My short summary is you should probably google and study radiant barriers, for reflectix is a radiant barrier. After studying this, you should develop a solution which makes sense to you. However I like talking so I have a long rant for you.
I encountered the same problem specifically regarding reflectix. Some will argue that it's not worth using at all. I do think it is extremely expensive (about the same cost per foot as 1/2" poly iso). But I used the crap out of it and am pretty fond of it. However I can share how I used reflectix, which by no means is correct. In fact it's probably completely incorrect.
I used reflectix primarily as a thermal break/air trap. My logic behind this was "the touch test". Is it cold? okay, lets throw some reflectix on the other side of that. Then smash it in with my insulation panels/fasten with furring strips. I know everyone says 1/8"-3/4" air gap or something along those lines. But for me I justify that the air bubbles in the reflectix are worth some space. Also my work is sloppy enough that I can be confident that there's an airgap with the reflectix no matter how hard I try to get rid of it. I also put long layers of reflectix all over the bus after I insulated the first 1". This was primarily to provide a thermal break between the steel bus with the hopes to eliminate as much thermal bridging as possible. Because the steel studs stick out more than my insulation boards do, I'm confident that there's an airgap there as well.
I made a post which sums this up for the most part, you can find it here.
Ultimately I have developed an ideology towards insulation as a maze for air. Air will get through one way or another, just gotta make it as hard as possible by making lots of twists and curves as well as lounge areas where it can take a rest. Close down the highways (thermal bridging), and make lots of blockades.
Hope this helps a bit.