The answer is the convective zone.
The energy pouring from the radiative zone gets trapped and cannot escape. Therefore giant convection currents are set up with hot matter rising and the cooler matter sinking. This results in large bubbles of ionized gas rising through the convection zone, reaching the surface in about one week and 3 days. When it becomes cooler than its surroundings, the gas sinks to become reheated and rise again.
In this fashion, rolling currents of hot and cold gas create a churning motion that carries heat from the bottom to the top.
