reflective blanket, over or under uq?

On the Yeti UQ, it come s with an optional reflective blanket, Its my understanding that it goes between the hammock and the UQ. How does that work, with condensation etc? Can the same thing be done with a mylar emergency blanket? How does the UQ still breathe if you do this?

In short, it acts a vapor barrier to limit breathability. In really cold temps, that can be helpful.

There is a great thread here about using vapor barriers.