[jeff5may]

I never said that radiant barrier is worthless. It does a very good job of reflecting heat and light back the way it came. It works best when you have a large amount of radiation, as in the case of suspended-tube underfloor heating, where normal feed temps exceed 130 degF. In this situation, you want a gap of 1 inch or so between the subfloor and the reflective barrier to help spread the heat through convection in the gap.

With a poured thin slab or stapled underfloor install with heat spreaders, feed temperatures are usually below 120 degF, due to the increased thermal coupling into the floor. For the same cycle time as above, there is less potential for heat loss due to the reduction in feed temp alone. The radiant barrier and airspace would still work, but would have less radiation at a lower gradient to contend with.

In both of these situations, more insulation is beneficial. Where that insulation should be put will vary. Depending on the site and design goals, extra insulation may or may not be necessary.