Reviewed and Revised on 10/31/2013
A radiant barrier (sometimes called radiant heat barrier) is a layer of metallic foil, reflective paint or any other reflective material which can effectively block the "radiation" form of heat. Aluminium foil is the most commonly used radiant barrier in attics in homes. The purpose of installing a radiant barrier is to reduce heat gain for a home especially during hot weather, and cut down on energy bills while increasing the home comfort.
What is radiant heat transfer and why are radiant barriers effective?
Heat transfer between any two objects can occur in three ways- conduction, convection and radiation. Radiant heat transfer involves the "radiation" form, its most simple example being solar radiation. Earth receives all heat from the sun in the form of radiation. Radiant heat transfer occurs when the heat is transferred between any two objects or surfaces, which are at different temperatures, in the radiation form. Radiations are nothing but electromagnetic waves and have the ability to travel through air as well as vacuum. Dark and rough surfaces have a tendency to absorb radiant heat, while smooth and shiny surfaces tend to reflect radiation. For this very reason radiant barriers which have smooth and shiny surfaces, are very effective in blocking and limiting the radiant heat transfer.
Radiant barriers have two properties which make them very effective in blocking heat gain.
High Reflectivity: Reflect high percentage of radiant heat back to the direction it came from.
Low Emissivity: Low ability to release the heat absorbed by them.
What is the effect of radiant barriers on heating and cooling bills?
The Florida Solar Energy Center and the U.S. Department of Energy (DOE) have performed experiments using both test attic sections and full-size houses. Test results have reported a 2-10% overall reduction in the cooling portion of summer utility bills; under some conditions, the reduction may be larger. Although radiant barriers can be used to reflect heat back into the house in winter, tests have shown that the percentage reductions for winter heat losses are lower than those for summer heat gains. Radiant barriers are generally not recommended for cold and very cold climates.
Here is a radiant barrier fact sheet from Oak Ridge National Laboratory for additional information. It also has a national map indicating energy benefits, from radiant barriers, for homes in different parts of the country.
More information from Florida Solar Center can be accessed here.
How are radiant barriers installed?
A critical thing to take care of when installing a radiant barrier is it must have an air space next to the reflective side for it to be effective. If you do not allow for this air space, the radiant barrier acts as a conductor and just passes the heat along from its hot surface to the next cooler surface. Therefore, if the material you purchase is reflective on only one side (has only one “shiny side”), this side should be facing the air space. It can not be sandwiched between two materials, as the air space is critical to its performance.
Several methods for installing radiant barriers in new construction include:
drape over the rafters or trusses in a way that allows the product to droop 1½ to 3 inches between each rafter.
attach to the underside of the roof deck (sheathing) with the foil side facing the air space or attic space.
attach to either the faces or bottoms of the rafters or top chords of the roof trusses (this method can also be used in existing homes).
Installation methods differ depending on the individual home, with issues such as dust and/or possible moisture accumulation, locations of bathroom/kitchen vents and recessed lights, safety, visibility of roof components, attic accessibility, etc. You should do further research to determine the best option for your home.
Additional resource from U.S. Department of Energy on Radiant Barriers .