Seattle, Washington area – how effective are radiant barriers when applied directly over existing fiberglass insulation? I understand that for radiant barriers to work, the surface must face an open-air space. In this application, one surface faces the attic cavity- an open-air space. The underside however, is resting on the fiberglass insulation with no open-air space. Does your research support this practice?
Research supports this application, however, RIMA’s position is that the preferred application is to the underside of the roof rafters. That’s because dust can collect on the top of the radiant barrier that rests on the floor and over time degrade its’ performance. How long this would take is very variable based on the environment of each house as well as other factors. RIMA only supports the floor application where attaching to the rafters is not an option.