How do radiation barriers save energy?

On a sunny summer day, solar energy is absorbed by the roof, heating the roof sheathing and causing the underside of the sheathing and the roof framing to become hot. These surfaces then radiate heat downward toward the attic floor. Radiant barriers reduce that energy flow. Since the amount of heat radiation striking the top of the insulation is less than it would have been, the insulation surface temperature is lower and the heat flow through the insulation is reduced. By reducing the energy reaching the attic floor, radiant barriers also reduce the attic air temperature.
The amount of energy exchanged between the roof deck and the attic floor depends on two factors.
• The temperatures of each surface
• The properties of the attic surface materials, called the "emittance" and “reflectivity.”
Energy exchange is greatest when the temperature difference is high, when the emittance is high, and when the reflectivity is low. The underside of a roof deck made of wood will typically have a high emittance. When a radiant barrier with a much lower emittance covers that wooden surface, the thermal radiation is reduced. When a radiant barrier with a high reflectivity is placed on top of the attic floor insulation, much of the heat radiated from the hot roof is reflected back toward the roof.
In the winter, radiant barriers can reduce indoor heat losses through the ceiling, especially during winter nights when the roof surface is coldest. However, they also reduce beneficial daytime heat gains due to solar heating of the roof. Depending on your climate, level of attic insulation, and other factors, the net winter effect can be positive or negative.