The attic should have good ventilation, insulation and, if the building's roof is in the sun, that is, not under the canopy of trees, a radiant barrier. While these items work together to control heat in different ways, one is not a substitute for the other. Heat moves from where there is more of it to where there is less. It moves by radiation, convection and conduction. The sun radiates heat to the Earth through space.
Conduction occurs when something hot touches something less hot.
The sauce pot touches the hot electric-stove element, and heat conducts from the element to the pot, and then from the pot to the food. Insulation retards the rate at which heat conducts. Attic insulation slows the rate of heat transfer between the attic and the room below. The quantity of thermal resistance insulation offers is stated as R-value. The R-value of an insulation depends on its thickness, density and composition.