A vacuum bottle will keep water hot for hours. The secret of this ability lies in the vacuum between the inner and outer bottles.
Heat naturally moves from high temperatures to low through metal, wood, water and other materials.
Air is also a conductor of heat. The vacuum layer prevents heat convection and conduction. Heat inside the vacuum bottle cannot escape, and the hot water stays hot. The SUNtube's outstanding heat-keeping ability makes use of this effective principle. Panel Type Solar Water Heater Heat escapes through the air SUN Tube Vacuum layer acts as a wall to prevent heat escape 2.