Thermal insulators reduce the rate of energy transfer by conduction, convection or radiation.
Trapped air is an excellent insulator because air is a poor conductor and, when trapped in small pockets, cannot convect. This is the principle behind loft insulation (fibreglass), cavity wall foam, double glazing, and wool clothing.
A vacuum eliminates both conduction and convection (no particles). Shiny surfaces reduce radiation. A vacuum flask uses all three strategies.
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