It is generally assumed that continents, which act as thermal insulation above the convecting mantle of the Earth, inhibit the Earth's internal heat loss. We test the validity of this intuitive picture using a combination of laboratory experiments and numerical simulations that are understood with scaling theory. We find that the partial insulation of the Earth's mantle due to continents does not generally reduce global heat flow and can actually enhance the rate at which the planet cools. A theory is presented that predicts the surface area at which heat flow is maximized. Remarkably, it appears that the surface area of continents through geological time has remained close to that which maximizes global cooling.