About Kurt Shickman

Kurt Shickman is executive director of the Washington, D.C.-based Global Cool Cities Alliance, which seeks to accelerate a worldwide transition to cooler, healthier cities.

There Is Evidence Cool Roofs Provide Benefits to Buildings in Climate Zones 4 through 8

FIGURE 1: Reflective roof requirements in ASHRAE 90.1 and IECC only apply in Climate Zones 1 through 3, shown here on the ASHRAE Climate Zone Map. SOURCE: U.S. Department of Energy

FIGURE 1: Reflective roof requirements in ASHRAE 90.1 and IECC only apply in Climate Zones 1 through 3, shown here on the ASHRAE Climate Zone Map. SOURCE: U.S. Department of Energy

Reflective roofs are a tried and true way to improve building energy efficiency and comfort, generate net energy savings and help mitigate summer urban heat islands. Reflective roofs work by reflecting solar energy off the roof surface, rather than absorbing the energy as heat that can be transmitted into the building and surrounding community.

The simple act of switching from a dark to a light-colored roof surface has a number of benefits. Buildings protected by these types of roofs require less energy to cool and help building owners and residents save money. Cool roofs on buildings without air conditioning can save lives during heat waves by lowering indoor temperatures. Cooler city air is safer to breathe and less polluted, which makes cities more livable and less vulnerable during heat waves. Increasing the reflectivity of urban surfaces can also offset the warming effect of green- house gases already in the atmosphere and help us address the challenges of climate change. Taken together, these benefits are worth billions of dollars to the growing number of people that live and work in U.S. cities.

The energy-savings case for cool roofs in warm climates is clear. Widely adopted model building-code systems, ASHRAE and the IECC, address roof reflectivity. ASHRAE 90.1-1999 added a credit for highly reflective roofs with IECC allowing compliance via ASHRAE in 2003. ASHRAE 90.1-2010 added reflectivity requirements for new and replacement commercial roofs in Climate Zones 1 through 3. IECC added the same requirements in its 2012 version. (Figure 1 shows the ASHRAE climate zone map for the U.S.)

There is, however, an ongoing debate about whether cool roofs deliver net energy benefits in northern climates that experience cold winters and warm to hot summers (Climate Zones 4 through 8). Do reflective roofs remain beneficial as the cold weather season kicks in? The same properties that allow reflective roofs to keep buildings cooler in the summer may also cause them to make buildings colder in the winter. Theoretically, buildings with cool roofs could require more energy to reach a comfortable temperature in winter—a consequence known as the “winter heating penalty.” Furthermore, building codes tend to require more roof insulation in colder climates than warmer climates, potentially reducing the energy-efficiency benefits of roof surface reflectivity.

FIGURE 2A: Annual energy-cost savings ($1 per 100 square meters) from cool roofs on newly constructed, code-compliant buildings with all-electric HVAC. SOURCE: Energy and Buildings

FIGURE 2A: Annual energy-cost savings ($1 per 100 square meters) from cool roofs on newly constructed, code-compliant buildings with all-electric HVAC.
SOURCE: Energy and Buildings

The “winter heating penalty” and the impact of insulation are considerations when installing reflective roofs in some cold climates, but their negative effects are often greatly exaggerated. The sun is generally at a lower angle and days are shorter in winter months than summer months. In fact, in northern locations winter solar irradiance is only 20 to 35 percent of what is experienced in summer months, which means the sun has a reduced impact on roof surface temperature during the winter. Heating loads and expenditures are typically more pronounced in evenings, whereas the benefit of a darker roof in winter is mostly realized during daylight hours. Many commercial buildings require space cooling all year because of human activity or equipment usage, thereby negating the little—if any—heating benefit achieved by a dark roof.

Two new studies, along with decades of real-world examples from the marketplace, indicate that reflective roofs are an effective net energy (and money) saver even in our coldest cities.

