Are you going to make your roof white or green?
That’s the burgeoning question individuals and cities are facing, now that the heat-absorbing problems with traditionally dark-colored rooftops are obvious. And two new studies attempt to answer that question from both an economic and environmental perspective.
White roofs are just what they sound like: roofs painted white — or something equally reflective — to bounce sunlight back into space. Green roofs involve planting a layer of grass, shrubs, flowers, or other greenery on a rooftop, with an insulation layer and a waterproof barrier placed underneath.
Both approaches have their advantages and limitations, so here’s a breakdown of what the researchers came up with.
Installation and maintenance. Green roofs aren’t that expensive in absolute terms, but obviously the investment is higher than the mere paint or roofing material needed for white or dark roofs. So, not surprisingly, researchers from Berkeley Lab and the University of Singapore determined white roofs win on that score. But green roofs do have their good points. Because they absorb rain, green roofs prevent wear-and-tear for the gutters and pipes that make up a building’s storm water management system. That makes them part of the rising tide of “green infrastructure” — the use of porous pavements, widespread street plantings, exposed soil, and storage to soak up rainwater, rather than allowing it to add to runoff, pollution, and sewage burdens.
Energy savings. Since darker roofs absorb sunlight, they help drive the heat island effect — the tendency of urban areas to get several degrees hotter than surrounding areas. That increases heating and cooling costs, and thus the use of fossil fuels to provide that power. The paper by the research team out of Arizona State University found white and green roofs can cut the heat island effect by almost 6°F. White roofs have the advantage in pure reflectivity, but that turns to a disadvantage in winter because they can’t help a building retain heat. Green roofs can, making them a better year-round temperature regulator. Due to the water they store, green roofs also help cool by humidifying the environment. So green roofs actually save more overall in terms of energy use. And buildings currently account for 40 percent of US energy use, and for about 35 percent of the nation’s greenhouse gas emissions.
Global warming. Despite green roofs’ energy savings advantage, they only reflect about one-third of the sunlight that white roofs do. That means white roofs do much more overall to reduce global warming. In fact, studies suggest that switching all the roofs in the tropical and temperate regions of the world to white or reflective materials would offset global warming by an amount equivalent to preventing 24 billion metric tons of carbon emissions. Which would be like removing half the world’s cars from operation for 18 years.
Precipitation. This was an odd detail: in places like Florida and the American southwest, white roofs reduced yearly rainfall — sometimes by as much as half. That’s probably because cooler air holds less water, and because the white roofs don’t help with humidity the way green roofs do. Given water scarcity issues around the globe, that’s a problem.
Overall, white roofs were considerably superior to green roofs from a pure cost-benefit analysis, thanks to the installation and maintenance costs.
If roof choice was our only option, or one of our only options for tackling global warming, there’d be a strong argument for just covering everything in white and calling it a day. Happily, it’s not, so the researchers decided that whether a white or green roof was preferable depended on circumstance. White roofs would probably be better for wetter and more temperate climates, green roofs for drier climates with bigger temperature swings. And, of course, a green roof is of little use to someone without roof access. But for someone who has that, the researchers also acknowledged that a readily accessible green space can have all sorts of advantages — aesthetic, emotional, etc. — that can’t really be quantified.