White Paint, Ten Degrees

A dark roof in the tropics absorbs the sun and re-radiates it downwards into the room below, which is why so much of the world's growing electricity demand is simply air conditioning fighting a roof. Cool roofs invert that: a highly solar-reflective coating sends the radiation back out, typically cutting indoor temperature by 2–3°C, and lowering ambient temperature across a neighbourhood when enough roofs are treated. Cool Roofs Indonesia, a team led by Beta Paramita at Universitas Pendidikan Indonesia, won the $750,000 Million Cool Roofs Challenge in March 2022 after coating 70 buildings across 15 cities — 36 low-cost housing units, 10 schools, two factories, an orphanage — and measuring indoor reductions of over 10°C in some pilots. The part that mattered most was industrial rather than scientific: with the manufacturer they set up local production of a certified reflective coating in Indonesia at a fraction of the imported price.