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....
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I hate to belabor the point, but it's basic chemistry/physics. Hot goes to cold. Hot is due to energy. Cold is due to a lack of energy. You get cold from sitting on the ground because the heat differential causes heat to want to leave your body. If you don't stop it, you will get cold.
As far as the double pad thing, I've heard some people say you'd be warmer to put the foam pad on top of the air pad.
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But Sarbar is right. That cold DOES come up out of the ground and sucks the heat right out of my rear-end and right back down into the ground. Can't you just see that little cold gremlin now? Picture it in your mind. That's really how it happens!
If you think you can, you can. If you think you can't, you can't. Either way, you're right.
Sarbar, eugene is right about the cold and hot. Cold really doesn't exist. It is just an idea to help us understand heat. It isn't a "thing", but a lack of a "thing". Dark is a similar concept. Dark doesn't exist, but is a way to describe a lack of light. When you think you feel cold creeping up on you, you are actually feeling the heat leaving the area. But, this doesn't have much to do with the discussion.
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