A.David Beal, building scientist at the Florida Solar Energy Center, responds: In cooling climates, radiant barriers can and do save cooling energy. Testing at the Florida Solar Energy Center and other laboratories have consistently shown that a radiant barrier can reduce the amount of heat entering a home through the ceiling by 25 to 40 percent. The amount of energy saved depends on the level of conventional insulation in the attic. For those with a thick layer of attic insulation, a 40% reduction in the small amount of heat coming through the ceiling is correspondingly small. For those with minimal attic insulation, on the other hand, a 40% reduction in heat flow through the ceiling is a much larger amount.

Two rules of thumb:

• If you have R-30 or better attic insulation, the payback period for the installation of a radiant barrier may be long, although it will save energy.
• In a cooling climate, a house with a radiant barrier and R-19 attic insulation, compared to R-19 with no radiant barrier, should see a reduction in cooling energy requirements of about 10 to 12 percent.

Radiant barriers are not recommended in heating-dominated climates. To my knowledge, there has been no testing of radiant barriers in a heating climate.

The easiest and cheapest way to install a radiant barrier in new construction is to install roof sheathing with a radiant barrier. Several manufacturers now offer OSB or plywood roof sheathing with a laminated radiant barrier. A radiant barrier system can also be installed under the bottom of the top chord of a roof truss, or to the bottom edge of rafters. Installing a radiant barrier on an attic floor is not recommended, since such barriers easily get dirty, reducing the performance of the radiant barrier significantly. For more information, contact the Florida Solar Energy Center at 407/638-1000.