Can I use Reflectix instead of r13 insulation? I am in SC and need to insulate a little strage unit.
OR is to be used in conjunction with it?
No, Reflectix has no R-value of its own and must be used with insulation, depending upon the application. If you're just trying to keep heat out of the roof on a hot day, then it will help. If you're keeping heat inside a heated space, it adds very little value of its own (approx R 1.0).
However, if you install it as the inside vapor barrier and a 3/4-inch air space from your R-13 unfaced fiberglass, in a 4-inch wall, you may get approx R 3 out of it.
Note that if there is no source of heat inside, then everything inside will eventually freeze, regardless of how much insulation you use.