Extreme heat in London could force major change to new housing developments

Photograph: Alisdare Hickson, CC BY-SA 2.0, via Wikimedia Commons

Londoners face a unique overheating risk in their own homes due to climate change impacting a densely built environment and an outdated planning and design system, the London Assembly has heard.

Extreme heat is becoming an increasing issue in London, with more than a tenth of the 3,271 heat-related deaths in 2022 – which saw record temperatures across the UK – in the capital.

This is due to the city’s increasing density of buildings and roads leading to the urban heat island (UHI) effect, where residents experience temperatures up to 10 degrees higher than in the suburbs or countryside as heat is absorbed there.

City Hall is currently drafting London’s Heat Risk Delivery Plan in a bid to cool down residents; this is despite summers already becoming hotter and drier every year. This could include ‘cool spaces’ around the city, thousands of water refill points and plans to plant thousands more trees.

While this may go some way to protect Londoners making their way around the city, people are still at risk of overheating in their own homes.

Currently, the London Plan encourages new developments to adopt “passive” cooling measures – such as trees, green roofs, and shading – rather than “active” measures, such as air conditioning (AC).

The 2021 London Plan states that the spread of active measures such as AC systems is “not desirable as these have significant energy requirements and, under conventional operation, expel hot air, thereby adding to the urban heat island effect”. Just five per cent of British households have AC units.

On Tuesday (10 March) the London Assembly Planning and Regeneration Committee was told that the “cooling hierarchy” present in the current London Plan – which means that active cooling is only implemented as a last resort – was “unhelpful”.

Ed Hezlet, Head of Energy at the Centre for British Progress, said: “It appears that the passive measures, particularly things like shading are very important. But fundamentally, the real health risks are the heatwave conditions and therefore, passive and active measures are doing slightly different jobs.

“We want passive measures to ensure that people aren’t locked in to properties that force them to use air conditioning more than they otherwise might, that would cost them money.

“But I would say that the active measures are very important with dealing with tail risks of very, very high temperatures, and I think the way the hierarchy is framed puts these two technologies in opposition much more than working together with one another”.

He said that as temperatures increase, a failure to encourage efficient active cooling systems means “people will go out and buy lots of inefficient devices”.

“Therefore, it makes a lot more sense to think about how we incorporate active cooling efficiently in advance”, Mr Hezlet added.

Dr Anna Mavrogianni, Professor of Sustainable, Healthy and Equitable Built Environment at University College London, noted research showing that AC installation rose almost sevenfold between 2011 and 2022 showed it was a phenomenon that “we cannot ignore”.

She supported Mr Hezlet’s point that active and passive cooling should work together, rather than in a hierarchy,

“Passive cooling strategies could potentially empower residents so if there is a blackout there are solutions there that could maintain comfortable indoor temperatures for a longer time and also they could reduce overall cooling loads,” she said.

“At the time when the an active cooling system might be switched on, this could be delayed so we would be looking at shorter periods of time when active cooling might be needed”.

Earlier in the session, Polly Turton from Shade the UK had warned against the “growing demand for active cooling”, which is “putting additional strain on our energy grid and causing a knock-on effect on infrastructure”.

She said the primary issue with overheating was tackling the “huge culture shift” in the development and design sector to more effectively “plan, design and manage our buildings in London” to have better cooling adaptations.

Dr Joel Callow, Founding Director of Beyond Carbon, said current regulations had trapped designers “in a system with contradictory aims”.

He added: “The planning system pushes for daylight criteria, which means larger windows, which goes against cooling criteria. Shading is not a historic part of our architectural vernacular – it’s very hard to convince planners to let us install shading, because it might not look like a British building.

“The principal challenge is that both planning and building regulations cannot both be met. A well-designed, modern apartment with all the windows closed should stay cooler than the outside temperature”.

Dr Mavrogianni noted: “There is a lot of emphasis on having large glazing areas with no provision for shading or solar control. Windows can be a significant source of solar heat gains – having an unopenable window can contribute to overheating”.