I know that radiant barriers (i.e., reflective films applied over or under insulated spaces) don't technically have R-values (because they primarily reflect heat, instead of primarily slowing convection and conduction as does traditional insulation), but they do have some insulating effect, which could be quantified as equivalent to an R-value.
E.g., if I put a radiant barrier under R-19 insulation, what equivalent insulation will that space experience? Holding conditions constant, an HVAC system will see a reduced load after the addition of the radiant barrier. One could find the pure insulation level that matches that load reduction – perhaps it's R-30? – and based on that say the radiant barrier is "functionally equivalent to R-11."
Amendment: Yes, there are good theoretical reasons why radiant barriers are not assigned an R-value. However, as one user notes, we only ever talk about "R". For example, where I live the standard for home insulation is R-30. Can I reliably achieve that through a combination of blanket insulation and a radiant barrier? MIMA, hardly an objective trade organization, did a test in Florida 30 years ago which, while highlighting shortcomings of radiant barriers, did note reasonably consistent heat flow reduction from properly installed barriers. I would assume that with the benefit of 30 years of knowledge and increased emphasis on insulation we can say something useful about the functional value of adding a radiant barrier to an insulation stack.