Radiant barriers are a favorite feature of residential construction in recent years. One study of a residence in the southwestern United States showed that a radiant barrier reduced the cooling load 30 to 50 percent. Another found that a radiant barrier with R-19 attic insulation is equivalent overall to R-26 insulation.
Similar effects are possible in commercial applications, though obviously, only the uppermost floors benefit. For industrial use, the radiant barrier is often made integral to the insulating system— either as the backing of insulation batts or glued onto the decking. In such instances, there are virtually no additional labor installation costs for new construction or insulation retrofits. The extra cost in such instances is negligible too, making radiant barriers a cost-effective alternative.
The problem with radiant barriers, for many years, has not been material or labor costs but technical feasibility. Designers and contractors, even when confronted with encouraging statistics, are still not convinced of their efficacy; thus they are hesitant to incorporate them into their projects. A brief discussion of the physics of radiant barriers may help to dispel this misunderstanding.
The thermal study of building loads is usually focused exclusively on the conduction of heat, as predicated by the R-values of the envelope components. The thicker the insulation, by this rationale, the higher the R-value, and the less heat that will flow. The insulation increases the thermal resistance, and more effectively isolates the interior conditioned spaces from the outside.
Conduction is the flow of heat between materials in direct physical contact: from air to the walls, through the envelope, then to the air inside, for example. There are two other ways heat is transferred: convection and radiation. Convection happens when the actual material moves, transporting heat along with it. Examples are warm air rising to the ceiling in the winter, or cold air drifting downward in the summer. Radiation is the transfer of heat by electromagnetic radiation, which requires no physical medium in order to occur.
Radiant-heat transfer is an abstract and intangible process, and what people don’t understand they don’t trust, much less use. Radiation-based heat transfer, however, is not so difficult to comprehend. Think of the warmth of the sun at its zenith, and the heat that washes every exposed surface of the earth. The sun is 93 million miles away, separated by the gulf of a perfect vacuum. Intuitively, no heat can be conducted or convected across this great distance, but obviously a huge amount does reach us on earth.
This thermal energy of the sun is transmitted by radiant energy, invisible high-frequency electromagnetic waveforms. When this energy reaches the earth, it’s absorbed by the air, heating it, and by the earth and any other material, heating them. Once this radiant energy is absorbed, it’s subsequently transmitted by convection and conduction. To better understand this process, consider a swimming pool. The sun creates a very warm surface layer of a few inches, where most of the radiant energy is absorbed. This warm layer heats the water below it by conduction, and thermal currents bring deep warm water to the surface by convection.
The important feature of this thermal system, as far as radiant barriers are concerned, is that not all the radiant energy is absorbed at the surface. The energy spans many frequencies, and the higher frequencies penetrate deeper before being absorbed. This means, for a roof system, that only a small percentage is absorbed by the shingles or builtup roof. The rest penetrates through the ceiling space or attic, into the occupied space where it’s absorbed by the walls, floor, and furnishings, heating them.
Another consideration is that radiant-heat transfer occurs within the building envelope itself. Part of the heat of the roof is re-radiated down into the building; the rest is conducted.
The radiant barrier is a reflective surface that blocks most radiation through the roof system, both the direct solar radiation and the re-radiated component. One analysis estimates a radiant barrier blocks 95 percent of the radiant energy, versus 5 percent for the usual vinyl-insulation facing. Another way to look at this is, according to another study, the Btu’s transmitted per square foot of horizontal interior roof surface are reduced from 74 to 2.5, which means that the radiant barrier makes the roof almost 30 times more effective at blocking heat transfer.
The bottom line is that radiant barriers can significantly reduce the cooling load. Conversely, by reducing the heating of the building’s outermost spaces, it can also increase the heating load in the winter. As a result, radiant barriers are best suited for warmer latitudes where the cooling season is predominant.
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Excerpt from Energy Conservation Projects.