Radiant barrier is a radiant heat reflective insulation material. Radiant barriers (also known as reflective insulation) are a type of thermal (heat) insulations that inhibits heat transfer by thermal radiation.eat transfer always occurs from a region of higher temperature to one of lower temperature. Radiant barrier (or reflective) insulation is heat/thermal insulation which reflects radiation heat (radiant heat), preventing transfer from one side to another due to a reflective (or low emittance) surface.