There is an interesting development in urban areas where lawns, shrubs and even trees are planted on rooftops in urban areas. The newspapers the Baltimore Sun had an article by Meredith Cohn titled “Look, up in the sky: green roofs“. The article was quite informative, but it included an error in the science of one of the benefits of green roofs. The text is
“For example, the roofs reduce runoff, so the city can approve them as alternatives to potentially more expensive and elaborate storm water management systems. And, she said, “They help us with the heat island effect because they’re cooler than traditional roofs, even more so than painting the roof white. We love them and are talking about ways we can encourage more.”
The use of green roofs are not cooler than painting a roof white in terms of the reduction of Joules of heat absorbed by the roof. The higher albedo of the white roofs result in a greater reflection of solar radiation and thus less heat absorbed by the roof top than is true for a green roof. This is why the house and other buildings in Greece and other hot summer climates are white. Green roofs are certainly an improvement over black asphalt roofs, however!
