Most radiant barriers are 97% reflective and this is what makes them so effective. If the radiant barrier loses the reflective coating it also loses it's ability to do its job. Installing radiant barrier on the attic floor allows dust to settle and cover the radiant barrier. While it will still have some effect it will be nowhere near what it's intended to be. Placing items on top of the radiant barrier also lowers its effectiveness. Besides the loss in effectiveness, placing radiant barrier on the attic floor creates a serious safety hazard if anyone ever needed to walk in the attic.