Reflecting is what it does.
'm brand knew here but I have a lot of experience with Reflectix having installed it in a few yurts.
So we loose heat in a few big ways when we're out there sleeping away in our slings of pure comfort: evaporation, convection/conduction (ie. cold wind or sitting on snow taking our heat) and radiation (electromagnetic waves). We keep dry to prevent the evaporation from cooling us off. UQ, TQ and pads are insulation that keep convection/conduction from cooling us off and create a barrier from cold. And yes Relectix will reflect back your body's radiant heat just like a space blanket... BUT it does not breath and it creates a barrier between you and your insulations which can make your down/pad etc. not as effective. And the farther away from your body it is, the less efficient it becomes at reflecting so it's not very useful as your outer layer.
Can it keep you warm? No doubt but I think best used as a cozy as others have said. Reflecting radiant heat is exactly how thermoses work. Keeps food warm and lets you hold a boiling pot!
All that being said, I've found it to work under my legs when I use a 3/4 UQ and I didn't have a pad.
Over simplified but that's the idea. Reflectix is amazing stuff. It's the only thing between the canvans of my friends yurt in Vertmont and as long as there is a radiant heat source (wood stove) it's sooooooooooooooooooo warm. The walls are less than a 1/2 thick!!
Hope that helps. Love this forum!