In your handbook you explain the difference between a reflective insulation and a radiant barrier essentially being that with the former, the foil is facing an enclosed air space, whilst with the latter the foil is facing an open-air space. Both systems are recommended for use in walls. My first question is, which system is most effective in a wall situation? Why can we only give an R-Value to the reflective insulation situation, and not the radiant barrier situation?
Answer:
Although radiant barriers and reflective insulation have different definitions, a reflective insulation can be installed as a radiant barrier and a radiant barrier can be installed as part of an insulation system. The definition of a radiant barrier, then, has more to do with the way radiant barriers work — they reduce heat transfer by reflecting heat waves before they can be absorbed by an opposing surface, thereby exciting the molecules in the material, causing heat to form and be conducted to the cooler side of the material. R-Values are a measure of the rate of heat transferring from the hot side of the material to the cold side. Since radiant barriers are only dealing with radiant waves, they cannot be measured for R-Values as there does not have a hot side or a cold side. Remember that radiant waves do not possess temperature, they can only create heat when they become absorbed by an opposing material surface.
When a radiant barrier is installed in an enclosed cavity, wherein there exists one or more air spaces facing the radiant barrier surfaces, an R-Value for the system can be calculated and/or tested because the system now incorporates a hot side and a cold side — like a wall with siding on the outside and drywall on the inside and a combination of air spaces and radiant barrier on the inside.