Any material or substance
that provides a high resistance to the flow of heat from one surface
to another. The different types include blanket or batt, foam, or
loose fill, which are used to reduce heat transfer by conduction.
Dead air space is an insulating medium in storm windows and storms
as it reduces passage of heat through conduction and convection. Reflective
materials are used to reduce heat transfer by radiation.
Extra insulation around the heating and/or cooling
ducts intended to reduce the loss of hot or cold air as it travels
to different parts of the residence.
Any material that when placed between the interior surface of the building and the exterior surface of the building, reduces the rate of heat loss to the environment or heat gain from the environment. Roof or ceiling insulation refers to insulation placed in the roof or ceiling of the top occupied floor in the building. Wall insulation refers to insulation placed between the exterior and interior walls of the building.
Source: Energy Information Administration