Cool Roofs Can Generate Healthy Energy Savings
By Naomi Millán, Associate Editor March 2008 - Energy Efficiency
With longer and hotter days just around the corner, facility executives might be considering ways to keep cooling costs down. If the roof made it to this year’s list of capital improvement projects, taking the extra step to create a cool roof could significantly impact energy cost savings — as much as $.20 per square foot of roof area, according to industry experts.
The idea of cool roofs has been around for decades but debate still swirls around whether they make sense in all locations. While there are many factors to consider before installing a cool roof, research shows a cool roof can make sense in almost any climate. Also, advancements in cool colors and databases which rate the coolness of certain materials make finding the right cool roof that much easier.
What is a Cool Roof?
The choices are plentiful when it comes to cool roofing. Cool roofing materials include membranes, metal roofs, tile and even asphalt shingles. Understanding the basic principles behind cool roofing will help sort out the options. The two properties that govern cool roofs are solar reflectance and thermal emittance. Solar radiation is made up of ultraviolet, visible and near infrared wavelengths. Of that radiation, 53 percent is in the near infrared end of the spectrum, which is not seen but absorbed as heat. Solar reflectance refers to the ability of a material to bounce that infrared wavelength back out before it can be absorbed as heat by the roof and transferred into the interior of the building.
Thermal emittance is the capacity of a material to release heat it already has absorbed back into the atmosphere before it is transferred by convection down into the building. While both reflectance and emittance are important, solar reflectance has a greater effect on a roof’s coolness.
Above a certain point of reflectance and emittance, a roof is deemed “cool.” Where that line is, however, is not universally agreed upon. Ideally, the roof needs to have the highest solar reflectance and highest solar emittance possible, says Hashem Akbari, group leader of the Heat Island Group and senior scientist at the Lawrence Berkeley National Lab. These properties are rated on a scale of zero to one, with one being 100 percent reflective or emissive.
Different jurisdictions and accrediting bodies have different definitions for how reflective or emissive a roof must be. The California Title 24 2005 standard for cool roofs, which is a prescriptive requirement for low-slope roof applications, requires the roofing product to have an initial thermal emittance of .75 and an initial solar reflectance of .70. Chicago’s Energy Conservation Code requires low-sloped roofs to have an initial reflectance of .25 on new roofs through 2008, and ENERGY STAR-compliant roofs after that. Currently ENERGY STAR requires a .65 initial solar reflectance, which degrades to no less than .50 after three years. Emissivity is currently not a factor for an ENERGY STAR rating, though emissivity values are now listed on the ENERGY STAR qualified products list. Long term, the EPA plans to consider the possibility of adding an emissivity requirement to ENERGY STAR.
What Are the Benefits?
Installing a cool roof does not have to be much more expensive than installing a conventional roof. For example, a painted metal roof is more reflective than a non-painted roof and will have a $.50 per square foot premium, says Greg Crawford, executive director of the Cool Metal Roofing Coalition and secretary of the Cool Roof Rating Council’s board of directors. But the premium between traditional pigments and infrared reflective pigments is nominal, he says. Some cool roofing alternatives, like single-ply membranes, can be cheaper to install than built-up roofs. Akbari suggests thinking about the cool alternatives within any roof system choice. “For example, in the asphalt shingle industry, black shingles have 5 percent reflectance and white shingles have a 25 percent reflectance, but there is a 0 percent cost premium between the two. You should think about the easy cost savings,” Akbari says.
Such a small premium for choosing a cool roof over a conventional roof means it might not be necessary to spend a lot to save a lot. Cool roofs save money by reducing the amount of heat that makes it into a building through the roof, thereby reducing the need for mechanical cooling to maintain a comfortable temperature. Because commercial buildings cool their space pretty much throughout the year, energy savings can be realized year-long. For commercial buildings, the estimated net annual savings of a cool roof vary between $.10 and $.20 per square foot of roof area, says Akbari.
The degree of savings depends largely on the location of the facility. In a study funded by the U.S. EPA, the Heat Island Group at the Lawrence Berkeley National Laboratory focused on the energy savings potential of light-colored roofs in 11 U.S. metropolitan areas and found savings were possible in all of them. (See chart.) The net energy savings were measured as the savings of cooling energy minus the penalties of heating energy. Ten residential and commercial building prototypes in each area were simulated. The warmer the climate, the greater the number of cooling days and the greater the cost savings. In places like Phoenix and Los Angeles, the potential for savings is $37 million and $35 million annually respectively. Cities in more northern climates would realize smaller savings, but savings nonetheless: $10 million for Chicago and $3 million for Philadelphia.
