An insulator is a material that resists electricity or heat from passing through it. There are two types of insulators: electrical and thermal. Some common insulators include wood, plastic, glass, porcelain and Styrofoam; Styrofoam and plastic are the most widely used in household applications. Styrofoam is often used to insulate floors, roofs and basements, as it reduces heating costs by retaining heat in homes. There are a number of properties that insulators have that prevent the conduction of heat and electricity.
The ability to impede electric current from passing through is known as electrical resistance; this resistance is measured in ohms. When one volt produces one ampere of current in an object, the resistance will be one ohm. All materials except superconductors have some resistance; conductors have low resistance, while insulators have high resistance levels. Heat insulators also have a very high heat resistance quotient; this prevents them from melting even when subjected to very high temperatures.
Breakdown voltage is also called dielectric strength. All insulators will conduct heat and electricity if subjected to extremely high voltages. By subjecting a material to very high voltages, the material's composition will change and it will lose its insulating abilities; the voltage at which this change occurs is known as the breakdown voltage. Different insulators have different breakdown voltages, and are used for different purposes. For example, plastic may be used as an insulator in households where the current is not very high, but it cannot be used for industrial purposes. Ceramic may be the best insulator in such applications, as it has a very high breakdown voltage.
In insulators, the valence electrons are tightly held together; this prevents them from moving. When the movement of electrons is restricted, no current can flow, making substances with this property -- for example, non-metals such as glass, wood and plastic -- excellent insulators. The reason most liquids and aqueous solutions are not good insulators is that when they are in this state they contain delocalized ions which allow electric current to flow; the same applies to wet plastic and wood.
Air permeability, the ability of a material to allow air to flow through its pores, is a necessary property for heat or thermal insulators. Good insulators have a high air permeability, since air is an insulating substance.