The following is from the web location http://www.rima.net/
It would apply if you live in a "warming climate", where you heat your house more than cool it.
Radiant barriers can also be used in floor systems above unheated basements and crawl spaces. The radiant barrier is either stapled to the underside of floor joists, creating a single reflective air space, or between the joists, followed by some type of sheathing, creating two separate reflective air spaces.
Laboratory experiments and computer modeling suggest that floor radiant barrier systems may exhibit R-values as high as R-7.5 to R-8.0 for reducing heat loss to basements and crawl spaces. Radiant barriers make an ideal choice for this application because, in addition to their excellent thermal properties, they are also vapor barriers that prevent ground moisture from migrating into the living space above.