During summer months, cities get hotter than surrounding areas. Trees and vegetation found in suburban and rural areas help reduce temperatures on sunny days. In contrast, tightly packed and tall city buildings tend to trap heat. To make matters worse, city buildings retain that heat and release it during the night due to their thermal mass.
On average, US cities can be up to 7°F hotter than surrounding areas during the day, and up to 5°F hotter during the night according to the EPA. This is not a static situation; the temperature increase seen by urban areas is expected to get worse due to global warming and the ever-increasing density and overall size of urban areas. For example, the following chart shows the projected number of days over 90°F that could be experienced by New York City due to the urban heat island effect and global warming:
While this view can seem alarming, there are many practical ways in which we can slow or reduce the anticipated rise in city temperatures. These include:
Improving the energy efficiency of the building enclosure. This can be achieved through the use of more and better thermal insulation, air barriers, and reflective roofs and walls. These measures can help to reduce energy use and slow the rise of carbon emissions. Roofing manufacturers such as GAF already offer insulation, air barriers, and membranes to help with these goals.
Increasing the use of blue and/or green roofs, to aid in rainwater storage and associated evaporative cooling.
Increasing the reflectivity of city pavements. This is the focus of this article because it offers an often overlooked way of reducing heat build-up in urban areas. Increasing pavement reflectivity provides a method in lieu of adding green space, which is often not possible.
How does city paving contribute to the urban heat island effect?
Roads, parking lots, sidewalks, airports, school playgrounds, and other paved areas normally use asphalt or concrete. Market share estimates are as high as 94% for asphalt, based on all US roads, and 60% for concrete, based on the US interstate network. In general, city streets are often surfaced with asphalt. The reflectivity of both concrete and asphalt paving is relatively low compared to modern reflective roof membranes such as PVC and TPO (between around 0.85 and 0.7):
These heat absorbing pavings lead to significant heating of the surrounding air:
How do pavement coatings help?
While higher reflectivity aggregates for asphalt paving and more reflective concretes exist, implementing these could take decades and require replacing the existing substrates. In contrast, commercially available pavement coatings are available that can increase reflectivity and have several other advantages:
Increased solar reflectivity: Some pavement coatings, such as GAF's StreetBond™ with Invisible Shade™ are specifically designed to boost solar reflectivity.
Increased pedestrian visibility and safety: Pavement coatings have a range of uses including improving pedestrian safety, such as intersection and crosswalk markings to bus and bike lanes.
Improve pavement performance: Pavement coatings protect the pavement from extreme weather thereby extending the performance of the pavement by slowing rutting or oxidation.
Encourage play: Coatings can make school playgrounds more engaging, as shown in the example below, which has been shown to increase use and activity.
Improve aesthetics: Parks, plazas, sports courts, civic centers, as well as playgrounds can all be given a differentiated and highly attractive look via the use of pavement coatin
What is the reflectance of typical pavement coatings?
GAF's white pavement coating has a reflectance of 0.77. However, while a white coating might be useful as a part of an attractive streetscape design, widespread use could create glare issues. In general, coatings designed to improve reflectivity range between 0.33 and 0.50 reflectance. Researchers at the Lawrence Berkeley National Labs have suggested that simply coating darker asphalt paved areas with materials having a reflectance of 0.35 (similar to lighter concrete) could make a significant improvement to urban temperatures. A GAF Cool Community Project in a Los Angeles, CA neighborhood, saw a 10 to 12 F reduction in surface temperature using a mixture of coating colors with a range of reflectivities.
Wouldn't increasing the reflectance of a pavement heat up the surrounding area?
No, there has never been any indication that this is the case. It has been suggested that reflective pavements would heat up the air more. However, air is a relatively poor absorber of radiated heat. It is much easier to heat air through contact with a hot surface such as a dark pavement than it is by solar radiation, whether directly or reflected.
Similarly, it has been proposed that reflective coatings would heat up nearby buildings by reflecting the sun's rays back up towards tall surrounding buildings. In general, while some experts suggest nearby buildings can be expected to have improved energy efficiency. others suggest it might depend on the exact building density and height of those buildings. Cool pavement coatings for play areas, walkways, and other large asphaltic regions of cities, the goal is generally to improve on pedestrian comfort.
Are there any other benefits of cool pavement coatings?
This article has already mentioned improved aesthetics as a side benefit of solar-reflective street coatings. In addition, Lawrence Berkeley National Labs researchers have identified many other benefits, including:
Energy savings and emission reductions. Cool pavements lower the outside air temperature during the summer, allowing air conditioners to cool buildings with less energy.
Improved comfort and health. Cool pavements cool the city air, reducing heat-related illnesses, slowing the formation of smog, and making it more comfortable to be outside. Pedestrians also benefit from cooler air and cooler pavements.
Improved air quality. By decreasing urban air temperatures, cool pavements can slow atmospheric chemical reactions that create smog.
Slowed climate change. Cool pavements decrease heat absorbed at the Earth's surface and thus can lower surface temperatures. This decrease in surface temperatures can temporarily offset warming caused by greenhouse gases.
Disadvantages of cool pavement coatings
Cool street coatings are exactly that – coatings. They will wear down and need to be re-applied periodically. Coated pavement areas with lower vehicular traffic, such as pedestrian walkways, school playgrounds, and city parks, typically see greater life spans.
Conclusions
Solar reflective pavement coatings can be a part of efforts to reduce temperatures in city and urban environments. They can contribute to lowered air temperatures and associated reduced air conditioning costs, improved well-being for pedestrians, and improved aesthetics of areas such as school playgrounds and public spaces.</li>
