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Realizing Radiant Barriers

Mcn  Eyb  Nci  Mar14 1

Radiant barriers easily increase building efficiency in a cost-effective way

Most people are unaware that they use radiant barriers at home on a regular basis. When prepping baked potatoes before putting them in the oven, did you know the aluminum foil you wrap them in acts as a radiant barrier? Even though the foil is a good conductor, it doesn't allow as much radiant heat to leave the potato as the bare skin would, so it does an excellent job at keeping the potato hot long after it has been removed from the oven. The property that allows for this is called emissivity, which is the relative ability of a material's surface to emit energy by radiation.

The higher the emissivity of a material, the greater the emitted radiation at a given temperature is. Materials with low emissivity emit very little radiation and usually perform well as radiant barriers. Old-style glass thermoses are another prime example. The shiny, polished aluminum on the inside of the container is a radiant barrier, which is able to keep its contents warm for hours on end.

Radiant barriers can do a lot more than keep potatoes and coffee warm, though, and they have a particularly important use in construction. Their main role in a building is to limit heat transfer by thermal radiation through walls and roofs to save on energy costs. The most common places they are installed are in attics, right below the roof and adjacent to wall cavities.

In conditioned buildings, walls, roofs and floors trade radiation with each other as well as other solid objects in the room. Since most building materials have an emissivity of 0.9, but radiant barriers usually are 0.1 or less, this trading of radiation is hampered across any cavity or airspace, resulting in keeping the warm air warm and the cool air cool despite the exterior conditions. Aluminized plastic film is the most popular material used for radiant barriers due to its low emissivity, flexible nature and low cost.

Looking at the big picture, radiant barriers are a relatively inexpensive way to increase occupant comfort by a significant amount. To most efficiently use them, it is smart to keep the active, or low-E, side of the barrier facing the cold side of the assembly. For example, in a warm climate, it would be most beneficial to place a radiant barrier inside a wall cavity with the low-E side facing the inside of the building, where the air is cooler.

Conversely, in the cold climate, it would be most beneficial to have the low-E side of the radiant barrier facing the outside of the building. This is the case for low-E windows, which can greatly reduce heating costs in cold climates. Double-faced radiant barriers are an excellent option for variable climates like the Midwest. In any case, it is also a good idea to use radiant barriers in conjunction with insulation for the best possible performance.

Most of the time, radiant barriers are not exposed to the exterior environment. But, when it is advantageous for an exterior surface to have a Thin insulation materials incorporating double-sided radiant barriers are installed in this roll-up metal door for improved insulation. (Photo courtesy of DBCI) low emittance, an aluminized zinc coating called Galvalume can double as both a radiant barrier and excellent corrosion protection for metal roof and wall panels, due to its relatively low emissivity of 0.15.

In efforts to consistently raise the bar in sustainable building, engineers and manufacturers continue to research new ways to incorporate radiant barriers in construction and increase the energy efficiency of buildings. Most roll-up overhead doors used on conditioned warehouses and other metal buildings are difficult to insulate. Therefore, thin and flexible radiant barriers are a good option for controlling temperature.

Windows are another area in buildings where radiant barriers have proven to be very beneficial. Because their main function is to let natural light into a building, installing radiant barriers into windows poses its own challenges. Double-pane windows have air cavities that make excellent hosts for near-transparent, low-E coatings, so the windows are still clear enough to see through. In the future, the industry will likely see different types of insulations that have radiant barriers incorporated into their makeup, which would take out the additional step of installing them separately.

Bob Zabcik is director of research and development at Houston-based NCI Building Systems. For more information, visit www.ncilp.com.