If you create a double vapor barrier with another layer of faced with vapor barrier fiber glass insulation, any moisture that does get through the first layer may condense on the second. This might cause water stains on the ceiling and could lead to severe structural damage. Install the second layer of unfaced insulation perpendicular to the first, covering the wood ceiling joists to reduce heat loss through the wood. Apply the second layer of insulation perpendicular to the first layer only when the joist cavity is full. Covering the attic joists with the second layer will reduce heat loss through the wood.
Radiant barriers are sheets of aluminum foil laminated to paper.
They are typically installed above the insulation in the attic. The results showed that adding additional fiber glass insulation is generally a much easier and less costly task than installing radiant barriers. Additionally, the actual reduction in heat flow achieved through properly installed radiant barriers is substantially less than that claimed by some manufacturers. Even with a good vapor barrier, proper ventilation is considered essential to prevent damaging condensation. Eave vents openings at the roof overhang combined with a ridge vent, roof vent, or gable vents, are effective ways to create a positive movement of air in and out of the attic.