Based on original content from Elizabeth Spencer; US Department of Energy
Radiant barriers are installed in attics—primarily to reduce summer heat gain and reduce cooling costs. The barriers consist of a highly reflective material that reflects radiant heat rather than absorbing it.
How They Work
Heat travels from a warm area to a cool area by a combination of conduction, convection, and radiation. Radiant heat travels in a straight line away from any surface and heats anything solid that absorbs its energy. When the sun heats a roof, it's primarily the sun's radiant energy that makes the roof hot. Much 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, including the air ducts and the attic floor. A radiant barrier reduces the radiant heat transfer from the underside of the roof to the other surfaces in the attic. To be effective, the reflective surface must face an air space. Dust accumulation on the reflective surface will reduce its reflective capability. A radiant barrier works best when it is 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 a radiant barrier can offer.
Radiant barriers are more effective in hot climates than in cool climates, especially when cooling air ducts are located in the attic. Some studies show that radiant barriers can reduce cooling costs 5% to 10% when used in a warm, sunny climate. In cool climates, however, it's usually more cost-effective to install more thermal insulation than to add a radiant barrier. In the DELMARVA, the radiant barrier installation pays you back at least half the year, during hot and humid days when your air conditioning loads are greatest. It doesn’t do much for your home energy savings during the really cold months but may assist in melting ice and snow accumulations in some cases. Installed in the attic of Tokori’s global headquarters, it shaves about 10-20 percent off the BGE bills from April through October.
If you choose to do the installation yourself, carefully study and follow the manufacturer’s instructions and safety precautions and check your local building and fire codes. The reflective insulation trade association also offers installation tips. It's easier to incorporate radiant barriers into a new home, but you can also install them in an existing home, especially if it has an open attic. In a new house, an installer typically drapes a rolled-foil radiant barrier foil-face down between the roof rafters to minimize dust accumulation on the reflective faces (double-faced radiant barriers are available). This is generally done just before the roof sheathing goes on, but can be done afterwards from inside the attic by stapling the material to the bottom of the rafters. Ideally, leaving a mid-span run of the material removable (Velcro or fasteners), will enable periodic inspection of the roof decking to check for leaks or signs of deterioration (it will be unobservable otherwise). When installing a foil-type barrier, it's important to allow the material to "droop" between the attachment points to make at least a 1.0 inch (2.5 cm) air space between it and the bottom of the roof. Foil-faced plywood or oriented strand board sheathing is also available. 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, so it is strongly recommended that you NOT apply radiant barriers directly on top of the attic floor insulation.