Attic insulation & Radiant Barrier work together in both hot and cold climates to make homes more energy efficient and comfortable. The best way to think of your home is like a giant refrigerator. It makes common sense that if you have two identical refrigerators and put one in the sun and one in the shade that the refrigerator in the sun will use MORE energy right? Why is this? The air temperature is EXACTLY THE SAME whether in the shade or the sun. What happens is that the SURFACE TEMPERATURES of the refrigerator in the sun will be higher due to the radiant heat from the sun. It may be only 70º outside, but the outside surface of the refrigerator could easily be 120º. As far as the refrigerator is concerned IT IS 120º. The same thing occurs in your attic. It may relatively cool - maybe 70º but if it's sunny, the roof will heat up and then radiate heat down towards the insulation. The result is that the TOP of the insulation can easily be 20º- 40º HOTTER than the outside air temperature or the air temperature inside the attic. This is one reason we often need to run our Air Conditioners even on "mild" days. Radiant barriers act like "shade" in the Summer. By installing a radiant barrier, the radiant heat that would normally be ABSORBED by the attic insulation is now REFLECTED away. Using the refrigerator example, if we wrapped the refrigerator with radiant barrier foil and put it in the sun the SURFACE TEMPERATURE would only go up a few degrees compared to the 20º- 40º without the radiant barrier. This is almost like putting it in the shade. Ultimately it's all about surface temperatures. The larger the difference between the inside and outside surface temperature, the more heat will flow in or out. You CAN add more attic insulation, and this is a good idea for most homes. See the DOE (Department of Energy) http://energy.gov for the recommended amout for your area. Regular fiberglass attic insulation SLOWS down heat via conduction. The problems is that it also tend to HOLD heat. Here is how it works. The Open-Ridge Method is the Standard Method is used for many roofs. Basically, the foil is stapled on the bottom of the rafters and run almost to the top of the rafters. A slot is left along the ridge and around roof vents to allow air to flow from the soffits to the top of the attic and then vent out the top of the attic.