On where to place the mylar: if under the pad you don't get condensation. On top of the pad and you run the risk of wet down by morning.
What happens with many inflatable pads is the cold of the ground sucks your heat right out of your back - a reason why snow campers sleep on 2 pads, one being a Ridgerest or blue pad that is solid and then an inflated pad for comfort.

And to the above poster that commented on cold not coming up from the ground...er, go sit on frozen ground. Yes, the cold is coming up! The mylar works by stopping your heat loss (by reflecting) but also you don't feel the cold beneath you as it is a barrier between your butt and the frozen ground.

Neo Airs have a metalized layer on bottom which serves both purposes....