The Temperature of Color

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FIELD TIP - The Temperature of Color - The Rest Of The Story

Author: MetalMag

Reflectance plays a key role in determining an objects heat build up under the sun in both the visible region ofthe solar spectrum and in the invisible infrared.

For example, identical objects of the same color can reach significantly different temperatures as they sit side byside 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 andstate regulatory agencies and code-writing bodies.

But reflectance is only part of the picture. The light that is not reflected is absorbed and thats the other part ofthe 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

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absorbed or reflected. As for #2, reflectance, we must consider that sunlight is composed of all the colors of thevisible 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 restof the spectrum to the observer. For example, a blue pigment will absorb all wavelengths of visible light exceptfor 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 heatbuild-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 temperaturearea. In a roof it goes from the roofs surface, to the underside, then to the roof deck, rafters, joists, interior walls,etc.

Convection is the transfer of heat from an objects surface to a surrounding fluid such as air, which is the type ofcooling provided by a fan.

Radiation is the emission of energy away from the roof in the form of far infrared light. Conduction andconvection 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 toradiate heat is known as emissivity (E) and is characteristic for any particular material. The higher theemissivity, expressed as a fraction of one, the more quickly an object will cool itself via radiation. An uncoatedaluminum roof, with an emissivity of 0.25, will build up much heat under the sun. The same roof, coated withpaint, 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 doesaffect the impact of emissivity on energy use. In a hot climate, highly emissive (E= 0.90) roofs generate energysavings. 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 temperatureincrease of less than 3F under typical conditions. Perhaps in recognition of this fact, the US EPAs Energy Starprogram 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 theircodes and incentives, generally > 0.75-0.80. In order to qualify for these programs, architects and contractorswho want to take advantage of the benefits of metal will need to specify that the roofing material be coated withKynar 500/Hylar 5000 or other durable paint finishes.

The process of incorporating infrared-reflecting ceramic pigments into durable coatings baked onto metal offersunsurpassed 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, resourceconservation and overall value for the owner and/or occupants. Thats why cool roofing solutions will play anincreasing 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 Companywww.shepherdcolor.com

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