The Rising Heat
We all know the UK is getting hotter. Global temperatures are rising, and the UK is seeing record-breaking summers and more frequent climate extremes.
The Heat is On... Inside
Why your
energy-efficient home might be cooking you this summer.
A deep dive into 'Indoor Mini-Heatwaves'
in Southwark, London.
The Blind Spot
Currently, a heatwave is officially defined by outdoor weather stations (28°C for 3 days in London). But we don't live at weather stations; we live indoors, where buildings create their own unique weather.
Our Approach
To solve this problem, we put environmental sensors to 40 houses in
Southwark London, during the hottest summer by record in 2023.
These weren't just thermometers. They measured minute-by-minute data on temperature, humidity,
and comfort levels to catch rapid changes that standard weather reports miss.
Introducing the 'Indoor Mini-Heatwave'
“An indoor mini-heatwave is a short, sharp spike in temperature exceeding the safe indoor threshold of 26°C.”
2
Outdoor Heatwaves
VS
5.35
Indoor Mini-Heatwaves (Avg)
During the study period.
Escaping the Comfort Zone
During the study, researchers tracked thousands of minutes of indoor climate data. The Blue Box represents where humans feel comfortable. The Red Dots show where residents actually lived during the heatwave—pushed far into dangerous levels of heat and humidity.
Key Insight: Nearly half of the time spent indoors during heatwaves was outside
the safe zone, pushing into 'Caution' and 'Extreme Caution' levels.
The Nighttime Lag
The heat doesn't sleep when you do. While mini-heatwaves often start around 3:00 PM, many persist late into the night.
Buildings act like batteries, storing heat during the day and releasing it after sunset, disrupting sleep.
The Efficiency Paradox
The study found that homes in good condition and with better Energy Performance Certificate (EPC) ratings actually got hotter. Modern efficiency traps heat inside during summer.
We need to redesign homes for passive cooling (shading, ventilation) and redefine 'heatwaves' to include indoor risks.
Publication & Team
Authors: Maoran Sun1, Jiayu Pan1, Qunshan Zhao2, Ronita Bardhan1
1 Sustainable Design Group, University of Cambridge, 2 Urban Big Data Center, University of Glasgow
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