I see many times the need to have an air gap opposite a reflective face, it seems to me that for example, a polyester fiber blanket has so much air in its volume, it amounts to an air gap. Is that incorrect or only partly incorrect in that conduction from the reflective surface through bulk insulation is quite low?
I had it in mind to lay on the attic floor, 50mm of polyester with radiant barrier on top, and another 50mm of polyester on top of the barrier to keep it dust free.
I would not recommend burying the radiant barrier between layers of polyester. The optimum air space for a radiant barrier is 3/4”, or in the case of an attic installation, one reflective surface should face the large open-air space of the attic itself. That would be regardless of where the radiant barrier is installed — on the underside of the roof rafters or on the floor on top of mass insulation. Any radiant benefit you would receive by burying the radiant barrier between layers of polyester would be minimal.