Installing a cool roof in cooler climates has been often debated because it is thought that the cool roof will increase heating costs during the cold months and that there aren’t enough warm months to capture significant savings. However, as the study shows, the heating penalty might be overstated. Even in cooler climates, where a cool roof really shines is at shaving peak cooling energy demands.
“That is when most facilities have the most problems and that’s what cool roofs really help to mitigate,” says Steve Ryan, ENERGY STAR roof products program manager. “They’re installing cool roofs up in Canada. You can’t get much more northern than that.”
In addition to saving energy costs, a cool roof could scale back the tonnage of the next HVAC upgrade because of the smaller cooling load within the building, Akbari says.
Cool roofs have positive effects for the climate outside of buildings as well by mitigating the urban heat island effect. In an urban setting, the large amount of dark non-reflective surfaces such as roads, parking lots and roofs, and the relatively small amount of green space cause the ambient temperature to be six to eight degrees warmer than surrounding areas. Not only does that directly increase cooling costs because the incoming air is that much warmer, but it also increases air pollution by creating and intensifying smog. For example, in Los Angeles, every degree above 70 degrees F increases smog by 3 percent.
There is also an expectation that a cool roof will last longer than a conventional roof because materials that stay cooler take longer to break down. In general, this is true, but to pit, for example, a single-ply membrane roof against an asphalt shingle roof and say the cooler roof will last longer is not fair simply because it’s not an apples to apples comparison, says André Desjarlais, group leader in Building Envelopes Research at Oak Ridge National Laboratory.
“There’s an expectation that a cool roof at a lower temperature will last longer,” Desjarlais says. “The reality is that a cool roof is a different critter than a non cool roof — different plastics, different polymers, different materials.” Desjarlais points out that if you could compare two roofs that are exactly the same except for their coolness, the cool roof would almost certainly last longer.
Factors to Consider
Even though there are ENERGY STAR-rated roof products, unlike household consumer goods that will perform as well in New York as in Hawaii, the savings from cool roofs depend on many factors. “It’s not a one-size-fits-all situation,” says Ryan. “It’s not like a refrigerator where you can just buy it and be guaranteed savings. Savings are going to vary with every roof.” For example, a building in a northern climate without a lot of cooling days and a high level of insulation will see a smaller amount of savings than a poorly insulated building in a hot climate.
The list of factors that affect the effectiveness of a cool roof is long and varied. “A cool roof is part of the overall building envelope,” says Crawford. “Its effectiveness depends on the style of the building, whether low-slope or steep-slope, multifloor or single-floor, the amount of insulation it has, its orientation, its fenestration.”
Building use will affect the effectiveness of a cool roof as well, says Desjarlais. Consider a commercial office building and a private residence in the same northern location, he says. The office building — which is full of people, equipment, lights and other heat-generating devices — may require cooling even in the wintertime. In the home, there is not much to generate an internal heat load, so heating the space would be more likely. “It depends not only on where you are but also how much internal energy is being produced in the building,” says Desjarlais. “The more internal energy generated, the greater the need for air conditioning, the greater the benefit of a cool roof.”
When making the decision to install a cool roof, facility executives should still follow the steps for selecting a good conventional roof. The focus should primarily be on its quality and longevity, not the coolness, says Desjarlais. “You look at a roof and you might want to have a cool roof, but you want to remember that the primary purpose is to keep you dry. Don’t forget all those other attributes of a roof. You just want to add coolness on top of that, as an additional feature. You want a good roof, then you want a cool roof. It’s a lot easier to make a roof cool than it is to make it good.”
Desjarlais says that in the desire to save energy with a cool roof, facility executives might rush into a poor choice. Some new roofs marketed as cool might not last 10 years while other less cool methods have proven track records. He cautions facility executives to ensure the roof system will stand the test of time and meet their needs, and then take the next step to make it cool.
As with any roof, proper maintenance is important. Making sure water, dirt and debris do not collect on the roof will extend its longevity and maintain its reflective properties. Some materials have a greater ability to shed dirt, which will cut down on the labor costs to keep it clean. As always, conducting a lifecycle cost analysis of roofing options is good practice.
In the end, a cool roof does not create more of a burden than a conventional roof and has the potential to generate significant savings. After evaluating their facility’s energy profile, facility executives might decide that it might be just the thing to take the bite out of their summer cooling bills.
Click & View Chart (pdf)
Cool Roof Resources
Cool Roofing Materials Database from the Lawrence Berkeley National Laboratory
More Than One Way to Cool a Roof
Cool roofs once meant white roofs, but the industry has come up with some alternatives. In the end, white will always provide the highest reflectance, but if white is not an option, consider these possibilities:
— Naomi Millán