Mylar blankets are:

  • a vapor barrier, making it uncomfortable and sweaty in temps upward of freezing
  • not durable, as in, the really cheap ones are essentially single use items
  • not a comfortable kind of warm - at least, not in the same way a nice sleeping bag can be.


However, it still has a place in an emergency kit, as you can use them if you come upon an injured hiker who's taking a chill in the middle of the day - they can also save your hide if you're caught out overnight without a full backpack. The Adventure Medical bivy with thermolite (around thirty bucks) will be more durable and probably more comfortable, and tends to be what members of our SAR team carry for the victim.

The NeoAir sleeping pad/inflatable mattress actually makes use of the reflective mylar in its baffles to keep the mattress compact and still effective. But it is not comfortable much below freezing unless you add a foam pad to it. Which is to say there are definite limits to a reflective warm layer. Hammock users sometimes use bubblewrap covered with reflective foil - Reflectix is also a home insulation and is additionally used to make heat-retaining cozies for backpacker's pots or dishes or freezer bags - but again, it doesn't keep you as warm as more traditional insulations.
_________________________
"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzuki

http://hikeandbackpack.com