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Vacuum flasks are thermally insulated double wall vessels. The narrow space between the two walls is evacuated which dramatically reduces thermal transfer. The near absence of air molecules minimizes thermal transfer by convection. Reflective surfaces in the vacuum space minimize radiant thermal transfer and the separation between the inner and outer wall minimizes thermal transfer by conductance. In summary, a vacuum flask thermally decouples a flask's payload from the external environment. The most common application of vacuum flasks is the storage or transmission of cryogenic fluids such as liquid nitrogen or liquid natural gas which can be maintained at extremely very low temperatures for extended durations.

Vacuum flasks provide very effective thermal insulation. The insulation is passive and the largest portion of thermal transfer into the vacuum flask occurs by conductance through the neck of the inner wall and by the electrical wiring or mechanical feed through entering the vacuum flask (thermal leakage). The small amount of conductance eventually raises the inner temperature within the vacuum flask which limits the payload's duration within hostile high temperature environments to a finite duration. Vaccum Flasks are often used as cola water bottle, insulated sports bottle or insulated coffee mug.

An additional form of heat gain in the vacuum flask is the heat generated by the electronics within the vacuum flask. Since the vacuum flask thermally decouples the contents of the flask from the external environment, the heat dissipated by the electronics can not escape from the vacuum flask and consequently increases the inner temperature within the vacuum flask which also limits the payload's duration within hostile high temperature environments.