Four heats in a home
A heat warning gives one number. A family under a tin roof lives with four, and the worst of it often comes after dark.
- Outdoor air. The temperature a weather station measures, in the shade. It is the number in almost every heat warning, but not what people feel inside a tin-roofed room.
- The roof. Sun-baked sheet metal gets far hotter than the air. Roofs of informal homes in Mumbai and Pune measured above 50 °C when the air was about 30 °C.
- Radiant heat. A hot ceiling gives off heat onto the people below, like standing near a stove, so a room can feel hotter than its air. This is what mean radiant temperature measures.
- Indoor air. The air inside, held in by thin walls, few windows and crowded rooms.
At night. After sunset the street starts to cool, but thin-roofed homes cool slowly. In low-income homes measured in Dhaka, Delhi, Faisalabad and rural Maharashtra, temperatures in tin-roofed houses swung 3–4 °C more over the month than the outdoor air, and stayed warmer indoors because they cooled so slowly at night.
7.6 °Chotter, at most, for people in Ahmedabad’s informal settlements than for people in formal housing, in a survey of 860 households.
Up to 9 °Chotter indoors than outside, in pilot measurements in informal settlements in Kenya, Tanzania and Zimbabwe.
2–5 °Ccooler indoors under reflective roof paint than under ordinary roofs, in Ahmedabad’s informal settlements.
Indoors is not always hotter at midday: in an experiment in The Gambia, metal-roofed houses were about 1.4 °C cooler than the outdoor shade by day, but warmer at night. The danger is nights that do not cool.
Eight questions for your heat action plan
- Does your heat warning count hot nights, not only daytime highs?
- Do you know which neighbourhoods live under metal, asbestos or plastic roofs?
- Have you measured indoor and night-time temperatures in a sample of those homes? Low-cost loggers can do it.
- Is there a cool-roof programme for informal settlements, and does it reach renters?
- Are cooling spaces open in the evening and at night, near where people sleep?
- Do outreach teams check on people who are alone, older or ill, including after dark?
- Do housing upgrades protect tenants from rent rises and eviction?
- Is the plan funded, with a named owner? In a review of 37 Indian heat action plans, 11 discussed funding.
For heat plans. Heat warnings measure the street. Heat plans can protect the room too: count indoor and night-time heat, warn for hot nights, and pay for cooler roofs where people sleep.
Sources
- Urban Solutions: how informal settlements stay cool (informal measurements). urbansolutionsjournal.substack.com/p/how-are-informal-settlements-staying
- Tasgaonkar et al., Scientific Data, 2022: indoor heat in low-income homes in South Asia. nature.com/articles/s41597-022-01314-5
- Frontiers in Built Environment, 2022. frontiersin.org/journals/built-environment/articles/10.3389/fbuil.2022.874751/full
- IIED, 2026. iied.org/sites/default/files/pdfs/2026-02/22697iied.pdf
- NRDC: community-led cool roofs in four Indian cities. nrdc.org/bio/anjali-jaiswal/cool-roofs-community-led-initiatives-four-indian-cities
- Malaria Journal, 2021: roof colour and indoor temperature in rural Gambia. malariajournal.biomedcentral.com/articles/10.1186/s12936-021-03951-4
- Centre for Policy Research, 2023. cprindia.org/briefsreports/how-is-india-adapting-to-heatwaves-an-assessment-of-heat-action-plans-with-insights-for-transformative-climate-action