Reflectance plays a key role in determining an object’s heat build up under the sun in both the visible region of the solar spectrum and in the invisible infrared.
For example, identical objects of the same color can reach significantly different temperatures as they sit side by side in the sun. An object containing infrared reflecting pigments will stay cooler than one containing non-reflective pigments of the same color.
This differential in solar reflectance is the basis of a number of ‘cool roof’ initiatives originating from federal and state regulatory agencies and code-writing bodies.
But reflectance is only part of the picture. The light that is not reflected is absorbed – and that’s the other part of the story.
When the surface of an object is exposed to light, the object will react in one or more of the following ways:
• Absorb a portion of the light
• Reflect a portion of the light
• Transmit a portion of the light
For an opaque object, such as a roof, we need not consider #3, since all the light incident upon a roof is either absorbed or reflected. As for #2, reflectance, we must consider that sunlight is composed of all the colors of the visible spectrum, as well as invisible infrared and ultraviolet light. Pigments impart color to paints, plastics, paper, ceramics, and many other materials by selectively absorbing certain wavelengths of light, and reflecting the rest of the spectrum to the observer. For example, a blue pigment will absorb all wavelengths of visible light except for the blue wavelengths (ca. 400-500 nanometers).
What happens to the light that is absorbed? It is converted to heat, leading to heat build-up. The degree of heat build-up is dependent upon how fast the heat can be dissipated. This occurs in three ways: conduction, convection, or radiation.
Conduction is the movement of heat through solid materials from a high temperature area to a low temperature area. In a roof it goes from the roof’s surface, to the underside, then to the roof deck, rafters, joists, interior walls, etc.
Convection is the transfer of heat from an object’s surface to a surrounding fluid such as air, which is the type of cooling provided by a fan.
Radiation is the emission of energy away from the roof in the form of far infrared light. Conduction and convection usually draw heat into the interior air of a structure. This is why an attic in your home is so hot in the summer – and air conditioning bills are so high!
The heat that is not dissipated through conduction or convection is radiated away from the roof. The tendency to radiate heat is known as emissivity (E) and is characteristic for any particular material. The higher the emissivity, expressed as a fraction of one, the more quickly an object will cool itself via radiation. An uncoated aluminum roof, with an emissivity of 0.25, will build up much heat under the sun. The same roof, coated with paint, will exhibit an emissivity of 0.85, and will re-emit (radiate away) a great deal of its heat.
Highly reflective roofs result in net energy savings in all but the very coldest climates. However, the climate does affect the impact of emissivity on energy use. In a hot climate, highly emissive (E= 0.90) roofs generate energy savings. In a temperate climate, emissivity has little or no effect. In a cold climate, low emissivity (E= 0.25) slightly reduces energy costs – providing the roof is also reflective.
Radiative dissipation of the heat generated by sunlight can only occur if that light is first absorbed.
Therefore, the emissivity of a roofing material has a limited effect on roofs with a high reflectance. For example, increasing the emissivity from 0.75 to 0.90 on a roof with a total solar reflectance of 70% results in a temperature increase of less than 3°F under typical conditions. Perhaps in recognition of this fact, the US EPA’s Energy Star program does not include emissivity in their criteria for energy efficient roofing materials.
Regionally, however, some states and communities, notably California and Florida, have built emissivity into their codes and incentives, generally Œ > 0.75-0.80. In order to qualify for these programs, architects and contractors who want to take advantage of the benefits of metal will need to specify that the roofing material be coated with Kynar 500/Hylar 5000 or other durable paint finishes.
The process of incorporating infrared-reflecting ceramic pigments into durable coatings baked onto metal offers unsurpassed performance in terms of long-term sustainable reflectance and emittance of cool roofing materials. This pigment technology can help roofs meet Energy Star criteria and other regulations without sacrificing color.
Cool roofing aids building designs that are environmentally friendly by enhancing quality of life, resource conservation and overall value for the owner and/or occupants. That’s why cool roofing solutions will play an increasing role in a world where these goals are becoming more important.
Now you know the rest of the story.
Jeffrey Nixon is marketing manager, coatings, for The Shepherd Color Company www.shepherdcolor.com
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