This very issue has been studied by several research institutes.An exhaustive test performed by James R. Hall, project engineer for the Tennessee Valley Authority concluded that ...dust appeared to have little effect on the effectiveness of the radiant barrier.  The percent reduction in ceiling heat flux was remarkably similar to that of a radiant barrier with no dust.  He stated that a radiant barrier's performance ...may not degrade nearly as much as would be expected from the significant increases in emissivity caused by small amounts of dust.Click here to read the report - summary conclusion is on page six.