The infra-red waves are traveling through the air and really don't heat much until they hit something solid and begin to warm that up. If they hit a reflective surface, some will be redirected back towards the source and some will be absorbed by the reflector as heat. The better the reflector, the less that reflector heats up and the less with which the insulation behind it must contend- if there was no reflector at all- then 100% of the infrared energy would be heating the insulation and thus, increase the load on the insulation. Even in the man hiding in the dumpster scenario- the author points out that as long as the body (the radiating source) is not in contact with the metal (thus allowing for conductive as opposed to radiative transfer) the IR detector will not pick up on his heat. In that case- the metal dumpster wall is acting as the sole barrier between the IR source (the body) and the detector. Air does not block IR radiation-- its essentially transparent to it-- air blocks conductive transfer of heat. This is important in that if the radiant barrier does heat up enough to re-radiate any significant IR, then air alone will not prevent the IR from striking the next surface in line between the source and the detector. However, provided that the radiant barrier is prevented from reaching temperatures significantly above ambient temperatures outside the room either by its effectiveness as a reflector versus absorber, or by continuously cooling it and controlling the destination of that collected heat, or a combination, then re-radiation will be close to the background ambient IR radiation and thus insignificant.
in addition to blocking conductive transfer of heat from within a room, insulation directly exposed to an IR radiative source without a reflective barrier is also serving the same function as the metal reflector... some, but not much, of the IR is reflected off the insulating material, and the rest is heating the insulation itself which is poor at conductive transfer. No matter the quality or material of the reflecting layer and its thermal conductive properties- as long as it is preventing some of the IR emitted from the source from ever reaching the insulation, it is beneficial and assists in the effectiveness of the insulation. It allows less insulation to be used than would be required without a reflective surface to achieve the same thermal barrier.
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