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.”
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.
Why does it matter whether there is air-conditioning ductwork in the attic?
Radiant barriers reduce energy losses from air conditioning ductwork in the attic in three ways. First, the outside surface of any ductwork located in the attic exchanges thermal radiation energy with the roof deck in the same way as the attic floor. Because the temperature difference between the roof deck and the duct surface is even greater than that between the roof deck and attic floor, this thermal radiation is significant. Second, the outside surface of the ductwork exchanges energy with the air in the attic space, and that energy loss is reduced when the attic temperature is moderated by the radiant barrier. Third, ducts leak. When return ducts leak, the air that is sucked into the ducts through the leaks feeds into your HVAC air handler, which then has to use extra energy to cool this warmer air. When the capacity of the HVAC System is nearing its limit, as on very hot summer days, this can make a noticeable difference in the comfort within the house.
When supply ducts leak, the air you’ve paid to condition goes into your attic and out with the attic ventilation. Furthermore, this diversion of the supply air can reduce the pressure inside your home, so that more outside air is pulled into any existing openings, such as cracks around windows and doors and openings between the attic and the conditioned space.) So in both cases, sealing up duct leaks can improve comfort and save energy.