An Engineering Extension energy feature
An Engineering Extension energy feature
By Mike Dorcey
Radiant barriers are reflective material that can be placed in an attic to impede the transfer of radiated heat (thermal
radiation) from the roof to the ceiling below, according to Byron Jones, director of the Institute for Environmental Research
at Kansas State University.
Heat is transferred from one object to another in one of three ways: convection, conduction or radiation, or a combination of the three.
The heat transfer we feel as the sun comes up over the horizon and warms us early in the morning is an example of radiated heat. That same sunlight radiates heat to the roofs of our homes, too.
Roofs absorb that heat and in turn radiate it into our attics. This reduces the effectiveness of natural ventilation in cooling our homes in the summer. It also adds to the heat load our air conditioners must handle in order to keep temperatures at a comfortable level in the living spaces below the attic.
We have all experienced the effect of putting a shield of some type between ourselves and a source of heat. It may have been an umbrella, a tent, a car, a building or a tree. Whatever it was, it acted as a barrier that blocked the radiated heat by either absorbing it or reflecting it.
Radiant barriers work on the principle of reflection. The more reflective the surface of the object we put between ourselves and the heat source, the better job that screen will do of reflecting heat. In our homes, then, radiant barriers block the transfer of heat (thermal radiation) from the roof to the ceiling by reflecting it back to the roof.
Radiant barriers come in a number of forms, most of which perform the job they were designed to do, according to Jones.
The most common--and most cost effective--forms consist of a thin material such as a plastic film or paper that has a reflective coating of aluminum on both sides. Some versions are made with several layers of materials. These cost more and may not be as cost effective in most applications, Jones said.
"Usually the barrier material is placed immediately below the roof deck (sheathing) or on top of the ceiling insulation," Jones said. "Under the sheathing is the best location because you can have condensation problems if you place a radiant barrier directly over insulation."
An even better installation of radiant barrier would be to staple it to the bottom side of the rafters, Jones said. This would provide an insulating air pocket between the barrier and the roof and still radiate heat back to the roof and out of the attic.
Radiant barriers also help in the winter by reflecting radiant heat escaping through the ceiling. But this benefit is generally not as great as in summer, Jones said.