SNOW’S IMPACT

In a study recently published in Energy and Buildings, researchers from Concordia University in Montreal evaluated the energy-consumption impact of adding cool roofs to a number of retail and commercial buildings in Anchorage, Alaska; Milwaukee; Montreal; and Toronto. The researchers looked at older, less insulated building prototypes, as well as newer buildings built with code-compliant levels of insulation. Unlike earlier work evaluating the impact of roof reflectivity on building energy consumption in cold climates, this new analysis also accounted for the impact of snow on the roof during winter months.

FIGURE 2B: Annual energy-cost savings ($1 per 100 square meters) from cool roofs installed on older buildings with all- electric HVAC. SOURCE: Energy and Buildings

FIGURE 2B: Annual energy-cost savings ($1 per 100 square meters) from cool roofs installed on older buildings with all- electric HVAC.
SOURCE: Energy and Buildings

Snow has two impacts on the roof that are relevant to understanding the true impact of roof surface reflectivity on energy consumption. First, snow helps insulate the roof. As a porous medium with high air content, snow conducts less heat than soil. This effect generally increases with snow density and thickness. Second, snow is white and, therefore, reflective. At a thickness of about 4 inches, snow will turn even a dark roof into a highly reflective surface (approximately 0.6 to 0.9 solar reflectance).

When snow is factored in, the benefits of cool roofs in cold climates be- come much clearer. Figure 2a shows the net energy savings and peak electricity reduction with and without snow for cool roofs installed on newly constructed, code-compliant buildings, assuming all-electric HVAC. Figure 2b shows savings from cool roofs installed on existing, older vintage buildings. The paper, available from the journal Energy and Buildings also includes results with gas HVAC systems.

INSULATION’S EFFECTS

Another argument often heard against reflective roofing in cold climates is that buildings in northern climates tend to have higher levels of roof insulation that reduce or negate the energy-savings impact of roof surface color. A new field study and model analysis of black and white roof membranes over various levels of insulation by the City University of New York and Princeton University and Princeton Plasma Physics Lab, the latter two of Princeton, N.J., clearly rebuts the “insulation versus reflectivity” tradeoff.

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A New Report Finds Sustainable Roofs Deliver Millions in Benefits to ‘Roof Aware’ Cities

“Roof Awareness” has come a long way during the years. It used to be that people only thought about their roofs when something went wrong. Building owners then started realizing that making smart choices about the roof could save money on energy costs. Roofs are now seen as essential platforms for cities to meet energy-efficiency and renewable-energy goals, to improve the health and quality of residents’ lives, and to achieve social equity. A new effort to better quantify those benefits and costs shows cities with good roof awareness are reaping millions in economic benefits.

TABLE 1: Summary of cost-benefit analysis results (NOTE: There is no internal rate of return, simply payback, or benefit-to-cost ratio for rooftop PV because we all rooftop PV systems are financed with a PPA [so there is no upfront cost to DGS]).

TABLE 1: Summary of cost-benefit analysis results (NOTE: There is no internal rate of return, simply payback, or benefit-to-cost ratio for rooftop PV because we all rooftop PV systems are financed with a PPA [so there is no upfront cost to DGS]).

With that change in role comes new challenges for evaluating what type of roof makes sense for building owners and cities alike. There are well-developed building models and field studies that give us great insight into how sustainable roofing—that is, reflective, vegetated or solar roofs—saves energy and energy costs. But there is not a single tool that could evaluate all the benefits that accrue from good roofing choices beyond energy savings, such as better health, enhanced air quality, greater stormwater management and improved social conditions. Until now, that is.

A recently released report—the “Affordable Housing Smart Roof Report”—from Washington, D.C.-based Capital E, a firm dedicated to accelerating the transition to a low-carbon economy, now allows city officials and owners of affordable housing developments to see and calculate the full lifetime costs and benefits of roof decisions. “This is the first model that helps the user puta dollar value on the various benefits of sustainable roofing options. We see this as a great tool for contractors looking to quantify the full benefits of sustainable roofing for their potential clients. It will also help city officials to enact policies that recognize the value of smarter roofing that may not be directly visible on the building owner’s books,” says Keith Glassbrook, a Capital E senior analyst and one of the lead developers of the new model.

