Heat moves three ways, so controlling it one way doesn’t necessarily affect the other two. It moves by:
A radiant barrier’s resistance to heat flow is variable and depends on conditions, including direction of heat flow, dust accumulation and other factors. In general, it is more effective at reducing vertical heat flow (such as from a roof to the attic floor). In general, white and light-colored materials tend to reflect more radiant heat than dark materials, but other characteristics of the material can greatly affect this.