Architectural Record BE - Building Enclosure

Cool Roofs for Hot Projects

Using cool roofs to save energy, address global warming, meet code, and have the coolest project on the block.
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Sponsored by the Cool Roof Rating Council
Sherry Hao, Jessica Clark, LEED AP, Celeste Allen Novak, AIA, LEED AP, and Sarah Van Mantgem

Color is made visible by light reflecting off a surface and into our eyes; however, sunlight has other properties that extend beyond the visible portion we can see. We have an intuitive understanding of the power of certain colors to reflect light as well as absorb heat, e.g., black leather seats in a convertible will be uncomfortably hot in the summer, and white clothing is a cooler choice at the beach. However, sunlight is composed of infrared and ultraviolet radiation, as well as the visible wavelengths (shown in graph above). In fact, about half of the sun's energy is infrared. "Cool color" roofing materials are created by integrating pigments that reflect infrared energy, even though their color may still absorb some of the visible spectrum. In this way, roofing products can be both "cool" and dark colored. This type of infrared reflective pigment has been used in conjunction with a variety of product types, including metal, tile, coatings and asphalt shingle granules, thus broadening the scope of cool roof applicability.

A research project by LBNL assisted manufacturers with developing new colors for cool roofing materials.11 These new "cool color" products demonstrate that cool roofs don't always have to be white in order to reap the benefits of an energy-efficient roof. While the energy savings of a "cool" rustic red roof will not be equivalent to that of a white or lighter colored roof, it will certainly deliver more energy savings than its traditional non-cool rustic red counterpart; this is the power of infrared reflective pigments.

The Southwest Energy Efficiency Project (SWEEP)

The Southwest Energy Efficiency Project (SWEEP) is a multi-state organization promoting energy efficiency primarily in the Southwest. SWEEP conducted a regional case study using the cool roof at the Thomas O. Price Service Center in Tucson, Arizona. This project highlights a retrofit of an existing 23,400 sq. ft office building. As part of the "Creating Cool" campaign, a cool roof was installed on Building One in order to offset the urban heat island effect.

The existing 28,000 sq.ft. aluminum roof included portions of a darker, blackened copper surface. The roof was retrofitted with a white elastomeric top coating in June 2001. The peak temperature reduction between the black roof and the cool roof was an average of 78 °F and between the raw metal roof and the cool roof surface, 46 °F. The average temperature reductions were between 17 °F and 10 °F between the black/metal roof and the cool roof surface. The building demonstrated a reduction in energy consumption of almost 50 percent with the cool roof, primarily due to a decrease in energy used for air conditioning.

The payback period for this investment in 2001 was approximately six years and the rate of return was 16 percent for the investment. The new roof had the added benefits of repairing existing roof leaks and extending the life of the roof from a UV-resistant coating that provided increased protection from solar radiation.

 

Price Comparison

As with any green product, a popular belief is that cool roof products are prohibitively more expensive than traditional roofs, such that the energy cost savings will never make up for the initial costs. Cool roof benefits can be realized simply by making smart color choices. A designer may choose a light grey coated metal panel rather than a dark grey, which can keep the building cooler without any additional costs. However, a cool roof need not cost more than a non-cool roof, and certain product types can be similar in cost to the conventional non-cool style. While some cool roof materials are more expensive than their non-cool counterparts, prices are decreasing as cool roofing becomes a more widely used option. Cool roof incentives and rebates can also help offset additional costs. Utilize the calculators mentioned in the "Quantifying Energy Savings" section to assist in calculating payback options.

In July 2010, the DOE released Guidelines for Selecting Cool Roofs, as part of their effort to broadly implement cool roofing technologies in the Federal Government. This resource provides an in-depth study of the economics of cool roofing.

Cool Roofs in Cool Climates

Another myth that hounds cool roofs is the notion that they should not be used in cold climates. However, there are several factors that can make cooling energy savings outweigh heating penalties in climates with cold weather. The sun is lower in the sky in the winter than in it is in the summer; the further north one goes, the lower it gets. Because of the shallow incoming angle, solar rays have to travel through a thicker cross-section of atmosphere during winter. As a result, the diffuse rays lose some of their potency. Additional factors that reduce the potential for heat gain during winter are that the days are shorter and there are fewer sunny days. It is also fairly typical to see snow-covered roofs in climates where a heating penalty might be a concern; this condition obviously negates any effect a roof's reflectivity may have on heat loss or gain.

While more energy can be saved through the use of cool roofs in hot climates than in cold climates, there can still be advantages to specifying cool roofs in cooler climates, especially where significant air-conditioning is needed in hot summer months. In addition, it has become increasingly evident that urban heat islands exist in cool climates, and the summertime benefits of installing a cool roof may outweigh the heating penalties during winter. It is important to remember that the majority of the cooler indoor temperature and energy savings from cool roofs are realized in the spaces immediately below the roof, and may be lessened in buildings with substantial insulation. The energy savings calculators developed by DOE and EPA are great tools to help assess the effectiveness of a cool roof in any climate.

 

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Originally published in Architectural Record

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