The three methods of heat transfer are conduction, convection or radiation. If you want to keep something hot, you need to retain heat by minimising the loss of heat by each of these methods. Conversely, to keep something cool, you need to minimise the gain of heat by each of these methods. The vacuum flask aims to achieve this. The construction of a vacuum flask is shown below.
Transfer of heat by radiation is minimised by using a double walled glass bottle, silvered inside and out. This means that heat infra red radiation is reflected back into the flask to keep liquids hot, or reflected out to keep liquids cool.
Transfer of heat by conduction is minimised by the use of insulating materials – cork and plastic – throughout.
Transfer of heat by conevection is minimised by reducing the ability of air and liquid to circulate. The warmer air and liquid will move to the top, and by standing the flask up, convection is reduced.