The best cold insulation materials are those that poor conductors of heat. Materials such as glass, plastic, polystyrene and fiberglass are poor conductors of heat and are therefore effective as insulation. Materials that are good conductors of heat, such as metals, are poor insulators.
When insulating to keep something cool, the true objective is to keep heat out. Heat flows from hot to cold, and good insulating materials slow this process. One of the most effective ways to insulate is to create additional space between the object and the heat source by adding air to the material. Air is a poor conductor of heat and can be used to widen the gap between the heat source and the object to be kept cool. The use of air as an insulating material is the principle behind the use of polystyrene and fluffy winter coats.