Why ‘cool roofs’ are critical to urban resilience



As the impacts of climate change become increasingly more apparent, investing in sustainable technologies to improve the resilience of our cities is more important than ever. While shiny solar panels often absorb much of the sustainability limelight, a roof’s simple coating can have profound impacts on a building’s environmental performance – as well as that of entire communities.

On a typical summer day, traditional “dark roofs” absorb significant sunlight – around 100 watts per square foot, or the power of one bright incandescent light bulb, according to Lawrence Berkeley National Laboratory. This effect heats both the building and surrounding air, which increases energy use in air conditioned buildings, making non-air conditioned buildings less comfortable. Meanwhile, hot dark roofs exacerbate urban heat islands by warming the air flowing over the roof, and contribute to climate change by radiating heat into the atmosphere.

With average global temperatures on the rise – last year was the hottest year on record, according to NASA, and the 10 warmest years on record worldwide have all occurred since 1998 – dark roofs are going to make less and less sense in the future. Increasingly, so-called “cool roofs,” which reflect and emit the sun’s solar energy back to the sky instead of absorbing and transferring heat to the building below, present an attractive option for facility managers and architects.

What is a cool roof?
By reflecting sunlight and cooling itself by efficiently emitting radiation to its surroundings, a cool roof literally stays cooler and reduces the amount of heat conducted to the building below. In buildings without air conditioning, this keeps the building cooler and at a more constant temperature. Likewise, if a building has air conditioning, it doesn’t need to work as hard.

One of the most common ways to create a cool roof is via the installation of a cool, white, elastomeric roof coating. As the Cool Roof Rating Council (CRRC) puts it: “Imagine wearing a white or a black T-shirt on a hot day. By wearing the white T-shirt, you will remain cooler than if you wore a black T-shirt because it reflects more sunlight and absorbs less heat. Cool roofs, like a white T-shirt, keep the internal temperature of the building lower.”

The application of a reflective roof coating is an effective measure to reduce internal temperatures in buildings – the coatings have a high solar reflectivity which helps to reduce the overheating at high temperatures, while effectively contributing to an extended lifetime of a roof.

Economic benefits of cool roofs
By reducing temperature fluctuations inside a building, a cool roof can offer reduced cooling energy costs while increasing comfort level. While energy savings vary depending on climate and geography, CRRC estimates that average energy savings range from 7 to 15 percent of total cooling costs. With the average U.S. residential monthly electric bill running around $110, according to the U.S. Department of Energy – Energy Information Administration (EIA), this can result in significant cost savings over the long term.

At the same time, cool roofs can increase occupant comfort and avoid the need for installing an air conditioner where there isn’t already one – and decrease the size and prolong the life of air conditioning systems. With new HVAC systems costing hundreds of dollars, these are no small peanuts. And as many cities increasingly call for more stringent green building codes, cool roofs can help homes and other buildings meet them. In some states, such as California, there are several incentives for purchasing qualifying cool roof products, ranging from financing to direct rebates.

White materials for flat roofs
While people often think of roofs as being sloped, in many cities, such as Philadelphia, low-sloped “flat” roofs predominate. In these areas, white materials are a popular cool option for building surfaces that can’t be seen from the street. On a typical summer afternoon, a clean white roof that reflects 80 percent of sunlight will stay about 55°F cooler than a gray roof that reflects only 20 percent of sunlight, according to Berkley Lab.

When it comes to white roofs in flat-roofed areas, there is a compounding effect – many buildings with white coatings can lower ambient air temperatures which help lower urban air temperatures by reducing the amount of heat transferred from roofs to the air, mitigating the urban heat island effect. Like all cool roofs, white roofs also help decrease heat absorbed at the Earth's surface, which can lower surface temperatures and reduce the flow of heat into the atmosphere – helping to offset warming caused by greenhouse gases. Meanwhile, there are a myriad of health benefits associated with improved air quality.

But creating a durable white roof isn’t as simple as applying a coat of white paint. The two basic characteristics that determine the "coolness" of a roof are solar reflectance (SR) and thermal emittance (TE). Both properties are rated on a scale from 0 to 1, where 1 is the most reflective or emissive. For some 20 years, Dow Building and Construction has answered the growing demand for energy-efficient cool roofs by supplying roof coating binders specifically designed to enhance the performance properties critical for a long-lasting roof coating: high elasticity, reliable adhesion, durability, UV resistance and dirt pick-up resistance.

A common problem that coatings applied to flat roofs face is the presence of ponds caused by irregularities in the roof. These ponds lead to biofilm growth and cycles of wetting and drying, both of which can cause damage to the coating and, ultimately, to the health of the roof. When these coatings fail they reduce the longevity of the roof and increase the costs associated with replacing the roof – not to mention the sustainability concerns associated with increased waste sent to the landfill.

One of the ways Dow is helping to promote and protect white roofs is by increasing the resistance to ponded water through the introduction our new CENTURION™ Acrylic Binder for Roof Coatings. This binder uses proprietary cross-linking technology to improve performance in ponded water situations. CENTURION™ greatly improve the water resistance of an acrylic coating and minimizes the changes in properties that these coatings often experience after exposure to water. This can expand the number and types of roofs that are suitable for an acrylic coating. And since it only needs to be refreshed every decade or so, CENTURION™, if consistently reapplied, can protect your roof for many years to come.

The future of roofs is cool
As the virtues of cool roofs have become more apparent and the business case more difficult to ignore, we can expect to see cool roofs increasingly mainstream. Already, this technology is growing outside of North America – and products and formulations created by Dow and others are being developed to meet specific climate needs that vary from region to region. In Saudi Araba, for example, the sweltering climate makes it a perfect candidate for cool roof technology, and white roofs in particular.

These aren’t just for homes – they are particularly applicable on buildings with large, flat surface areas such as malls and warehouses. And as cities across North America and the world increasingly require cool roofs as an option for reducing energy consumption and mitigating the urban heat island effects, they will become a critical tool for promoting urban resilience.

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