Referring to thermal building insulation in particular. Insulating materials are employed in order to reduce unwanted heat-loss or heat-gain which. Insulation is recommended for pipes, cavity walls, external walls, dry linings and lofts. This reduces the amount of energy required to heat up or cool down the building. Therefore insulation can increase energy efficiency.
Thermal insulation is the reduction of heat transfer (the transfer of thermal energy between objects of differing temperature) between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with specially engineered methods or processes, as well as with suitable object shapes and materials. Heat flow is an inevitable consequence of contact between objects of differing temperature. Thermal insulation provides a region of insulation in which thermal conduction is reduced or thermal radiation is reflected rather than absorbed by the lower-temperature body. The insulating capability of a material is measured with thermal conductivity (k). Low thermal conductivity is equivalent to high insulating capability (R-value). In thermal engineering, other important properties of insulating materials are product density (ρ) and specific heat capacity (c).
Source: Wikipedia - Thermal insulation