Radiant barrier

Radiant barriers or reflective barriers inhibit heat transfer by thermal radiation. Thermal energy may also be transferred via conduction or convection, however, and radiant barriers do not necessarily protect against heat transfer via conduction or convection.

Reflectivity and emissivity

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 (also called the "emittance"). 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. A related material property is the reflectivity (also called the "reflectance"). This is a measure of how much energy is reflected by a material at a given wavelength. The reflectivity is also expressed as a number between 0 and 1 (or a percentage between 0 and 100%). At a given wavelength and angle of incidence the emissivity and reflectivity values sum to 1 by Kirchoff's law.

Radiant barrier materials must have low emissivity (usually 0.1 or less) at the wavelengths at which they are expected to function. For typical building materials, the wavelengths are in the mid- and long- infrared spectrum, in the range of 3 - 15 micrometres.

Radiant barriers may or may not exhibit high visual reflectivity. This is because while reflectivity and emissivity must sum to unity at a given wavelength, reflectivity at one set of wavelengths (visible) and emissivity at a different set of wavelengths (thermal) do not necessarily sum to unity. Thus, it is possible to create visibly dark colored surfaces with low thermal emissivity.

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

Applications

Space exploration

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, so a radiant barrier is much more effective than it is on earth, where heat transfer can still occur via convection and conduction, even when an effective radiant barrier is deployed.

Where to install radiant barrier insulation

"Reflective" or "low emissivity" material can be used with equal effectiveness on the outside or the inside of a wall that needs to be insulated.

Textiles

• Fire proximity suit
• Space blanket
• Thermo-lite (medical)

Construction

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 reflected back toward the roof 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.

This is different from the cool roof strategy which reflects solar energy before it heats the roof. Although a cool roof has been demonstrated to be more effective , a radiant barrier can be a less expensive retrofit.

Radiant barriers 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.

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. Often pre-laminated radiant barrier sheathing will have vapor barrier properties because the lamination adhesive will fill in the perforations. It is often better to use a perforated, reflective radiant barrier and staple it to the roof decking before installing it on the rafters.

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. The radiant barrier foil material can be either stapled to the bottom of the roof rafters or laid out over the existing insulation. Both methods provide the required air space. If put over existing insulation, care must be taken to insure moisture is not trapped in the insulation by using a perforated product that is NOT a vapor barrier (in a heating climate, i.e., when the house normally is warmer than the ambient air). In hot humid climates a non-perforated (vapor barrier) type radiant barrier is NEVER recommended in the interior of the structure.The ONLY appropriate placement of a vapor barrier type radiant barrier in hot humid climates in on the outside of the exterior sheathing (hot side of the insulation).

See also

• Aluminized cloth
• Bivouac sack
• Emissivity
• Fire proximity suit
• Insulation
• Reflectivity
• R-value
• Space blanket
• Aluminize

External links

• List of Emissivity Values for Common Materials
• How a radiant barrier works
• Radiant Barriers: A Question and Answer Primer
• This entry incorporates public domain text originally from the Oak Ridge National Laboratory and the U.S. Department of Energy. [1]
• Effect of Radiant Barriers on Heating and Cooling Bills Department of Energy
• Video-How Attic Radiant Barrier Works

Reflecting on Space Benefits: A Shining Example

Comparative Summer Attic Thermal Performance of Six Roof Constructions, by Danny Parker and John Sherwin, Florida Solar Energy Center (FSEC), FSEC-PF-337-98 [2]