New York rooftops 40 degrees cooler painted white, reckons NASA/Columbia study
It's long been suggested that white rooftops could help reduce the heat bubble microclimates that surround our cities simply by reflecting solar radiation directly back into space, and in 2010 we reported on NCAR efforts to demonstrate the effect through computer modeling. A new study goes one better, putting the theory into practice and pitting three white materials against one another on three New York rooftops. The results of the study appear to be overwhelmingly positive, with white roof coatings reducing peak rooftop temperatures in summer "by an average of 43 degrees Fahrenheit (about 24 degrees C)."
It's hoped that the deployment of white roofs could help reduce the warming effect of urban microclimates that can see city temperatures several degrees higher than surrounding areas. The effect is thought in part due to the dark materials with which we build our cities, that reflect much less light than natural landscapes. New York's microclimate can result in nighttime temperatures 5 to 7 degrees Fahrenheit (3 to 4 deg. C) higher, according to preceding research by Stuart Gaffin of Columbia University, who conducted this study into roof surfaces with the assistance of NASA scientists.
The three materials tested were an ethylene-propylene-diene monomer (EPDM) rubber membrane, a thermoplastic polyolefin (TPO) membrane, and an asphaltic multi-ply built-up membrane coated with white elastomeric acrylic paint (white paint on a typical roof surface, in other words). The latter acrylic paint-coated membrane is a low-cost material being promoted by Mayor Michael Bloomberg in the CoolRoofs program - part of a drive to reduce New York's greenhouse gas emissions by 30 percent by 2030.
The surface temperatures of each surface were monitored against a black control surface using infrared radiometers. In addition, pyranometers were used to measure shortwave radiation - both arriving at the surface (the incident radiation) and that leaving the surface (reflected radiation), with which the surfaces' emissivity (its ability to radiate energy) could be determined.
A comparsion of dark and white surface temperatures over the New York summer of 2011 shows a marked reduction in surface temperature (Image: Stuart Gaffin/Columbia University)
Results indicated that temperature differentials between the white test surfaces and the black control surfaces were most marked on sunny days (well duh, you might say), but on average the test surfaces were, at daytime peak, found to be 42.5 degrees Fahrenheit (23.6 deg. C) cooler.
The report concludes that all three materials showed "very similar" performance, each with an albedo (a coefficient of reflectance) of about 0.65, in contrast to a typical albedo of 0.05 for near-black roof surfaces. A crucial caveat is that the acrylic paint surfaces tested were brand new, while the "professional" EPDM and TPO membrane installations had aged three and four years respectively. A significant downside of the acrylic paint is that its albedo halved over a two-year period.
The report claims that this proves that professional membranes are more effective in maintaining reflectance over time. That said, even accounting for the two-year performance drop of acrylic paint, the report finds that it still provides "a significant boost" over ordinary roof surfaces.
The paint boasts obvious economic advantages. No reroofing is required, and the paint can be applied by home-owners or at a cost of US$0.50/square foot ($5.38/sq m) through a volunteer organization.
At first glance the more expensive coatings fared less well in other metrics. The EPDM membrane had a lower emissivity than expected (and, indeed documented in the product specification). The report points out, however, that this may actually be an advantage in winter in respect of avoiding heat energy penalties.
The results were published in a report entitled Bright is the new black - multi-year performance of high-albedo roofs in an urban climate in Environmental Research Letters yesterday and can be downloaded free of charge. It's worth bearing in mind that a three-roof study in a single city is necessarily limited in scope, and it would be fascinating to see if a similar margin could be replicated in other cities. The report also highlights that other weather conditions, such as wind-speed, were not taken into account in this study.