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The urban heat island - it’s actually pretty cool

The urban heat island (UHI) is misunderstood, much maligned and quite probably a good thing explains Director, Sustainability and ESG, Barny Evans in his latest blog on what the UHI is and why it’s often misconstrued as wholly negative.

I was supporting a major housing development recently in a semi-rural location in central England when I noticed the local policy requires the minimisation of the urban heat island – it got me thinking about the topic and how misunderstood it is.

What is the UHI?

The UHI is the effect of higher temperatures experienced in urban areas compared to rural areas. The effect is most noticeable in the evenings. The causes are the internal heat absorption / generation of cities; vehicles, cooling systems and particularly the large amount of concrete/metal/asphalt. These surfaces absorb a lot of energy and then reradiate it later, hence the effect being most prominent in the evening when they have absorbed a lot of energy. 

Why is it not necessarily a negative?

In most of the UK, the UHI is not significant; unless you are in a major city or airport, the difference is small. In the centre of our big cities, the difference can be big - several degrees. This contributes greatly to the issue of overheating in cities, particularly in new build flats. When I looked at this issue in 2015 and undertook a survey, 1 in 10 Londoners said their homes were uncomfortably hot most of the time and 80% of people suffered from overheating sometimes (this is not all due to the UHI, the focus on heat networks in London has been a major contributor).

Since then, much has changed; most authorities have policies for new development to minimise it and England and Wales have Building Regulations that require homes are designed to minimise the risk of overheating. Sophisticated modelling is used to assess methods to reduce the risk, and this encourages solar control glazing, shutters, external blinds, larger openable windows, etc.

We can all agree that we should be designing homes to minimise the risk of overheating, but what if by reducing the broader UHI we are throwing the baby out with the bathwater?

If you look at the statistics, around 2,000-4,000 people a year die of overheating in the UK, whilst an average of 30,000 die due to the cold. The less extreme issues of discomfort and health effects have fewer clear numbers but are likely to be similar. The immediate implication is that the UHI may save more lives than it costs. Allowing for climate change, a study (even with the unrealistically high RCP8.5 scenario) suggests the UHI still saves lives.

At the less extreme end, there is the issue of quality of life. Most British people would prefer to spend more time outdoors; we see this in warmer weather, when we see so many more of us outside, even though we could stay in. That has huge benefits in terms of socialisation, physical activity and human happiness. The UHI helps city dwellers have more fun.

Of course, the corollary of that will be increased risk of uncomfortably hot homes. The answer is not difficult, however, and we are halfway there. We should be designing new urban flats to minimise the risk, and the new Building Regulations offer a method to ensure we are using passive measures to reduce the issue- we really have got to adopt external shutters; so effective and so stylish!

Time to embrace cooling in urban flats

This brings us to the controversial need to embrace cooling in urban flats. This is strongly discouraged by authorities and resisted more broadly by people like me, advising on sustainable development. However, I want to make the case. As almost all new development and retrofit now uses heat pumps, they can come with the ability to provide cooling as standard. With an ambient loop internally, you can even reclaim some of the expelled heat. Cooling could be restricted to when there is evidence of a threshold of risk. Interlocks can ensure that it is only used when windows are closed and limiting the cooling to, say, 24°C would mean the cooling load would always be very small.

Cooling is actually very easy to integrate into a net zero energy system in the UK. Unlike heating demand, which rises as solar energy generation declines, it is almost always very sunny when it is hot in this country. That means that when the cooling is needed most there is a lot of solar power, or it will be happening when daily energy storage can meet the need.

I would rather we use that extra bit of clean energy so someone can have a good night’s sleep. The cooling also protects if climate change leads to even warmer conditions than predicted.

Part of my reason for writing this piece is as a general plea for more logical thinking about policies and positions. The UHI is just a small example of where we need to focus on data and consider trade-offs. When you think about it, the urban heat island is a pretty cool thing in our country.

For more information, please get in touch with Barny Evans.

This article was originally published in Building.

25 September 2024