In an existing house is it better to install a barrier between the rafters or on the floor over the insulation? It is loose insulation not bats.
RIMA-I always recommends attaching the radiant barrier to the rafters of the roof. There are some instances where this is not possible, either because the pitch of the roof is too shallow to allow access to the rafters or the insulation on the floor is too thick and high to be able to work on top of it to get to the rafters. In those cases, it would be permissible to place the radiant barrier on top of the floor insulation — NOTE you must use a perforated material to allow moisture from the mass to escape to the attic air and ventilation. Based on studies done over the years by several agencies and universities, when newly installed, a radiant barrier laying on top of the mass on the floor will have a significantly greater reduction of the heat flux at the ceiling line than a rafter installation, which would translate into better performance. However, over time the floor applied radiant barrier will accumulate dust and that will tend to degrade its performance. How long this will take is determined by the amount of dust that collects in a given home in a given environmental condition.