I need to know the efficacy of radiant barrier installed on flat concrete roof tops?
The radiant barrier should be used below the concrete roof with the reflective side (low emittance side) facing toward the interior. The reflective side of the radiant barrier should not touch concrete. Concrete absorb 80 to 90% on the incoming solar energy. If the concrete is coated or painted white, then the absorption will be decreased to 20-30% of the incoming energy. This idea was demonstrated in Pakistan about 30 years ago. The optimum solution is to do both: (1) make the top side of the roof reflect incoming solar energy and (2) install radiant barrier below the concrete rook deck.