Radiant barriers are designed to reflect heat back into a space however, they are only effective when they are clean and shiny and have an air space. When a radiant barrier is installed on floor joists with fiberglass insulation, it traps moisture between the barrier and the subfloor. As air enters the space through gaps in the system and the rim and band, it is carrying moisture. The fiberglass soaks up this moisture and it is absorbed by the wood. The barrier does not allow the insulation to vent, so the moisture continues to build up resulting in mold and rot.