials reduce heating from sunlight and shed heat quickly. US studies have shown that cool roofs are up to 40C cooler than conventional roofs, reducing heating of the building and the surroundings.
- Cool streets – The same use of reflective and emissive materials works at street level. In addition, porous or water permeable surfaces allow evaporative cooling.
For new-build we can copy the traditional methods from hotter parts of the world, such as the Mediterranean and North Africa:
- Deeper eaves and verandahs
- Smaller windows, particularly on the south side of buildings
- Thick solid walls that heat up slowly
- White walls
It is hard to change the walls and roofs of existing buildings, but we can use:
- External window shutters
- Solar control coatings for windows
- Brise soleil, awnings and other bolt-on shadings
We must start working on resilience now
Transforming our towns and cities to a low-carbon future is vital. It will help to reduce climate change with all its risks. But we also need to deal with the climate changes that are already with us and will develop over the coming years.
Overheating is a real challenge for the urban environment. It is killing people now, and the problem will get worse. The vast majority of the global built environment will not cope with the predicted increase in the number and intensity of heatwaves.
Governments need to make it a policy priority to improve the resilience of the built environment to heatwaves.
Research should be carried out on the best response to the risk of overheating in different parts of the world.
The construction industry needs to develop the capability to deliver solutions to overheating rapidly, in volume and cost-effectively.
Citizens need to understand the threat to their towns and cities and be prepared to adapt their buildings and the way they live.
More air-conditioning using current technology cannot be a solution.