A Thermos container is made of glass to prevent heat loss by conduction. The second form of heat transfer is by convection. A convection heater warms a room by causing warm air to move to all parts of a room. No air means no convection. The third form of heat transfer is by radiation.
Black objects radiate heat better than shiny objects.
The outer wall of the inner Thermos container is silvered to reduce heat loss by radiation. By the way, a Thermos will keep a drink cold as effectively as it keeps soup hot.