What’s the difference between a cool roof, a white roof, and a green roof? What about solar?

There are a number of ways to use roofs to decrease the environmental toll of our built environment, and begin to use urban infrastructure as an agent of adaptation and environmental services. White roofs, cool roofs, green roofs, and photovoltaic installations can all be effective ways to improve the environmental performance of roofs.A cool roof is a roof that uses a highly reflective building material, regardless of the color. Cool roofs can help cool buildings, cities, and the planet by reducing the percentage of sunlight converted to heat by the building surface. White roofs are one type of cool roof. Cool colored roofs are typically treated with a coating that makes them reflective. Because white roofs have a higher reflectivity of any color, they are generally a better choice than cool colored materials. One caveat is that a any colored roof made out of metal or that has a metallic coating, will still absorb a lot of heat (because it will have a high thermal emittance) and is therefore not considered “cool”.

Green roofs are living vegetative systems located on rooftops. Their benefits include cooling the building through shading and insulation, reducing peak storm runoff, and potentially growing food. Green roofs do not, however, provide enhanced reflectivity and thus would have a negligible effect on global temperature if they were widely implemented.

Solar photovoltaic (PV) installations provide neither the reflectivity benefits of a cool roof nor the storm water management benefits of a green roof. However, PV installations can generate clean energy, an important benefit in our global effort to transition to a low-carbon energy economy. Because PV installations typically do not take up the entire surface of the roof, and sometimes require highly reflective roofing to ensure sufficient system efficiency, cool roofs and PV are complementary technologies.

Cool roofs, green roofs and PV are all excellent options for improving the environmental performance of a building. Which system or combination of systems is most appropriate for an individual roof will need to be evaluated on a case by case basis. We strongly believe that each of these options have an important role to play in reducing the environmental impact of our cities and see plenty of room for each solution to thrive.

We do want to point out, though, that cool roofs are by far the cheapest option of the three. They are at least an order of magnitude less expensive on a dollar per square foot basis than both green roofs and PV. When resources are available, we certainly encourage PV and/or vegetative roofs. However, we believe that cool roofs are a solution that can be more readily adopted by a larger percentage of cities globally.

Cool Roofs and Cool Pavements Toolkit

The GCCA Cool Roofs and Pavements Toolkit is a hub of information on urban heat islands and mitigation technologies. The toolkit includes a searchable knowledge base of more than 500 studies, pictures, videos, presentations, and programmatic documents. You can also check out the Forum to ask questions, find answers, and interact with others.

For basic background, the toolkit also has downloadable primers that explain the science of cool, reflective surfaces and highlights how cities implement policies to reduce excess urban heat.

The Toolkit is updated on a regular basis, so check back often to find the latest cool news and science.

CoolRoofToolkit.org

Extreme Heat Hits Northern Cities Too

Several recent reports have shed light on the many ways climate change is affecting our way of life, and these reports have people talking about strategies for dealing with extreme heat and the resulting health problems. We expect extreme heat events down in Atlanta, GA or Los Angeles, CA. But we’re also hearing of concerns over the urban heat island effect and extreme heat in northern cities like Minneapolis, MN and Chicago, IL.

Atlanta is better able to handle these extreme heat events, with most buildings and homes equipped with air conditioning units. But cities further north may not have the cooling infrastructure to handle more extreme heat waves. Schools (which don’t have air conditioning) are shut down and children stay home. People living on the top floors of un-air conditioned buildings are in greater danger of illness or even death from this extreme heat.

Extreme heat is also affecting places like London, England, and studies tell us that unless something’s done to mitigate the impact of climate change, mortality will increase significantly. London could be looking at 800 deaths per year by 2050. Another study tells us that London could see their heat-related mortality rate jump 257% by 2050 unless steps are taken to address the effects of extreme urban heat.

