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At the simplest, thermal insulation is any material used to reduce the transfer of heat. By keeping heat in or out as desired, energy use is reduced.
Typical materials used as thermal insulation in walls and roofs of buildings include glass fiber batting, rigid foam panels, spray foam which solidifies in place, loose fill cellulose.
Some considerations that must be considered when selecting insulation include cost, durability, environmental friendliness, flammability, toxicity (e.g. formaldehyde gas from some formulations of spray foam).
Thermal insulation is also commonly used in applications where temperatures significantly higher or lower than the ambient temperature are desired, such as refrigeration or solar collectors.