ctgottapee is mostly right. Radiant barriers do work but their effect is modest and generally not worth the cost. Foil on foam is fine though, because it doesn't generally add to the cost. GreenBuildingAdvisor has a couple great articles on the folly of foil bubble wrap and radiant barriers:

http://www.greenbuildingadvisor.com/...ed-bubble-wrap

http://www.greenbuildingadvisor.com/...search-problem

On a different note, AC, it appears you are creating a stackup with two layers of foam that have vapor barrier facers, and there will be fiberglass and wood joists stuck in this vapor barrier sandwich. This is generally advised against. Any moisture that finds its way between the two layers of foam will take a long time to dry out.

Also, have you planned for a thermal barrier under the final layer of polyiso? Code requires foam to be covered with a thermal barrier such as 1/2" drywall, because it is relatively flammable. Code aside, it is a life safety hazard.