The thing I see people miss is that Reflectix actually has a very small R-value - and the R-values they quote are very misleading. They somehow translate their heat loss reduction that is gained from reducing RADIANT heat transfer, which really has no correlation to R-value at all by definition. The air space in the bubbles is actually only R-1.1(from conduction). This is from the manufacturer and I don't even buy that. Undisturbed air space has an R-value of about R-1 per inch. By definition, R-value only covers conductive heat losses. However, the whole premise of Reflectix, is really to block radiant heat transfer. The only way you get a true value from it is if you are somehow stopping RADIANT heat transfer. In order to stop radiant heat transfer (which is heat transfer from a hot surface to a cold surface across an air space), you MUST have an air space. It barely helps if you wrap a MLT in Reflectix without an air gap. Additionally, adding additional layers does not really help things much, since you have no air gaps between layers and are relying only on the conductive heat loss from having an air space in the bubble.
The air gap must also be air tight. So, you need to tape all seams and ends of the sheets . This will then be reflecting the radiant heat back to the MLT. The key is that air gap must be air tight, or Convection will ruin the whole gain by the Reflectix application.
So when you do your project, make sure to create an air gap and make that air space, air tight. You will see much better results than without.