Millions of India’s urban residents are living in “heat traps” where indoor temperatures peak long after sunset, according to a study.
The research, released recently by Climate Trends at the India Heat Summit 2026 warns that India’s current heat governance, which relies almost entirely on outdoor daytime data, is failing to capture the severe health risks faced by residents who endure indoor temperatures above 31°C throughout the night.
The study, titled Nighttime Thermal Stress in Low and Middle Income Housing in India, used high-resolution sensors to monitor 50 households in Chennai between October 2025 and April 2026. The findings reveal a worrying new picture of urban life: indoor spaces reached their peak temperatures not at noon, but between 8 p.m. and 9 p.m., as reinforced concrete (RCC) structures released heat stored during the day.
The 8 PM Heat Peak: Why Urban Homes Stay Hot After Dark
Reinforced cement concrete (RCC) used in roofing and walls acts as a thermal battery, prolonging indoor heat exposure long after the sun has set. Because the building materials continue to release stored heat, temperatures remain above 34°C well into the night. This “nocturnal warming” is intensified by relative humidity, which remained consistently above 75 percent during the study, hindering the body’s ability to cool itself through evaporation.
The ‘Binary’ Cooling Gap: How Income Determines Heat Resilience
Researchers identified a “binary cooling gap” driven by income rather than environmental exposure. While nearly all monitored houses used similar heat-retaining concrete materials, high-income households mitigated the stress through universal access to air conditioning. In contrast, every low-income household in the study relied solely on ceiling fans, which were unable to provide relief in high-humidity conditions.
The following data highlights how different construction and housing types influenced these thermal extremes:
|
Construction Type
|
Location Type
|
House Type
|
Mean Temp (°C)
|
Max Temp (°C)
|
Min Temp (°C)
|
|---|---|---|---|---|---|
|
Pucca (Concrete)
|
Commercial
|
Shared Housing
|
42.7
|
49.6
|
35.3
|
|
Pucca (Concrete)
|
Commercial
|
Apartment
|
41.5
|
47.6
|
34.2
|
|
Pucca (Concrete)
|
Residential
|
Independent
|
33.7
|
48.5
|
21.3
|
|
Pucca (Concrete)
|
Residential
|
Shared Housing
|
33.6
|
53.4
|
21.3
|
|
Semi-Kuccha
|
Mixed-Use
|
Independent
|
29.8
|
34.6
|
24.2
|
Eight Months of Continuous Heat: The Reality of ‘Baseline’ Exposure
The scale of exposure is chronic. The worst-affected households recorded up to 5,800 hours of exposure above 32°C—the equivalent of eight months of continuous heat—even during the cooler months outside of peak summer. Residents reported widespread disrupted sleep and persistent fatigue, confirming that indoor environments offer no meaningful respite for physiological recovery.
Expert Perspectives from the Heat Summit
Aarti Khosla, Founder and Director of Climate Trends, emphasised the urgency of shifting policy focus toward the home. “The study brings into focus how nighttime heat retention must also receive attention,” Khosla said. “That its residents must also face chronic heat exposure indoors, and have it affect their sleep and recovery periods, is a matter that seeks urgent interventions.”
Addressing the summit virtually, Pralhad Joshi, Union Minister for New and Renewable Energy, acknowledged the crisis. “Rising temperatures and extreme weather events are realities affecting our cities, villages, economies, and our daily lives of citizens,” Joshi said. “Heat stress has emerged as one of the defining challenges of our times.”
Dr. Naveen Puttaswamy, Associate Professor at SRIHER and study co-author, highlighted the hidden toll on residents. “Most people spend the majority of their time indoors, where they face the dual threats of heat stress and poor air quality,” he noted. “These exposures carry uncertain physiological trade‑offs and add psychological as well as economic strain on individuals and households.”
Study Limitations
While the study provides rare high-resolution indoor data, researchers noted several limitations. The study was restricted to a small sample size of 50 residential units and was geographically specific to Chennai, meaning thermal patterns might differ in cities with drier climates. Furthermore, the absence of continuous outdoor temperature measurements limited the ability to directly compare exactly how much hotter the indoors were relative to the outdoors at any given moment.
Beyond Outdoor Data: The Case for Policy Reform
The report argues that India’s 300+ existing Heat Action Plans are insufficient because none currently mandate indoor temperature monitoring. The authors are calling for updated building codes that require cross-ventilation and the promotion of “climate-responsive” materials like compressed stabilised earth blocks to replace heat-absorbing concrete.