Reflectix is generally a poor performing product. There are a number of good evals online. As I recall, it relies on a huge air gap and is more about thermal "reflection" (reducing heat gain) than actual "insulation" (I could be wrong/mis-characterizing). In any case, better options include foam, classic insulation (fiberglass/denim), and air gaps. However; they all involve 'materials' so the trick is probably finding one that doesn't bother you.
I installed mylar behind windows that I was covering over to reflect the suns heat but again, it offers no insulation value.