You have to remember how heat is transferred, convection, conduction, and radiation. With still air the only two that are important are conduction and radiation. Conductive loss would be hot air on the interior and cold air on the exterior. The heat transfer from the hot side to the cold side is dependent on the R value of any insulation between the two sides. Radiant heat does not heat air, just objects in line of sight (think bonfire). A radiant source will heat the wall and then the heat loss would be by conduction through the wall. Short answer, if you are using a heated air heating system radiant barriers aren't effective. If you are using a radiant source like a pot bellied stove or a barrel heater, then they would be of value as they would reflect the radiant heat back into the room. So the answer to your question is like so many others, it depends.