The sun is the greatest source of heat and we are in an everlasting battle to control that heat in our homes. The heat we experience from the sun is made up of different parts i.e visible light heat, ultraviolet heat, and infra-red heat. When out in the hot sun the heat you feel is 3% from ultraviolet light, 57% from infra-red light, and 40%from visible light. This is the same heat that strikes the roof of our homes and heats the inside. When the sun hits the roof most of the heat energy is absorbed by the roof and spreads throughout the roof towards the cool interior of the house. After the absorbed heat will no longer spread owing to the space for air on the bottom of the roof’s deck, it moves to the next object after changing back to radiant heat.
Years ago, when we did not insulate homes, most radiant heat would come through the ceiling into the home. When you start to improve the home, you can work on different types of heat transfer to keep the heat out. When you use a white colour coating on the roof, you reduce the heat coming from the light. White roofs have no impact on radiant heat gain. When you insulate the home, you're slowing down the conduction of heat into the home by adding a high R-layer between the heat and the home's interior.
Note that under roof insulation blanket will slow down the conductivity of heat and not stop it completely and that is why a home that's not air-conditioned in the summers will get extremely hot inside.