Radient Barrier vs Mass-based Materials
The roof structure (envelope) of a building absorbs heat energy from UV (visible wavelength light) and infrared (IR-non-visible wavelength) during daylight. Painted a white color, it would reflect a high percentage of UV radiation. However, IR will still penetrate the envelope and heat up the roofing material, sheathing, framing and insulation until it's saturated and then release it to the cold side. This will continue radiating heat energy even when the solar heat load is "off" at sunset until it reaches a point of equilibrium with the temperature of the interior and exterior. That roof envelope structure and conventional insulation is a heat battery storing and releasing heat. Having a solar battery is a good thing in cold climates, and even works depending on the volume of mass and type (adobe, concrete, etc.), in hot climates. However, during the hot season the typical wood frame roof envelope typically soaks up that heat energy and redistributes it. Radiant Barrier Versus Mass-based Materials
Insulation coating blocks the majority of the UV and IR wavelengths, significantly reducing heat loading of the roof membrane and the heat battery effect. And when the heat load is turned off at sunset, there's no battery (mass) to continue radiating until exhausted.