I hear people ask about foil insulation a lot and the first thing I have to do is stop them and give an explanation about what insulation is.
Insulation, in the context of what you are going to stuff into your attic, is a material that slows down the transfer of heat. Its really that simple. Fiberglass insulation, cellulose insulation, spray foam insulation and even rock wool are ways to slow down the movement of heat. We either want to keep heat out or in. Insulation can do this.
What people refer to as foil insulation or reflective insulation, is not really insulation at all. It does not slow down the transfer of heat. There is no R-Factor to it.
What foil does best, when installed properly, is to REFLECT the heat away. There is no slowing involved. As a reflective surface, the idea is to create a radiant barrier in your attic that the infra-red rays cannot get past. You will have a barrier that stops heat from radiating into your home (hence where the name comes from).
Foil bubble insulation, whether it is Reflectix, ARMAFoil, RadiantGuard, Polarlum, Ecofoil, or any other brand can actually qualify as insulation. Unlike the regular foil, it has bubble wrap sandwiched between the foil. This bubble wrap, whether a single layer or a double layer slows down the flow of heat. Admittedly, the R-Value of bubble wrap is not impressive, but it does end up being a thermal break. this being the case, you can argue the case of foil bubble being insulation.
The bottom line is that a radiant barrier attic needs both the foil to block the radiated heat and insulation to stop the flow of convective and conductive heat. That is the only way to ensure that you really are stopping the heat from getting into or out of your home.