The Loma Prietan - March/April 2010

The Climate

Paint the Town ... White!

story and photo by Alex Cannara

This can of paint, applied to your roof, can do more to preserve the climate than almost anything you can do inside your house.
This can of paint, applied to your roof, can do more to preserve the climate than almost anything you can do inside your house.

What can any property owner do to have an immediate, good, permanent effect on the climate? Paint the roof white. Or maybe silver, or even a light pastel.

The California Energy Commission's (CEC) calculations show that increasing a typical home's reflectivity by 40% is equivalent to not driving a car for a year. Worldwide, because 3% of Earth's land is covered by human structures, increasing structural reflectivity 40% equals not driving any of the planet's 600 million fuelled vehicles for a decade.

The key to accomplishing this cooling effect is to arrange that the frequencies (colors) of the sunlight hitting the roof don't change when they're reflected. The big issue is how visible light is handled. A light-colored roof reflects visible light as visible light. This light then passes through the air and out into space; not much is absorbed. A dark roof absorbs visible light and turns it into heat. Like any hot surface, the roof then radiates infrared (IR) that goes right back into the atmosphere, to be largely absorbed, trapped as additional climate heating.

To prevent turning sunlight into heat, a color near to white or silver is best. A typical home's 120-square-yard roof reradiates 50 kilowatts of IR if it's a dark color, but less than half that if light gray. The less coloring and roughness a surface has, the better. Beware 'cool colors' — they may reflect IR (52% of sunlight) a bit better, but their visible coloring means they're absorbing complementary colors and converting them to IR, adding to the climate problem.

Light Roofs have Other Advantages

Changing from a dark roof to a light roof typically reduces a roof's temperature by over 50ºF and reflects more ultraviolet (UV) light. Changing to a white or silver roof gives an even greater improvement. Since UV and heat are major factors in roof deterioration, cooling your roof increases its life. Since painting a roof is far less costly than replacing it, both in terms of dollars and in terms of emissions and wear on the environment, this is a huge advantage. Also, a cooler roof can lead to a cooler house or to reduced air conditioning costs.

Getting a Light Roof

The first thing to do is to go outside and look at your roof. If it's already white or a very light color, pat yourself on the back and skip the rest of the steps. Otherwise...

Composite shingles or roll roofing. Arrange to paint it white or silver. It'll cost about $1,000 plus labor for a 2,000-square-foot roof but your roof will last decades more, saving you over $1000 per year in replacement cost; saving replacement energy, over three tons of materials and dumping; and lowering greenhouse heating by the equivalent of several tons of carbon dioxide. Paint with something equivalent to Henry 555 aluminized paint; one gallon will cover 200-300 square feet.

Tar and gravel. If the gravel is a dark color, exchange it for a more reflective color (common ones are white quartz, or 'taffy'). The surface under the gravel can be painted, as with a composite roof, to ensure it will last longer, even if the gravel moves.

Metal or flat tile roof. Usually, these can be cleaned and painted as with a composite roof. Oil-based paint (as Henry 555) is likely best

Wood shingles. Go to a friend's house, look at that roof, and offer to help him or her. When your time to re-roof comes, consider changing to a type that can be made reflective. For the interim, plant shade trees.

Spanish tile. If it's a uniform, light color, great. If not, plant shade trees.

Reflectivity makes the atmosphere seem to have less greenhouse gas (GHG) content than it does. This doesn't give us a pass on GHG reductions, but it buys us time to address emissions reductions,. The benefit, however, applies to all greenhouse gases, while saving energy mostly just offsets carbon dioxide emissions. Since carbon dioxide is only responsible for about 16% of global warming, reflectivity improvement buys us a lot more and buys it instantly.

Dr. Cannara is an engineer, a Chapter member, a member of the Menlo Green Ribbon Committee, NRDC and the Silicon Valley Action Network.