Conventional fibrous insulation is designed to resist the movement of convective and conductive heat. This movement allows heat to move faster through the walls. Placing a fibrous insulation in this cavity keeps air from moving and thereby reduces energy use. Since it has very low density there is very little conductive heat moving through. As seen above, insulation only slows two types of heat.
Radiation is no different.
During the winter your warm ceiling conducting heat through your insulation which is radiating heat to the cold roof. Since insulation is designed to slow convective heat by keeping air from moving and limit conduction by having a low density, radiant heat readily passes through it. However, placing a sheet of material above insulation in a cold climate has been proven to create other problems like the formation of ice and trapping moisture. Radiant Barrier Chips allow moisture to pass but not infrared heat. When the sun strikes the roof and subsequently emits infrared heat towards your insulation, it becomes a heat sponge all day getting hotter and hotter.