The good news is that more people are beginning to understand that – even in cooler climates such as London –cool roofs can bring down the temperature in buildings, increasing comfort and reducing the chance of heat-related illness and death. It also brings down energy consumption, which means less carbon in our atmosphere.

You can learn more about extreme heat around the United States, by visiting NOAA’s extreme heat tracking site HERE.

Cool Roofs in UAE

A recent article about the benefits of cool roofing installed at Masdar in the United Arab Emirates points out a lot of what we already know about cool roofing saving energy etc. The article is worth a read, but especially for the great photo of the researchers walking on the white roof with bare feet! Good visual evidence of the cooler surfaces created by reflective roofs.

Five Ways We’re Already Killing Ourselves with Climate Change

Yesterday, the Obama Administration released the Third National Climate Assessment. This is the most comprehensive look at how human activity is changing our climate – and it’s not good news. Grist lays out 5 ways climate change is already harming us, and urban heat comes into play in two of the five points.

Urban heat kills, and it sends thousands to the emergency room during extreme heat events every year. This report tells us that not only are these events already becoming more frequent, but overall temperatures are on the rise, making them more deadly. The study also shows us that humidity is on the rise, making these extreme heat events even more unbearable – especially among vulnerable populations.

We may be sending less smog up into the air from our tailpipes, but the added heat and sunlight brought on by climate change mix with other pollutants, increasing the amount of ozone in our atmosphere. Droughts add dust and wildfires send up soot, adding to the level of pollution. The urban heat island effect makes these problems even worse for people living in our cities.
Cool roofs and reflective pavements help reduce urban heat and they save lives. And as cities add reflective surfaces to their mitigation arsenal, they’ll also save money.

Hot Town – Summer in the (Twin) Cities

Climate change and the urban heat island effect are heating things up in cities around the world. That’s not surprising in places like Los Angeles and Atlanta, but they’re finding it too hot to handle in northern cities as well.

Researchers are collecting data on urban temperatures in Minneapolis / St. Paul, Minnesota (Twin Cities), and they’re finding cooler temperatures in areas with more vegetation and reflective surfaces. They’re hoping this data will help them map out strategies for mitigating the effects of climate change in northern climates. Part of their strategy includes installing more green and white rooftops and planting trees throughout the Twin Cities.

A National Science Foundation grant will help set up studies in other cities around the country to measure the urban heat island effect.

You can read more about it at:
Can’t take the heat? Get out of the city

Cool Surfaces News Roundup: 2013

The Global Cool Cities Alliance keeps a comprehensive list of media covering cool roofs, cool pavements, and a wide range of urban heat island-related issues. Periodically, we will highlight trending topics in a feature we call Cool Retrospective. For our inaugural issue, we look back at 2013.

2013 saw a number of cities make huge strides in mitigating the impacts of excess urban heat by adopting new cool surface policies. New studies identified the economic advantages of cool roofing and highlighted other critical benefits of reducing urban heat islands to health and to addressing environmental and social justice issues. Overall, awareness of the cool roofing opportunity continues to grow.

Read the Retrospective on our Cool Roofs and Cool Pavements Toolkit knowledge base here.

GCCA Releases Cool Surfaces Retrospective 2013

Periodically, the Global Cool Cities Alliance will highlight trending topics covering a wide range of urban heat island-related issues in a feature we call Cool Retrospective. For our inaugural issue, we review cool developments in 2013.

2013 saw a number of cities make huge strides to mitigate the impacts of excess urban heat by adopting new cool surface policies. New studies identified the economic advantages of cool roofing and highlighted other critical benefits of reducing urban heat islands.

You can read all about it HERE.

Using the Roof Savings Calculator

The Roof Savings Calculator (RSC) is a joint initiative of Oak Ridge National Laboratory, Lawrence Berkeley National Lab, and White Box Technologies that models the energy use impact of various residential and commercial roof structure choices via a simple online interface. Building owners, contractors, and manufacturers use the RSC to support roof decision-making, and as part of marketing materials. It is important, therefore, to understand the current state of the RSC.

