Ra·di·ant :: Pronunciation: \ˈrā-dē-ənt\:: Radiant Barrier

Radiant barriers or reflective barriers inhibit heat transfer by thermal radiation.

All materials give off, or emit, energy by thermal radiation as a result of their temperature. The amount of energy radiated depends on the surface temperature and a property called the emissivity. Emissivity is expressed as a number between zero (0) and one (1) at a given wavelength. The higher the emissivity, the greater the emitted radiation at that wavelength.

Radiant barrier materials must have low emissivity (usually 0.1 or less) at the wave lengths at which they are expected to function.

To perform properly, radiant barriers need to face open space (e.g., air or vacuum) through which there would otherwise be radiation.

In 1954, NASA invented a lightweight, reflective material composed of a plastic substrate with a vapor-deposited coating of aluminum. The material, now commonly known as a “space blanket”, is used to protect spacecraft, equipment, and astronauts from thermal radiation or to retain heat in the extreme temperature fluctuations of space. In the vacuum of space, heat transfer is only by radiation.

“Low emissivity” material can be used with equal effectiveness on either side of a physical barrier that needs to be insulated. Although reflective insulation is often thought of as a “mirror” to heat waves, a more accurate analogy is a material that is both “deaf” and “mute” to heat waves. Reflective/low-emissivity material reduces radiant heat transfer by not “hearing” heat waves, i.e. not absorbing radiant heat, on the exterior side of a wall or roof, or by not producing it on the interior side. Either way, the material reduces the transfer of heat.

Solar energy is absorbed by a roof, heating the roof sheathing and causing the underside of the sheathing and the roof framing to radiate heat downward toward the attic floor. When a radiant barrier is placed directly underneath the roofing material incorporating an air gap, much of the heat radiated from the hot roof is stopped and the low emissivity of the underside of the radiant barrier means very little radiant heat is emitted downwards. This makes the top surface of the insulation cooler than it would have been without a radiant barrier and thus reduces the amount of heat that moves through the insulation into the rooms below the ceiling.

Radiant barrier can also reduce indoor heat losses through the ceiling in the winter.

On residential homes radiant barrier is typically installed one of two ways: Radiant Barrier Decking or Radiant Barrier Attic Foil.

In new construction, a product called Radiant Barrier Decking is used. This product is made by laminating a highly reflective piece of aluminum foil to one side of OSB board or plywood. The foil side will face the attic which creates the required air space. It is important to note that if the structure is located in a humid area, the radiant barrier should be perforated to ensure proper passage of moisture.

In existing homes, Radiant Barrier Attic Foil is installed inside the attic. This product is a tarp-type material with a layer of aluminum foil laminated on both sides to create a double-sided radiant barrier.