Do radiant barriers, reflective insulation make sense (Part 3)
Heat travels from a warm area to a cool area by a combination of conduction, convection and radiation. However, unlike most common insulation systems, which resist conductive and sometimes convective heat flow, radiant barriers work by reflecting radiant heat away from the living space. Radiation is the transmission of electromagnetic rays through space. Traveling at the speed of light, these rays are invisible, and they have no temperature, only energy. In this case when we refer to radiation, we are describing Infrared rays. All objects radiate infrared rays from their surfaces in all directions, in a straight line, until they are reflected or absorbed by another object. Finally, radiation, which accounts for 50 percent to 80 percent of all heat transfer, will pass through air (or a vacuum) with ease, just as it travels the many million miles that separate the earth from the sun.
Heat control with aluminum foil is made possible by taking advantage of its low thermal emissivity (the rate at which radiation is given off) and the low thermal conductivity of air. The surface of aluminum has the ability to reflect 95 percent of the infrared rays which strike it. Since aluminum foil has such a low mass to air ratio, very little conduction can take place, particularly when only 5 percent of the rays are absorbed.
When the sun heats a roof, it’s primarily the sun’s radiant energy that makes the roof hot. A large portion of this heat travels by conduction through the roofing materials to the attic side of the roof. The hot roof material then radiates its gained heat energy onto the cooler attic surfaces. A radiant barrier reduces the heat transfer from the underside of the roof to the other surfaces in the attic. To be effective, it must face an air space. Dust accumulation on the reflective surface will also reduce its reflective capability.
The radiant barrier is a thin sheet of reflective material applied to kraft paper, plastic film, cardboard or plywood. Radiant barriers are most effective in hot climates, especially when cooling air ducts are located in the attic. Some studies have shown that radiant barriers can lower cooling costs 5 percent to 10 percent when used in a warm, sunny climate. The reduced heat gain may even allow for a smaller air conditioning system. In cool climates, however, it’s usually more cost-effective to install more thermal insulation as the radiant heat absorbed in winter can reduce heating cost.
A radiant barrier’s effectiveness depends on correct installation, so it’s best to use a certified installer. If you choose to do the installation yourself, study and follow the manufacturer’s instructions and safety precautions and check local building and fire codes.
A radiant barrier works best when it’s perpendicular to the radiant energy striking it. Also, the greater the temperature difference between the sides of the radiant barrier material, the greater the benefits. Radiant barriers that aren’t facing an air space don’t work. If sandwiched between other materials, it becomes a conductor, not a reflecting insulator.
Note that reflective foil will conduct electricity, so workers and homeowners must avoid making contact with bare electrical wiring. If installed on top of attic floor insulation, the foil will be susceptible to dust accumulation and may trap moisture in fiber insulation, limiting its effectiveness. It’s strongly recommended you not apply radiant barriers directly on top of the attic floor insulation.
The effect of a radiant barrier on a building assembly may be significant or insignificant, depending on whether the assembly is well-insulated. A poorly insulated assembly will benefit from a radiant barrier. According to a research report published by Oak Ridge National Laboratory, tests have shown that in attics with low insulation levels, radiant barriers can reduce summer ceiling heat gains by between16 and 42 percent compared to an attic with the same insulation level and no radiant barrier. Ceiling heat gains represent about 15 to 25 percent of the total cooling load, resulting in a 2 to 10 percent reduction in the cooling portion of a summer utility bills.
If you live in a cold climate, radiant barriers make less sense. According to the U.S. Department of Energy’s Energy Efficiency and Renewable Energy Clearinghouse, if you have properly installed R-38 or R-49 attic insulation, a radiant barrier contributes less than a 1 percent reduction in energy consumption.
Bottom line, radiant barriers only make sense in hot sunny climates, they have to be installed correctly with an air space backing, and with the correct level of current building code insulation, they only make a very small difference.
Contact Paul Scrivens at greenhomeenergyadvisors.com.