GCCA recently drafted an update on the process to redevelop, fix bugs, and validate the RSC. We ultimately recommended that, although the RSC is live, it is better to hold off on using the RSC until the full validation process is complete. You can get the whole picture on the Knowledge Base.

Responding to ‘Ballast Cooler Than You Think’

The recent article by Annie McCarren called ‘Ballast Cooler Than You Think’ published in Environmental Design and Construction provides an overview of the “cool” properties of thermally massive ballasted roofs. We largely agree that ballasted roofs can be a good roofing option under the right circumstances. There are, however, a few inaccuracies in the article that are worth dispelling because they recur frequently in discussions of cool roofing concepts.

The extra thermal mass of a ballasted roof can make a lot of sense, particularly in climates that are hot in the daytime and cool at night. Some codes and standards do allow ballasted roofs to count as exceptions to cool requirements, though the threshold weight required for the tradeoff is pretty high and will probably not be a cost-effective substitute for reflective roofs. For example, California’s Title 24 building code allows stone ballasted roofs weighing 17 pounds per square foot or a roof with pavers weighing 25 pounds per square foot to qualify for the exemption, which is heavier than most ballast rock on the market. Ballasted roofs may also increase nighttime cooling energy demand in areas with hot days and warm nights.

There are a few inaccuracies to address, however.

First, reflecting sunlight is not the same thing as reducing heat flow into the building (with insulation) or delaying heat flow into the building (with thermal mass). Increasing roof reflectance rejects solar energy as light, cooling the building and the outside air. Adding insulation to the roof reduces heat flow into the building, while raising the roof surface temperature and warming the outside air. Adding thermal mass to the roof makes it slower to warm during the day and slower to cool at night. Ideally, a designer would opt for a high-performance roof system that optimizes the solar reflectance, thermal resistance, and thermal mass of the roof to cost-effectively gain the benefits of all three approaches.

The next few issues arise when the author says: “A cool roof can be a great choice for buildings in warm, southern climates where the number of cooling degree days exceeds the number of heating degree days. The term ‘cooling degree days’ refers to the number of days that a building would need to run an air conditioning unit versus a heating unit to stay at a set temperature.”

One problem here is that the author neglects to mention that cool roofs are a viable roofing option in northern climates. While cool roofs certainly make a great choice in warm southern climates, they can also provide net energy benefits in places where energy usage for heating exceeds cooling energy consumption. There are plenty of peer-reviewed studies to support the notion that cool roofs can provide net energy savings in northern climates. Perhaps more importantly, the well-established and rapidly-growing marketplace for cool roofing in northern regions (covering billions of square feet of roof space) demonstrates that the author is presenting an overly narrow view of the energy benefits that cool roofs are providing to building owners across the country. The case for cool roofs is even stronger when the health benefits of cooler cities (such as cleaner air and lowered heat health risks) are factored in.

A second problem with the author’s statement is the misuse of heating and cooling degree days as a means to determine if reflective roofing makes sense from an energy savings perspective. The article incorrectly defines heating and cooling degree days. Heating/cooling degree days gauge the need for heating or cooling that is driven by the difference between outdoor and indoor air temperature. For example, annual cooling degrees days (base, or threshold temperature of 65°F) would be determined by calculating for every hour of the year the outside air temperature and subtracting the base temperature (in this case, 65°F), summing all positive values, and dividing this total by 24 (hours in a day). But because cooling degree days and heating degree days are based on outdoor air temperature and do not account for the sun’s ability to heat buildings, they paint a misleading picture. To illustrate this point, consider a cool sunny day during which the outdoor temperature never exceeds 65°F (meaning zero cooling degree days). The building may still require air conditioning to remove its solar heat gain. In fact, cool roof savings/penalties are not proportional to cooling/heating degree days. Rather, they scale with the solar energy that strikes the roof during hours when the building is cooled/heated.