Efficient thermal-energy storage requires effective thermal insulation. From a consideration of the possible heat-transfer modes, the requirements for a thermal barrier can be deduced. In a good conventional thermal insulation, most of the heat transfer takes place by conduction through the gas (air) in the insulation voids, the conductivity of the ‘still’ gas representing the limiting performance of such a material. It follows that a better performance can be obtained only by reducing this gas-conducted heat by having voids smaller than the molecular mean-free path (‘microporous’), by replacing the air by a low-conductivity gas or by partially evacuating the insulation.