A collaboration between the University of Glasgow’s Urban Big Data Centre (UBDC), London residents, the Bureau of Investigative Journalism (TBIJ) has shown that some homes routinely go over recommended temperatures, even on comparatively cool days. 

The Hot Homes project was introduced in response to concerns that housing in the UK is not designed to cope with rising temperatures.

In 2019, the UK’s Climate Change Committee warned that homes in the UK ‘are not fit for the future’ and that attempts to adapt housing stock for higher temperatures are ‘falling far behind the increase in risk from the changing climate’. But until now there has been little data gathered to show how people experience heat in their homes.

For six weeks, from late July to early September in 2023, TIBJ and UBDC fitted 40 homes in Southwark – one of the hottest places in the UK according to our analysis of Met Office data – with temperature and humidity sensors. These were the findings:

  • Every single home monitored went over the 25C limit (the World Health Organisation maximum recommended indoor temperature for London)
  • During the mini-heatwave in early September (4-10 September), 85% of participating households saw temperatures over 27C. Ten of the homes experienced temperatures over 30C.
  • One household saw a sustained heat period with no break for 22 days, with the temperature averaging 27C - despite this not being an especially hot summer. Other homes saw sustained heat periods for 15 and 16 consecutive days. 
  • Some homes were as much as eight to ten degrees hotter at night over the full monitoring period than the outside reading at local weather stations.
  • The Heat Stress Index factors in relative humidity as well as temperature and describes how temperatures are likely to be experienced by human bodies. Using this, almost all of the participating homes experienced conditions that felt like 27C (described as ‘Caution’ in the Heat Stress Index), and almost half experienced conditions that felt like 32C (described as ‘Extreme Caution’ in the Heat Stress Index, with increased risk of heat exhaustion and heat stroke).
  • Many recent builds were affected and represent some of the hottest homes in the study
  • Five people who participated described their home feeling like a sauna, three described their home feeling like a ‘greenhouse’ and others used the terms ‘heat chamber’ and ‘oven’.
  • In the first week of our monitoring period, which was not a heat period, 80% of our participants experienced at least one symptom of heat exhaustion, and several people had to seek medical help that they attributed to the heat in their home.
  • Two thirds of participants who responded to our surveys at the start of the monitoring period said the heat at home was affecting their physical or mental health. During the heat period in early September this rose to more than four out of five people.
  • Almost 70% of our participants said ventilation issues were causing their homes to overheat, while 71% said that lack of shade was a major factor.
  • Another aggravating factor is communal heating systems, or district heating: networks of pipes that deliver hot water and heat to individual properties, and which residents often cannot switch off. Our study found the homes affected by these systems to be more than 1C hotter on average, compared to homes without these systems, this summer. (TBIJ reported earlier this year on the communal heating issue.)

Dr Qunshan Zhao, senior lecturer in urban analytics at UBDC, said that while heat has not historically been a major problem in the UK, it is now becoming an “upcoming and urgent issue”. He also believes it is urgent to establish better citizen sensor monitoring systems in homes to fill the existing data gaps in indoor environment.

“I think the most surprising part of this analysis is how many households experience indoor heat stress in London, even in a not extremely hot summer.”

Professor Anna Mavrogianni from University College London’s Institute for Environmental Design and Engineering, said the findings, “clearly indicate that overheating in London homes is not a future problem, but a current and ongoing issue, in particular in more energy efficient, newly built constructions, which will be exacerbated as our climate becomes warmer.”

“Prolonged exposure to high indoor temperatures, especially when there is no night-time respite from heat, can result in a range of adverse health effects, especially for the most vulnerable populations groups, such as babies and young children, and older people, or people suffering from cardiovascular, respiratory or mental health conditions.” 

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First published: 6 December 2023

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