Building on Solar Reflectance: Meeting cool roof standards with concrete and clay tile

Cool roof mechanics
A tile cool roof is a multi-layered system. The topmost surface is the one garnering the most attention, but other elements of a tile roof make significant contributions to reducing heat transference to the building below.

From the top down, the layers are:

• tile;
• air channel;
• underlayment;
• roof deck;
• radiant barrier (sometimes included, often as a foil layer laminated to the bottom of oriented strand board [OSB] decking); and
• insulation.

The material properties of clay and concrete tile themselves act in several ways that help keep the roof cooler.

Reflectivity
‘Reflectivity’ is a surface’s ability to make incoming radiation bounce off, still in the form of radiation, rather than be absorbed by the surface and transformed into heat. As is taught in elementary schools, lighter colors reflect more light and stay cooler in the sun because they also reflect infrared. However, there are some materials with fairly dark colors—generally in the orange and red range—that also reflect a significant proportion of infrared. Several shades of clay tile naturally have this property. Most clay tiles achieve their color from chemical changes in the clay caused by the firing process. The color is integral, and very stable, even after decades.

There are also special pigments that are formulated for high IR reflectance, regardless of the lightness or darkness of visible light reflectance. However, they tend to be expensive. (Recent research, for example, showed promising results from a fluorescent pigment made with synthetic ruby.) Some clay tile is made using high-IR-reflective materials that are coated onto the clay before firing, thus resulting in higher IR reflectance.

Concrete color is achieved by mixing integral pigments into the concrete during casting. Concrete tile cool roof colors tend to be lighter shades, although concrete tile made with high-IR-reflective pigments are also possible.

Emissivity
‘Emissivity’ refers to a surface’s ability to shed radiant energy (as distinct from conducting away heat). Concrete and clay tile both have high emissivity, so a portion of the radiation not instantly reflected is still sent back upward, more slowly, in the form of radiation.

SRI ratings take into account both reflectance and emittance. There are clay and concrete tile colors available with SRIs in the 40s and 50s, which satisfy LEED v4 high-slope cool roof requirements. There are many more colors with SRIs in the 20s and 30s, meeting Energy Star high-slope standards.

LEED is becoming a requirement for public- and private-sector buildings in an increasing number of jurisdictions, and net-zero houses will be mandated in some areas (such as California) in the near future. It is likely manufacturers will make more high-SRI colors available. Lighter-colored roofs are already showing more popularity than in previous eras.

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