Beyond cool
Beyond their cool roof aspects, both concrete and clay tiles have other sustainable qualities.
Regional materials
Clay and concrete tile are manufactured in many parts of the country. Raw clay is locally sourced throughout the United States as a waste product of other mineral extraction operations. It is not only used for clay tile, but also as one of the two main raw materials of Portland cement powder, which is used to make concrete. Since most clay and concrete tile products are sold in the region where they are manufactured (to reduce costs associated with shipping), they often qualify as regional materials. However, not all colors and styles are equally available in all regions.
Both cement manufacturing and clay-tile firing are high-heat processes, with the associated CO2 emissions. However, modern manufacturing technologies have significantly lowered the energy consumption and increased the efficiency of these processes; in the best plants, they are more sustainable than they have ever been.
Long life cycle
Both materials have very high durability and long service life, reducing material consumption inherent in replacing them. Neither suffers noticeable degradation in performance over time unless they are physically damaged, and it takes considerable abuse to do so.
Concrete tiles are generally expected to last at least 50 years. Integrally pigmented concrete may exhibit slight color loss, especially during the first few years, as some less-adhered pigment particles in the surface layer get washed out by rain or snow. This slight lightening of color can result in a slightly higher three-year aged SRI than the initial SRI, possibly resulting in improved cool-roof performance.
Clay tile often lasts for centuries. The material generally does not exhibit much change in color or other performance factors during its service life. Also, like concrete, clay is recyclable at the end of its service life.
Photo © Steven H. Miller
Design considerations
Roof pitch is both an aesthetic and performance choice. Steeper pitches shed water and snow more readily, and may stay cleaner because rain tends to wash them easily. In regions with intense rainfall or high snow loads, lower slopes may not be suitable.
The ORNL study found some lighter-colored tile experienced a slight reduction in reflectance due to soiling by air pollution and airborne dust over the course of two years of exposure. This effect was more pronounced at test sites in a highly populated urban area of California and in the desert than it was in a more rural (and more moist) region of Tennessee. A steeper-pitched roof was also observed to show less loss of reflectance than a lower-slope (2:12) roof. The hypothesis is wind action kept the tile cleaner. For pitched roofs in urban areas, or regions such as the Western United States where dry and dusty conditions prevail, slopes of 4:12 and greater may be advisable to limit reflectance loss.
A cool roof’s efficiency varies with the climate. Locations experiencing a large differential between daytime and nighttime temperatures benefit more from the thermal mass effect. Regions with a high proportion of rainy or overcast days will not enjoy as large a reduction of cooling costs or energy consumption, because they have lower need for cooling.
Conclusion
Concrete and clay tile cool roof systems are highly effective at reducing heat buildup both inside and around buildings. Their effect extends beyond the building walls to include minimizing contributions to urban heat islands. As code requirements gradually increase the proportion of cool roofs in urban areas, heat islands become less intense, and their multiplier effect on energy consumption and carbon emissions decrease.
In many ways, the various styles and colors of cool roof tile assemblies take the handcuffs off design professionals, allowing them to create architectural roofs compatible with high sustainability goals. As those goals continue to evolve with more stringent standards, clay and concrete tile manufacturers can be expected to respond with even more efficient and effective products.
| LEED POSSIBILITIES |
A cool roof may contribute to credits in the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) Building Design and Construction v4 (as well as credits in LEED for Schools, LEED for Neighborhoods, and LEED for Homes). For LEED v4, possibilities include:
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Rich Thomas, LEED AP, is product manager for Boral Roofing, a manufacturer of clay and concrete roof tile. He has been involved in the roofing industry for 28 years, focusing on energy-efficient roof systems as a key aspect of roofing design. Thomas can be reached at rich.thomas@boral.com.
Steven H. Miller, CDT is a freelance writer and photographer, and a marketing communications consultant specializing in the construction industry. He can be reached via e-mail at steve@metaphorce.com.