TABLE 2: Present value summary of costs and benefits for the three technologies on all low-slope DGS roofs (NOTE: All PV is financed with a PPA so there is no upfront cost to DGS; results may not sum due to rounding).

TABLE 2: Present value summary of costs and benefits for the three technologies on all low-slope DGS roofs (NOTE: All PV is financed with a PPA so there is no upfront cost to DGS; results may not sum due to rounding).

Building the Tool

Rather than reinventing the wheel, Capital E identified existing tools, models and methods from the huge base of existing science for each item in its cost-benefit analysis. The model integrates these individual, detailed components into a form that is accessible and easy to use for non-scientists and that provides straightforward results in dollars per square foot.

The model is an extension of an analysis undertaken for the Washington, D.C., Department of General Services (DGS) as part of its Smart Roofs Initiative. The initiative is designed to help Washington achieve its aspirations to become the greenest, healthiest, most equitable city in the U.S. by using the roofs of city-owned buildings more thoughtfully. DGS owns and controls more than 400 buildings in Washington, including office buildings, schools and hospitals. The city is using this portfolio (28 million square feet of buildings with approximately $62 million in annual energy expenditures) to drive deep improvements in energy efficiency and achieve other objectives.

Like a growing number of cities, Washington, D.C., is committed to using its roofs to deploy photovoltaic panels to generate electricity, cool roofs to reflect sunlight and reduce unwanted heat gain in summer, and green roofs to cut stormwater runoff that results in water pollution and requires construction of expensive water-treatment plants and other grey infrastructure. Tommy Wells, a former councilmember and current director of the District Department of the Environment, summarized the reasons in the Smart Roof cost-benefit report’s press release: “Past research shows that ‘smart’ roof strategies that reduce extreme temperatures in buildings can literally save lives. This new report provides additional justification for cool, green, and solar roofing solutions by showing that they also make compelling financial sense as we work to make D.C. a healthier and more sustainable city.”

Washington has been among the most advanced cities in the nation in deploying sustainable roof technologies. But because there was no established methodology for quantifying the full cost and benefits—including health benefits—for any of these technologies, Washington to date had not been able to quantify the full costs and benefits of these roof choices or compare the merits of each to make informed decisions about which technologies to deploy and at what scale. The analysis undertaken by Capital E to remedy this issue concluded that DGS’ Smart Roofs program can deliver between $47 and $335 million in benefits to the city over 40 years, depending on the roof technology chosen.

More Analysis

A parallel analysis was funded by the New York-based JPB Foundation, which seeks to enhance the quality of life in the U.S. by creating opportunities for those in poverty, promoting pioneering medical research, and enriching and sustaining the environment. JPB Foundation launched its analysis based on the success of this initial analysis by DGS. This time, the model was adapted to evaluate actual affordable-housing buildings in Baltimore; Los Angeles; Philadelphia; and Washington, D.C. The sample buildings, which were part of the National Housing Trust, Washington, or Columbia, Md.-based Enterprise Community Partners Inc.’s portfolios, included steep- and low-slope roofs, high- and low-rise structures, as well as some attached row houses. The project team for this study included the National Housing Trust; Washington-based American Institute of Architects; Washington-based Global Cool Cities Alliance; Enterprise Community Partners; and U.S. Green Building Council, Washington. In each city and building type evaluated, the model found sustainable roofs would generate more benefits than they cost (first cost and maintenance) and would, in some cases, have an immediate payback.

The results were variable by building and city but they confirmed that sustainable roofing was the superior economic choice compared to traditional dark roofs in the cities studied.

The JPB Foundation analysis shows there is no one-size-fits-all solution to maximize value with sustainable roofing. For example, green roofs made the most sense in Washington, D.C., because of the city’s stormwater rules. On the building in Baltimore, cool roofs were the best choice. The results were variable by building and city but they confirmed that sustainable roofing was the superior economic choice compared to traditional dark roofs in the cities studied.

A second phase is currently underway by JPB Foundation to extend the model to large areas of cities to capture the impact of sustainable roofs at a community scale, as well as other technologies, such as reflective pavements, and to better quantify some of the social benefits of cooler, more enjoyable surroundings.