i was thinking more about this "air gap".
if you have a conducting material facing another conducting material with a small air gap in between, you would not expect the air to act as much of an insulator, but rather functionally behave as a conductor. convection would just be conduction, in context. it would basically be the same thing. however, if you had an insulator in the other side, the gap would just fill up with heat until it got to equilibrium. the gap would then become a part of the insulator, but only because the gradient is reversed. the gap itself would slow the reflection down, not help it.
yet there may be some scenarios where you do want an air gap between aluminum and an insulator that have to do with preventing moisture building up, rather than implementing the radiant barrier. that air gap would have no effect on the reflective or insulating properties of the aluminum-foam combination, but it would prevent water damage behind the foam. those issues did not cross my mind, but they are not relevant to me, because i am insulating a basement. i don't know if i would have thought of moisture as a concern if i was trying to block an a/c from below me rather than above me (i would not need to do this if the a/c was below me. i would just open the windows full blast and blow the heat in with fans. placing thermal barriers on the floor wouldn't make any sense, as they would block the heat from rising. instead, i'd be trying to push heat down from outside. i might have guessed that would create moisture.), and don't want to be pretentious, but that's not what i'm doing.
i think this page is helpful in going over the different contexts and why it's important to be careful.
in the context of what i'm doing, i want to eliminate intentional air gaps and push the insulation tight against the wall. my analysis was correct. however, any inevitable pockets of air that continue to exist will allow for humidity to escape upwards. if i could theoretically completely eliminate these gaps (i know i can't), i would create humidity issues. but, i can't, and i'm not worried about that.