How Building Materials Radiate Heat

The ways that building materials interact with the thermal radiation that reaches them is of great importance to designers. Three properties that describe these interactions with radiant heat are reflectance, absorptance, and emittance. Each of these can be influenced by the interior design of a space.

Reflectance (Fig. 15-3) refers to the amount of incoming radiation that bounces off a material, leaving the temperature of the material unchanged. If you think of radiant heat acting like visible light, a heat-reflective material is similar to a mirror. The electromagnetic waves bounce off the reflective material and don't enter it, so its temperature remains the same. New white paint will reflect 75 percent of the IR radiation striking it, as will fresh snow.

Absorptance (Fig. 15-4) is just the opposite of reflectance. An absorptive material allows thermal energy to enter, raising its temperature; when the sun shines on a stone, the stone becomes warmer. All the radiant en-

Figure 15-3 Reflectance.

Figure 15-4 Absorptance.

Figure 15-3 Reflectance.

Figure 15-4 Absorptance.

ergy that reaches a material is either absorbed or reflected. Dark green grass will absorb about 94 percent of the IR radiation shining down on it. Clean asphalt and freshly tilled earth will both absorb about 95 percent.

The color of a building's surroundings and surfaces influences how much radiation is reflected, and how much is absorbed. A building painted white reflects about three-quarters of the sun's direct thermal radiation, but only about 2 percent of the longer wavelength IR radiation that bounces back onto it from its surroundings. The less intense radiation from the surrounding area tends to get absorbed by the building. For example, the heat bouncing back onto a building from a light-colored concrete parking lot outside is relatively likely to be absorbed by the building.

Once a stone has absorbed the sun's heat during the day, it will radiate that stored heat out to cooler surrounding objects through the night air. This ability of a material to radiate heat outward to other objects is called emittance (Fig. 15-5). The amount of energy available for emittance depends upon the amount absorbed, so a highly reflective material would have less absorbed energy to emit.

Black surfaces absorb and then emit the sun's heat. Sun-heated lawns and pavements emit almost as much heat to the building as if these surfaces were painted black. A building with a bright metallic exterior reflects most of the radiation emitted from the earth back out into space.

Materials can emit radiation only through a gas that is transparent to IR wavelengths (or light waves) or

through a vacuum. They can't radiate heat if they are sandwiched tightly between other layers of construction materials. Metal foils are good heat conductors, but work as a mirror-like insulation to prevent radiation from being emitted when there is a space with air on one or both sides. Metal foils are often used inside walls. In cold climates, they are installed facing the warmer interior, to keep heating energy indoors. In hot climates, they are used facing the sunny outside to keep the building from heating up.

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Responses

  • Sagramor
    Do building materials radiate?
    2 years ago
  • Tomba
    How building materials radiate heat?
    2 years ago
  • Ruairidh
    How heat can absorb and emit in construction materials?
    2 years ago

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