Mar 23, 2012
White roofs are more cool
Policymakers should phase out the use of black asphalt on roofs in hot climates. That's according to researchers in the US who have analysed different roofing membranes for their effectiveness at reducing the urban heat island effect.
"The era of asphalt roofs needs to be phased out," Stuart Gaffin from the Center for Climate Systems Research at Columbia University, US, told environmentalresearchweb. "We have shown that in New York City, professionally installed polymer membranes are maintaining their temperature control effectively and are meeting the Energy Star Cool Roofing performance standards requiring a three-year aged albedo above 0.50."
Gaffin and his colleagues also studied the effectiveness of a white elastomeric acrylic paint that was used as a "retrofit" on top of an existing standard dark asphaltic membrane. They found that while the painted asphaltic surface showed high emissivity at the beginning of the test period, it lost about half of its initial albedo within two years of installation.
"We do not know what caused the deterioration," said Gaffin. "Despite the reduction in albedo, I still think any dark roof is worth painting because this is such a low-cost technique and can also reduce the temperature cycle of a roof and prolong its life. Our two-year aged acrylic surface albedo still represents a significant boost in albedo over the original black surface."
The researchers hope their analysis will help influence policy so that at the time of roof replacement for any urban building, only white membranes may be considered. White membranes are the same cost as dark membranes and outperform painted surfaces.
"I am convinced that raising the planetary albedo in a safe way will become a major research topic and will include strategies such as painting roofs white or even using slightly brighter crops in agriculture," said Gaffin. "There are still many unanswered questions, though. For example, we still do not know how effectively the light is being reflected back into space. It may be being absorbed back into the atmosphere by particles in the air, especially in urban environments."
The scientists published their work in Environmental Research Letters (ERL).
About the author
Nadya Anscombe is a freelance science journalist based in Bristol, UK.