To be a good thermal insulator, a material must be resistant to the conduction, convection and radiation of heat. A good insulator is made from an opaque, nonmetallic material without a crystalline structure, and its insulating properties may be enhanced by creating small air pockets inside its structure.
The reason metals conduct heat so well is that metallic atoms have a poor hold on their outermost electrons. This allows the electrons to flow from one atom to another easily, and they carry energy as they move. When one end of a metallic object is heated, this increase in temperature excites the electrons and causes them to flow, carrying the heat along the object and heating it evenly. Insulating materials such as plastics resist the flow of electrons, reducing heat flow through the material.
Air pockets are another way to increase insulation. Under normal circumstances, air can transmit heat through convection, flows of air created by differences in temperature and pressure. If the air pockets inside a material are separated from one another, however, these flows cannot happen. The heat also cannot conduct through the trapped air, reducing its ability to flow through the material. Winter coats and other insulated clothing often utilize this technique, as do the insulating materials inside the walls and attics of residential homes.