Urban Heat Island
Not all of the heat absorbed by roofs transfers to the building’s interior. Much of it radiates back into the air. This is not the same as reflected sunlight, which bounces back into the sky. Once roofing material absorbs energy from the sunlight, it is stored as heat energy and can transfer to the building interior and the surrounding air via the three methods of heat transfer: conduction, convection and radiation.
When there are many dark colored roofs in a city or neighborhood, all of this heat contributes to the urban heat island effect, which is the increase in temperature in cities compared to the vegetated land just outside cities.
Roofs are not the only culprits. Asphalt, concrete, stone and gravel all absorb more heat than vegetation does. Consider a bird’s eye view of a dense city. Most of the horizontal surfaces are rooftops. What is not a roof is likely a street or parking lot. Urban heat island is an environmental concern because of increased energy costs, increased smog, human health concerns associated with heat, and over all decreased comfort.
Addressing the roofs of buildings citywide is an important part of urban heat island mitigation. Adding vegetation to cities, both on the ground and on roofs is part of the solution. Transpiration in plants helps cool the air and plants provide shade, which lowers the temperature. For these reasons and others, green roofs are becoming popular. However, green roofs are also more expensive and require extra maintenance, and they are not always practical, either because of the cost, need for upkeep, or building structure limitations. An inexpensive and highly effective alternative is a cool roof.