Thermal insulation refers to materials used to reduce the rate of heat transfer, or the methods and processes used to reduce heat transfer. Heat is transferred from one material to another by conduction, convection and/or radiation. Insulators are used to minimize the transfer of heat energy. In home insulation, the R-value is an indication of how well a material insulates. Reflectors are used to reduce radiative heat transfer. Many different materials can be used as insulators. Many organic insulators are made from petrochemicals and recycled plastic. Many inorganic insulators are made from recycled materials such as glass and furnace slag.