The sun never sets on environmental issues.
In fact, in dense urban areas, matters can heat up once the moon rises. That's when the phenomenon known as urban heat island (UHI) casts it longest shadow, as rooftops and other solar-friendly surfaces begin radiating the heat they've gathered throughout the day.
The EPA indicates that the annual mean air temperature of a city of 1 million or more can be 1.8 to 5.4 degrees Fahrenheit higher than its rural surroundings. At night, the difference can be as great as 22 degrees, triggering rising air conditioning costs, greenhouse gas emissions and heat-related illness.
There is also the issue of smog – that tell-tale haze that hovers over summer skylines, posing additional hazards to human health.
The degree to which pavements, walkways and the like contribute to UHI isn't fully understood, though USGBC targets such surfaces for LEED certification.
The terms – as outlined in LEED's Heat Island Effect: Non Roof Credit 7.1 – are fairly clear cut. Owners can earn a single point toward certification by locating a minimum of 50% of parking spaces underground, in covered, structured parking, or by undertaking "hardscaping" measures that incorporate shading devices – think photovoltaic canopies – and reflective surfaces. More specifically, the measures call for:
- Providing shade (within five years for landscape installations) and/or light-colored/high-reflectance materials and/or open-grid pavement – essentially pavements incorporating vegetation – for at least 30% of the site's impervious surfaces, including parking lots, walkways and plazas, or:
- Providing an open-grid pavement that is less than 50% impervious for at least 50% of the parking area.
Results aren't as easy to summarize.
Owing to the canyon-like character of urban settings, solar energy doesn't pierce pavement to the degree it does rooftops, says Howard Marks, director of environmental health and safety with the Lanham, MD-based National Asphalt Pavement Association. Nor, says Marks, do pavements exert the same influence on energy consumption as roofs do. "Roofs are thin and readily transfer heat inside the building," he says. "Pavements don't."
While acknowledging that pavement design can assist in reducing surface temperatures, Marks maintains that science has yet to demonstrate that pavements – or the remedial measures prescribed by USGBC – have a determinative impact on heat island effect.
Marks has his supporters, including Jay S. Golden and Kamil E. Kaloush, a pair of researchers at Arizona State University who have argued that "planting more trees, increasing reflectivity or changing pavement types grossly overstates and simplifies the value of those mitigation strategies."
"It's difficult to break down UHI into granular detail to account for the unique contributions of roofs, pavements, trees and shrubs," acknowledges David Shepherd, director of sustainable development with the Skokie, IL-based Portland Cement Association. "From a bird's eye view, it's easy to deduce that each plays a role. If we didn't see base temperature differentials in cities such as Phoenix, home of the quintessential heat island, we'd be more apt to challenge that assumption."
Pore Better or Pore Worse?
Additional controversy surrounds the exclusion of permeable pavements from Credit 7.1. Though relatively novel, such systems are endorsed by the EPA, with the proviso that "further field testing and validation would help to quantify and clarify the range of impacts and benefits of permeable pavements on urban climates."
Originally designed for stormwater control, permeable asphalt and concrete promote cooling via water absorption and evaporative cooling. While the water passes from air voids to underlying soil, residual moisture evaporates once the pavement warms, extracting heat in a manner similar to natural green landscape cover.
"Side by side, some porous pavements perform better than reflective ones," says Marks.
The problem, says Shepherd, is that moisture availability varies by region and season.
Add to that the lack of an accepted means to assess the performance of permeable pavements, and it becomes clear why USGBC has cast its lot with high-reflectance materials, whose performance is measured on the basis of solar reflectance and thermal emittance. The resulting value, known as Solar Reflectance Index (SRI), denotes surface temperature relative to a standard black and standard white surface. LEED 7.1 requires that reflective materials carry an SRI of at least 30.
Not Quite Black and White
That's a criterion asphalt can't easily meet, given that SRIs vary from 100 for standard white surfaces to zero for standard black ones, as calculated using ASTM E 1980, Standard Practice for Calculating Standard Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces.
Some owners have responded by adding light-colored pigments and sealants to asphalt's surface. According to Marks, other potential solutions include:
- Chip seals and sand seals with light-colored aggregates – essentially surface treatments consisting of single or multiple applications of asphalt and aggregate on existing pavement. This low-cost option is commonly employed to treat weathered pavements.
- Surface gritting with light-colored aggregate – a method that spreads aggregate over newly placed asphalt and presses it with a roller. The increased surface friction also promotes safety.
- Colorless synthetic binders and light-colored aggregate – an approach common to sports venues.
By virtue of its coloring, concrete is considerably more reflective than conventional asphalt, and carries SRIs ranging from the low 30s to low 50s, according to Shepherd. Concrete made of white portland cement can carry values of 70 or higher, he says.
Better known as a component of precast concrete cladding, white portland cement is similar to its gray portland counterpart in all respects except color, which it achieves with reduced amounts of iron and magnesium, the substances that give gray portland cement its color.
Due to weathering and the accumulation of dirt, the reflectances of conventional asphalt and concrete are something of a moving target. Asphalt tends to lighten as its binder, typically petroleum derivatives, oxidizes and its aggregate, typically sand and stone, is exposed as a result of wear. Concrete, on the other hand, tends to darken as a result of foot and vehicular traffic.
Accordingly, SRIs for concrete can fall to 25 in as few as five years, while SRIs for asphalt can rise to 20 in seven years.
To ensure values don't drop below the prescribed threshold, concrete may require pressure washes on a periodic basis, according to Shepherd.
Differences between the asphalt and concrete extend to cost (see chart). According to the Federal Highway Administration, owners can expect to pay a 10% to 15% premium for permeable asphalt and a 25% premium for permeable concrete.
Costs for conventional varieties skew along similar lines, with concrete priced two to three times higher than asphalt on an installed, square-foot basis. However, conventional concrete typically enjoys a longer service life, roughly twice that of conventional asphalt.
As ever, there are wild cards to consider, including climate, season, site accessibility, underlying soils, project size, and projected traffic, all of which drive the bottom line.
Like other facets of cool pavements, little is set in stone.
John Gregerson is a contributing editor based in Chicago.