by Rich Thomas, LEED AP, and Steven H. Miller, CDT
To many design professionals, the term ‘cool roof’ means a flat roof with a white membrane—a high-reflectance roof that performs well, but needs to be hidden from view. This is a very unfortunate misconception. There are other roofing options that can also contribute to a building’s architectural statement while meeting cool roof standards. Tile roofs—both concrete and clay—are cool by design in ways that include solar reflectance, as well as going beyond it.
Pitched tile roofs not only present a designable surface to the world, but also have inherent material and structural properties that combine to make the assembly highly effective at dealing with solar energy: high thermal emissivity, high thermal mass, natural insulation, and passive ventilation. An air channel beneath the tile—formed by the tile’s structure—not only reduces heat transference during summer days, but also mitigates heat loss during the colder seasons.
Tile cool roofs allow architects to design roofs that help combat ‘urban heat island’ formation and make the building cooler, more energy-efficient, and less costly to operate without sacrificing architectural style.
What is a cool roof?
Every day, energy from the sun, in the form of electromagnetic radiation, bombards the Earth. The visible wavelengths reaching us are sunlight, but about half the radiation is infrared (IR) wavelengths longer than those in the visible spectrum. When infrared impacts water or solid objects that absorb it, it is transformed into heat. This heat is the source of all fuels on the planet (i.e. our major source of stored energy) and the climate conditions allowing Earth to be inhabitable—it grows all vegetation, is responsible for the heating and cooling cycles driving the weather, produces wind energy, and delivers rainwater.
On a clear day in the temperate zone, sunshine falls with an intensity of about 10,765 watts/m2 (1000 watts/sf). If a roof absorbs and holds most of that energy, it gets very hot; if it reflects a significant percentage, it will be cooler. Equally important, if the roof can disperse the heat it absorbs, or in other ways prevent it from being transferred to the building below, the building will be cooler in hot weather. The temperature immediately below a cool roof could easily be 25 C (45 F) lower than below a conventional roof.
The exact qualifications of a ‘cool roof’ depend on whose definition is being used. The U.S. Environmental Protection Agency’s (EPA’s) Energy Star program rating is based on solar reflectance (SR), either “initial” (new product) or “three-year aged.” The U.S. Green Building Council (USGBC) Leadership in Energy and Environmental Design (LEED) program uses both reflectance and emittance, as expressed in the Solar Reflectance Index (SRI). The most recent version of LEED (LEED v4) allows either initial or aged SRI to be used. California’s Title 24 uses initial Thermal Emittance (TE) and aged solar reflectance (SR), or aged SRI.
As shown in Figure 1, all three systems differentiate between low-slope (i.e. a pitch of < 2:12) and high-slope (i.e. > 2:12) roofs. Energy Star and Title 24 have similar SR/SRI thresholds. LEED v4 has much higher requirements to contribute to LEED credits.
In addition to reflectance, both concrete and clay tile have cooling properties stemming from material composition and tile system design. Rating systems do not account for these effects, but there is significant impact upon the roof’s ability to prevent or otherwise mitigate the heat transference.
Why are cool roofs important?
A roof’s heat can have an impact on both the building and the surrounding area. In the summer, even with a normally insulated roof system, heat transmitted through the roof increases the demand for air-conditioning, raising electrical energy consumption. That energy consumption increases air pollution and carbon dioxide (CO2) emissions associated with climate change. It also costs more, making the building more expensive to occupy. A cool roof limits heat transference to the interior, reducing all these impacts.
A hot roof also raises the temperature of the air above it. Like any hot object, it gives off heat to things coming into contact with it (i.e. conduction). Heated air rises and causes wind currents that spread the heat around (i.e. convection). A city full of hot roofs becomes a large concentration of hot air, with studies showing it remains hotter, both day and night, than rural areas nearby. Buildings within these urban heat islands may see increased summer demand for air-conditioning (even after dark when temperatures would otherwise cool down), which multiplies the damaging environmental effect. A reflective roof mitigates these impacts on the building and the surrounding area.
A cool roof’s contribution to lowering energy consumption was assessed by Oak Ridge National Laboratory (ORNL), in collaboration with Lawrence Berkeley National Laboratory (LBNL), in a two-year residential study. (For more, see William A. Miller and Hashem Akbari’s 2005 ORNL report, “Steep-slope Assembly Testing of Clay and Concrete Tile With and Without Cool Pigmented Colors.” Visit info.ornl.gov/sites/publications/files/Pub6113.pdf.) Published in 2005, the research found a tile cool roof “reduced the peak heat transfer penetrating the roof deck at solar noon by about 70 percent” as compared to a typical residential asphalt shingle roof. A similar effect could reasonably be expected on one- or two-story commercial buildings, although the exact reduction could vary considerably depending on the configuration of the roof and above-ceiling plenum.