Chu Raises Environmental and Energy-Conserving Benefis of White Roofs

(image: features.csmonitor.com)

“President Obama’s energy adviser has suggested all the world’s roofs should be painted white as part of efforts to slow global warming.” begins a recent article in the London Telegraph.  Speaking at a climate change symposium in London, US Energy Secretary, Steven Chu said the simple expedient would not only benefit the environment, but would result in huge savings of energy and money normally spent on air conditioning.

Air conditioning accounts for about 1/6 of all the electricity consumed in the U.S. White roof coatings could reduce that figure by 10-15 percent. More white or light colored surfaces could also slow global warming by reflecting heat into space rather than allowing it to be absorbed by dark surfaces where it is trapped by greenhouse gases, increasing temperatures.
Although white roof technology is already popular in warmer parts of the US– it’s the law for new commercial construction in California – the emphasis has typically been on flat roofs. But there is reason to believe that white or light colored roofing on homes, even with sloping roofs, could make a big difference as well. One study conducted in California was detailed in an article in the Christian Science Monitor.  It showed that a 1,000 square foot area of rooftop painted white has about the same one-time impact on global warming as cutting 10 tons of carbon dioxide emissions. If a roof is white, most sunlight reflects back into space and doesn’t heat the earth. But if a roof is a dark color, the sunlight converts to heat rather than bouncing off as light. That thermal energy then radiates off the roof back toward space, where it is trapped and absorbed by CO2 in the atmosphere. “As a result, the world’s thermometer reads just a little higher than it did before,” the article said.

But if a white roof saves money by reflecting heat in the summer, won’t it lose money by reflecting it in the winter?  Some, but the loss is probably more than offset by summer AC savings. The Cool Roof Rating Council admits to a “winter penalty” when cool roofs are installed in northern climates. An article in the April edition of Maintenance Solutions magazine quotes the CRRC as saying, “Heating costs may increase slightly for buildings located in cooler regions.” The organization pointed out that sunlight hours are longer in the summer than they are in the winter, thus more energy is consumed to cool a building than to heat it.

The technical term for painting your roof white is known as “negative radiative forcing.” Light, reflective surfaces bounce 10-20 percent of the summer heat back into the sky, reducing urban heat island effect and significantly reducing summer cooling costs, thus its more common name — a “cool roof.” An online report from WCCO radio in Minnesota describes a visit to a local company – ER Systems in Rockford, MN — that makes and markets a material for painting roofs.

An ER executive equated 140 degrees on a black roof with 97 degrees on a white roof.  “We think of the roof as low-hanging fruit. It’s something easy to modify and you get a big benefit out of it. There are billions of square feet of black roofs across the country, bouncing heat into the air and making air conditioners work harder.”  ER Systems makes a white elasomeric paint for roofs. “It’s like paint, but fancier and a lot thicker, he said.”

Workers practive negative radiative forcing (painting a roof white) on a home. The process can reduce cooling costs by 10-15 percent. (image: mnn.com)

For more information on how white roofs can benefit homeowners by lowering energy costs, check out this HEAT Zone post from April 30th.