What you are describing is a radiant barrier. The basic physics behind this is that a reflective surface both absorbs and radiates heat at a slower rate than a non-reflective surface. You also see this effect in the fact that dark objects absorb / radiate heat faster than light objects. An example of this is the films that are used in Low E glass.
Reflective barriers ONLY deal with radiant heat issues. They have nothing to do with conductive or convective heat movement. So wen they talk of percentage of improvement, they are referring only to this area of concern. This is even if they speak in terms of "R" value, which is a measurement of conductive heat loss (this is done, not to necessarily to be deceptive, but to put it terms that the average person understands). Reflective barriers are only effective where conduction thru solid matter is not an issue (in other words, the radiant barrier must have a air space and not be between 2 solid units, as in between 2 pieces of wood).
The primary application for reflect barriers is in reflect solar or radiant heat away from the living space in hot climates. In cold climates, with the exception of windows, radiant barriers are usely not used because the primary concern is on conductive and convective heat movement.