A white roof reduces energy usage, improves the comfort of homes, and helps reduce the greenhouse effect.
A dark colored roof absorbs much more heat than a white roof. By absorbing the sun's rays, the material of the roof converts visible spectrum light to heat energy.
This matters to climate change in two key ways:
- This re-radiates into the atmosphere as infra-red, heating the atmosphere. (By comparison, much more of the visible light reflected by a white roof will pass out of the atmosphere.)
- Dark roofs absorb huge amounts of heat into buildings, especially when it's hot. This means more air conditioning is used to cool buildings, homes and workplaces becomes less comfortable, and those at risk of heat stress are in greater danger. Dark colors also radiate heat more easily in cold conditions, meaning greater heat loss from buildings and higher heating costs.
Hot roofs also affect the local microclimate, increasing the urban heat island effect.
Estimates for the effect on electricity usage from painting the roof white are around 20 percent on a hot day - more if combined with other measures such as a passive solar retrofit. Reductions from peak usage on hot days are important because they also reduce the need for extra electricity generation capacity.
Programs
New York City's Mayor Bloomberg began a program to hire and train young people to paint New York's roofs white. Every black roof in New York should be white; every roof in Chicago should be white; every roof in Little Rock should be white. Every flat tar-surface roof anywhere! In most of these places you could recover the cost of the paint and the labor in a week. It’s the quickest, cheapest thing you can do. In the current environment it’s been difficult for the mayors to get what is otherwise a piddling amount of money to do it everywhere. Yet lowering the utility bill in every apartment house 10 to 20 percent frees cash that can be spent to increase economic growth.