Aluminum reflects 97% of the radiant heat from the sun. However, this is not without a price; the aluminum will get extremely hot. For example, have you ever touched a foil-wrapped baked potato fresh out of the oven? It will burn your hand. Instead, if you pick up the potato with a hot pad (insulation) the potato will not burn the hand. The same holds true for an attic when the radiant barrier is not insulated. The surface temperature of the aluminum will transfer into the attic, lessening the effectiveness of a radiant heat barrier. Thermadeck offers a radiation barrier (aluminum) backed with a strong insulator (polystyrene, a strong, insulating aromatic polymer) to ensure that any heat that is blocked by the radiation barrier will be quickly disappated.
When the radiant barrier is properly insulated, the surface temperature is transferred much more slowly. If given enough time, it will eventually pass into the attic. Air does not convey heat effectively by conduction, therefore it is a good insulator.
The one inch of airspace combined with the one-inch thick polystyrene will stop any measurable heat from entering the attic via conduction. One of the best ways to demonstrate the effectiveness of polystyrene's ability to stop conduction of heat is to compare the temperature of coffee in a metal cup against coffee in a foam cup...you can feel the difference.
This insulating layer is the real key to the Thermadeck system. Without it, the homeowner would see a little bit of energy savings, but becasue it's there, there is a huge leap in savings.
When the radiant barrier is properly insulated and ventilated, the surface temperature will not have time to heat up the attic. The heat naturally moves upward through a vented ridge system, effectively moving up and out before it can reach the attic. Ventilation is all about proper location.
The location of the soffit vents make a way for air to flow up and out of the roofing ridge vent instead of entering your attic. By adding extra soffit vents, and with the help of the ThermaDeck design, air is allowed to pass in and through a one-inch gap between the decking and the polystyrene foam backing. This increases the flow of hot air transferred from the sun's radiant heat out of the roofing ridge vent. Because heat naturally rises, with these holes in the proper place (within the 1" area), the air that enters into the soffit vent will go straight through the holes to travel up and out of your roof (by convection) without actually having to come into the attic space. With traditional attic design, air was allowed to enter the attic. Although it certainly helped by moving air throughout your attic, something was just missing. Billy couldn't accept the fact that the air had to enter the attic. He put his focus on keeping the air outside where it belongs.