DISPELLING MYTHS

COOL ROOFS AREN'T JUST FLAT AND WHITE:
SAVING ENERGY FROM TUCSON TO DETROIT, FROM L.A. TO D.C.

Solar energy distribution graph illustrating that infrared radiation makes up a large portion of the solar spectrum. Graph courtesy of Berkeley Lab Heat Island Group

 

Many designers believe that only white roofs meet the criteria for cool roofs, that flat or low-sloped roofs are the only ones that benefit from the specification of a cool roofing material, and that a cool roof is only beneficial in hot climates. Researchers have created a new pallet of roofing materials that combine the properties of color with reflective surface materials that challenge some of the basic assumptions of what constitutes a cool roof. Numerous roofing materials are now rated with relatively high reflectance and emittance values, including high profile products, colored products and those applicable for steep sloped roofs. This provides more choices for designers to employ aesthetic designs.

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 the Lawrence Berkley National Laboratory assisted manufacturers with developing new colors for cool roofing materials.^x 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" forest green 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 forest green counterpart; this is the power of infrared reflective pigments.

Advancements in pigment technology increase solar reflectance across a broad spectrum. Photo courtesy of American Rooftile Coatings

 

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 out weigh heating penalties in many cold climates. 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. The energy savings calculators developed by DOE and EPA, mentioned in the "Calculating Energy Savings" section of this course, are great tools to help assess the effectiveness of a cool roof in any climate.

Heat loss through this snow covered roof in a Northern climate is no more marked by "cool" roofing than by traditional roofing. Photo courtesy of A3C Collaborative Architecture

 

As we all know first-hand, 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.

Temperature Studies: Cooler than What?
Former Vice President, Al Gore, states that the planet has a fever. Studies have shown that cool roofs can be as much as sixty degrees cooler in surface temperature than non-cool roofs.^xi A cool roof can reduce the temperature of a building enough to reduce the energy load. When Dan Jacobs, AIA, LEED AP, principal of A3C Collaborative Architecture of Ann Arbor, Michigan, decided to prove the advantages of a cool roof, he collaborated with Professor Motaba Navvab, Ph.D. of the University of Michigan - Taubman College of Architecture and Urban Planning Sustainable Design Research Laboratory to evaluate cool and green roof performance values.

Sustainable Design Research Laboratory, Ann Arbor, MI

A green retreat allowed the architect to test various types of cool roofs and sustainable design strategies, including this sloped metal cool roof over the conference room. Photo courtesy of A3C Collaborative Architecture

In 2006, A3C Collaborative Architecture decided to convert their offices into a learning lab for numerous environmental systems and products and created the UrbEn Retreat, a rooftop meeting space that overlooks a demonstration cool and green roof. The project covers several thermal studies of cool roof performance in South East Michigan. While several studies have been done of other areas of the country, none have factored in the lake effect and weather conditions of Lower Michigan, where there tend to be only three days of clear sky during December, January and February. The weather can typically be described as cold, dark, grey and snowy. A3C is working with the University of Michigan Architecture School�s Sustainable Design Lab to assess the performance of cool roofs over a full year. While results are not yet fully compiled, the initial returns show that cool roofs provide a benefit to building owners in the region by reducing rooftop thermal gain during the summer. Measurements from the first day of summer and the hottest day of summer show that the white membrane cool roof provides an eighty percent reduction in heat transfer through the roof as compared to the black EPDM roof, providing a substantial benefit to the building owner (See figure below). Data for the spring and fall months indicates that there is no heating penalty, or heat loss through the roof, that can be directly attributed to the cool roof. Similarly, when the roof is covered with snow during the winter months, the data indicates that there is a negligible and equal heat loss through the cool and non-cool roof surfaces. Measurements from this study show that cool roofs can not only be effective in cool climates, but are also applicable for both sloped and flat surface configurations.

Measurements throughout the day monitored cool roof performance. Click on the diagram to view more details. Photo courtesy of A3C Collaborative Architecture

 

THE FUTURE COOL ROOF

Continued research and development of cool roofing materials will provide even more opportunities to mitigate urban heat islands, save energy, reduce CO2 emissions and improve air quality and occupant comfort. Additionally, new products will emerge that will broaden the range of applicability and client suitability. Cool roofs may be one of the easiest and most effective energy savings measures to implement, in both new construction and existing buildings, without compromising project design or performance.

Co-written by Celeste Allen Novak, AIA, LEED AP and Sarah Van Mantgem, Energy Consultant and Cool Roof Rating Council Administrator.

The Cool Roof Rating Council is a non-profit organization that maintains a credible, third-party rating program for measuring and labeling the radiative properties of roofing products. The Council publishes these ratings via its online directory as a public service for use by architects, code officials, building owners and other interested parties. www.coolroofs.org.