Heat energy will transfer by using three modes; conduction, convection, and radiation. To protect the heat loss by convection, radiation, and conduction in a thermos bottle, the bottle is double walled, evacuated, and silvered.
The silvered coating in the thermos bottle is used to reduce the transformation of heat energy by radiation. Thermos bottle is evacuated because the vacuum portion protects the transformation of heat from inside to outside by the process of convection and conduction.
The two places in the thermos bottle where conduction may occur; at the top of the thermos bottle where the two wall mix and the insulated portion of the bottom. Heat will not transfer between the inner and outer walls by the process of convection because the presence of very little air between the airs. That is why the thermos bottle is double walled, evacuated, and silvered.