New York City tested a simple fix for scorching summer rooftops in 2011, when volunteers painted dark roofs white to see whether reflective color alone could cool buildings, according to The Times of India.
Data from Columbia Climate School showed a stark difference. On one hot summer day in 2011, a standard black roof surface heated to about 77 degrees Celsius, while a nearby roof coated in white reflective paint stayed at around 53 degrees Celsius.
Over the course of the summer, the painted roof recorded peak surface temperatures that averaged 24 degrees Celsius lower than the black roof.
Why white roofs stay cooler
Scientists from Columbia University and NASA's Goddard Institute for Space Studies monitored several painted roofs across New York City using sensors that continuously tracked surface temperature. Rather than relying only on lab tests or simulations, they observed how the roofs responded to real weather over several years.
Dark roofs absorb most of the sunlight that hits them, turning it into heat that warms the building below and radiates into the surrounding air. White roofs have a much higher albedo, reflecting more incoming solar radiation and absorbing far less heat, keeping both the roof surface and the building underneath cooler.
Because roofs cover a large share of a city's surface, the effect matters citywide. Along with roads and pavements, dark roofs store heat during the day and release it slowly after sunset, contributing to the urban heat island effect that keeps cities warmer than surrounding rural areas. Higher urban temperatures raise demand for air conditioning, strain electrical grids during heat waves, and increase the risk of heat-related illness.
Benefits beyond the roof
Cooler roofs produced benefits that reached beyond individual buildings. Because reflective roofs absorb less heat, less heat transfers indoors, reducing the need for air conditioning in hot weather, which can cut energy use and cooling costs. Cooler roof surfaces may also last longer, since they experience less thermal stress from repeated heating and cooling.
According to Environmental Research Letters, a single reflective roof cannot meaningfully lower a whole city's temperature, but widespread adoption across thousands of buildings could reduce neighborhood heat and cut cooling energy demand. The findings showed cool roofs work best when applied at large scale rather than as isolated projects.
The New York experiment took place during one of the city's hottest summers. On July 22, 2011, New York recorded a peak electricity demand of 13,189 megawatts, underscoring the strain extreme heat can place on urban power systems. The NYC CoolRoofs program also showed reflective coatings could be applied at relatively low cost, with volunteers painting the roofs of schools, apartment buildings and commercial properties without replacing the existing roofing.
A simple idea with practical limits
The project also showed white roofs are not a universal solution. Their biggest benefits appear in hot climates or places where air conditioning is widely used. In colder regions, reflective roofs can slightly reduce useful winter heat gain, though research generally shows that for many buildings, summer cooling savings outweigh winter losses.
Researchers also compared reflective roofs with other approaches, including urban vegetation, green roofs and solar panels. Green roofs support biodiversity and act as a sponge for rainwater, while trees provide shade and clean the air. A reflective roof remains one of the most accessible methods, requiring only minor changes to existing structures.
According to Nature Climate Change, increasing the reflectivity of roofs and paved urban surfaces can help lower city temperatures and reduce the energy needed to cool buildings, and reflective surfaces could help ease climate change effects and the urban heat island effect, especially when adopted at large scale.
More than a decade later, the New York experiment still demonstrates how a simple roof coating can cut heat. A layer of paint did not solve the city's heat problem, but it showed that simple engineering ideas can sometimes produce surprisingly powerful results.
