The surface of aluminum foil reflects 95 percent of the infrared rays that hit it, absorbing only 5 percent. Because of the low mass-to-air ratio that aluminum foil has, it conducts almost no heat, making it a superior insulator.
Because of its excellence as an insulator, aluminum foil is a common surface with radiant barriers. Holding up a piece of aluminum foil in front of one's face shows these properties. After a short amount of time, one feels heat bouncing back toward his face. This is the heat energy from the person's body bouncing back from the foil. The human face has an absorption rate of 99 percent for infrared rays (far more than the 5 percent of foil). As a result, the person feels heat coming back from the foil.
Construction engineers use aluminum and aluminum foil to manage heat effectively. Because air does not conduct heat well, and aluminum foil doesn't send much out in terms of its own heat, aluminum foil is a popular material for controlling heat. For example, the space shuttle Columbia had aluminum bits embedded in ceramic tiles that would reflect heat before the craft could absorb it, and moon suits feature foil surfaces over trapped air to allow temperature modification.