Cool Roofs for Hot Cities
PVC roofing mitigates the urban heat island effect
In cities, where roof tops dominate and open spaces dwindle, the lack of cool roofing is becoming a key contributor to rising temperatures. The effects of rising temperatures in urban areas have negative outcomes for a wide range of quality-of-life issues, including health, well-being, and economic impacts. A study of nearly 1,700 cities found these combined effects, if left unaddressed, would cost a city approximately 1.7 percent of its annual economic output by 2050 and 5.6 percent by 2100*, easily measured in the billions of dollars.
This “urban heat island effect” (UHIE) results from the concentration of dark surfaces in cities, such as dark roofs and asphalt paving, that absorb energy from the sun and ultimately heat up the entire area. The net effect is that cities are typically five degrees to ten degrees hotter than the surrounding suburban and rural areas. This urban heat island effect occurs in all climate zones.
There are solutions that building experts can adopt today to mitigate UHIE, minimizing heat gain and reducing temperatures in buildings, in cities – and on the planet. The choice of roofing can play a major role in these reductions, leading to a proliferation of “cool roof” benefits for cities. Two key benefits associated with cool roofs are a reduction in both UHIE and cooling energy consumption.
To understand the impact roofing system selections can have on urban heat, it’s important to know the key measurements related to the effects of solar energy acting on roofing surfaces. There are three: solar reflectivity, emissivity, and solar reflectance index (SRI).
Solar reflectivity refers to the amount of incoming solar energy a material reflects back into the atmosphere. It is usually expressed as a percentage. For example, roofing with a reflectivity level of roughly 80% means that 80% of the solar energy that is hitting the roofing membrane is reflected back. The balance of energy that does not get reflected is absorbed by the roof. Part of that absorbed energy will be transmitted into the structure, while some of it will be emitted back to the environment.
Emissivity refers to the amount of energy a material releases that it has absorbed, and it is also typically expressed as a percentage. The higher the reflectivity and the higher the emissivity, the “cooler” the roofing material is.
Solar reflectance index (SRI) is a calculation that takes into account both reflectivity and emissivity, combining them into a single number, which makes it much easier to assess and to compare roofing materials. The higher the SRI number, the cooler the roofing material.
Below is a list of the SRI of some common roofing materials.
Black EPDM: -1
Smooth bitumen: -1
White granular surface bitumen: 28
Dark gravel on BUR: 9
Light gravel on BUR: 37
White thermoplastic (PVC): 104
At one end of the scale, EPDM and smooth bitumen are at the low end (-1), while white thermoplastic (PVC) surpasses the other materials with an SRI of 104. PVC also scores high in solar reflectivity (83%) and emissivity (92%).
Cool PVC roofing
What is PVC roofing? It is a high-performing, low slope, single-ply roofing solution. Reflective PVC roofs have protected and kept buildings cool in climates around the world for decades. They can also be an important contributor to reduction of the urban heat island effect and its consequences.
Cool PVC roofs provide significant benefits in the hot summer months. Even in cool climates, summer cooling-cost savings of PVC can outweigh wintertime heating-cost savings from darker roofs. The solar radiation cool roofs reflect away during summer help save $29.73 per million BTUs, while dark roofs absorb solar energy during winter and help save just $9.10 per million BTUs. PVC roofs have demonstrated that they can provide cooling energy savings, peak demand electricity reduction, and net energy savings.
With such a dramatic reflectivity, emissivity, and SRI, specifiers might be surprised to hear that the cost of PVC roofing is often comparable to other roofing materials while also having one of the best track records for durability and life cycle performance.
“While sometimes the initial costs might be slightly higher than some technologies, PVC roofing lasts a minimum of 30 years,” said Bill Bellico, an 18-year roofing industry veteran who serves as vice president of marketing for Sika Sarnafil and is the marketing chair for Chemical Fabrics and Film Association - Vinyl Roofing Division. “When you take the low maintenance needs and longevity into account, PVC roofing is a smart choice for the lowest life-cycle costs even before you start factoring in its energy savings and positive environmental impact.”
Cool PVC roofing has the added benefit of being suitable and highly effective for re-roofing existing structures along with new construction, making it an ideal choice for quickly increasing the number of cool surfaces in an urban setting, thereby helping to reduce the urban heat island effect.
Know your roofing
The U.S. EPA has phased out its Energy Star specification for roof products, but there is still third-party certification readily available to document PVC roofing’s energy-saving properties: the Cool Roof Rating Council (CRRC)’s CRRC-1 Program.
In existence since 2002, the CRRC-1 Program’s ratings are based on a product’s surface radiative properties (reflectivity and emissivity) and range from 0 to 1, with 1 being the most reflective or emissive. The ratings inform on how efficient the product is at reducing building energy use, increasing occupant comfort, and mitigating the urban heat island effect.
Over 3,000 roofing products are published in the CRRC Rated Roof Products Directory, an online, publicly available database that policymakers, design professionals, building owners, and others have relied on for years for third-party data. The directory gives consumers the ability to search for and compare roofing products that comply with code requirements, green building certifications, and rebate programs. The ratings are also on CRRC labels found on product packaging.
Beyond energy savings
The Smart Surfaces Coalition helps cities make “smart” surface decisions by demonstrating the value of surfaces that better manage the effects of sun and rain. The coalition consists of 40 leading national and international organizations with a shared commitment to promoting these surfaces as the global urban design standard.
A recent coalition study of the effect of implementing smart-surface solutions in Baltimore, Maryland, contained some stunning projections on the broad impact adoption of smart surfaces could have on the city. Highlights include: a peak summer temperature reduction of 4.3 degrees in the hottest areas of the city and a 2.5-degree average reduction city-wide; a 17 million metric ton reduction in carbon impact; the creation of 3,600 jobs in the first 20-years and 78,700 job years created over 30 years; a $13.5 billion net present value from adoption of Smart Surfaces; and 15 times more benefits than costs required to implement and maintain the strategy.
“A city that is too hot from its dark impervious surfaces and lack of trees negatively impacts everyone’s quality of life – particularly lower income and minority neighborhoods and particularly children,” said Greg Kats, CEO of Smart Surfaces Coalition and recently named an honorary member of American Institute of Architects (AIA). “Cities that do not use smart surfaces that are reflective, porous, green, etc., are locking in more heat, more smog, more flooding and mold – all of which leads to poorer health. That means more days off work, earnings reductions, a lower tax base for the city, fewer people buying things to drive the local economy, and less jobs created. It’s an entire ecosystem that suffers.”
With such pressing need for change, the Chemical Fabrics and Film Association - Vinyl Roofing Division believes it is imperative that architects, specifiers, and builders use science to guide their roofing material choices. The alternative can have grave and long-lasting impact for cities, their constituencies, and the shared planet. These scientific measures are widely available and easy to interpret. In particular, the solar reflectance index (SRI), which considers both reflectivity and emissivity, makes it easy to assess and compare roofing materials.
Obviously, planners, specifiers, and buyers have much to consider. Design aesthetics, of course, are critical. So, too, are cost and durability. Aesthetics are assuredly subjective; however, the science supports that choosing better-for-the-earth materials does not need to have a negative impact on either cost or durability. Savings, in both material cost and long-term wearability, can be realized when considering cool PVC roofs and their positive effect on cities.
*“A Global Economic Assessment of City Policies to Reduce Climate Change Impacts,” by F. Estrada, W. Botzen, and R. Tol, published on May 29, 2017, by Nature Climate Change. For more information, visit https://doi.org/10.1038/NCLIMATE3301.