Definition

An insulator is a material that blocks heat or electricity from passing through easily, such as rubber, plastic, wood, glass, or wool, and trapped air is one of the best insulators, which is why down jackets, foam, and double-pane windows all work by trapping tiny pockets of still air. Thermal insulators slow conduction because their particles transfer energy poorly, while electrical insulators have electrons tightly bound to atoms so charge cannot flow; a good thermal insulator is usually a good electrical insulator too, though not always. In band theory, an electrical insulator has a filled valence band separated from an empty conduction band by a large energy gap, typically above $3\ \text{eV}$, so thermal energy cannot promote electrons into conducting states, giving resistivities above about $10^{8}\ \Omega\cdot\text{m}$ versus $10^{-8}$ for metals; every insulator breaks down at a sufficiently high field strength, about $3 \times 10^6\ \text{V/m}$ for air, which is what lightning and spark plugs exploit.