The term “Urban Heat Island” is used to describe the phenomenon of dark materials re-radiatingretaining heat generated from the sun to and warming their surroundings. Scientists have linked the urban heat island effect to dark colored pavement, roofs and parking lots.
It is a proven fact that higher surrounding air temperatures hold more low-level ozone – which is a contributor to air pollution. This is further exacerbated by the increased production of green house gasses from energy generation using fossil fuels. As the ambient temperature goes up, so does the air’s ability to hold low-level ozone. At the same time, the population increases their energy use by turning up their air-conditioners. It is a vicious cycle, but this is where cool roofs can help.
Research at the Lawrence Berkeley National Laboratory’s (LBNL) “Heat Island Group” has shown that for sprawling urban areas such as Atlanta Georgia, the average surrounding air temperature has gone up by as much as 5 – 10 degrees over the last 15 years. Through the use of infrared photography, LBNL’s research shows that this increase in air-temperature is directly attributable to the conversion of green, vegetated areas into roads, roofs and parking lots. Although there haven’t been many mandatory programs for specifying lighter colored pavements, cool roofing with both a high reflectance and emittance has been mandated in some urban areas located in hot (low heating degree-day) climate zones. In these mostly southern areas, light-colored painted metal roofing is the best option. Northern climates will see the most benefit from either dark-painted or unpainted metal roofs.
It should be mentioned that various paint pigment manufacturers have developed groundbreaking technology for “infrared reflecting pigments”. These special pigments can increase the reflectance of darker colors to match those of visibly lighter colors. In actual weathering tests, darker paints that utilize these new pigments show a drastic increase in reflectance over visibly similar colors that don’t use the pigment. By utilizing this new technology, metal roofs that are dark in color – even burgundy and green - can achieve code-specified levels of reflectance. It is important to remember that all painted metal roofing products have a high-emittance – in the range of 0.80 to 0.90 - regardless of the color.
Compared to all other cool roof products, metal roofing stands out in terms of long-term performance. Both painted and unpainted metal roofs exhibit sustainable Reflectance and Emittance values in even the harshest environments, like south Florida. One reason for their outstanding performance is their ability to “self-clean” with rain water, and their inherent resistance to mold and mildew growth. These factors, in addition to the ability to expand and contract with fluctuations in the roof temperature, makes metal an excellent choice for cool roofing.
