Don't leave home without one
This little peice of Mylar can mean the diference between shivering all night as the cold zaps all your energy and being able to get back up at first light to make your way back to safety. I've been caught out without a "space blanket" before. If you don;t believe the heating powers of a vapor barrier, go get a large trash bag and hudle with it wrapped around your shoulders, within minutes you should feel the trapped heat building. Added bonus to tthis blacket are the survival instructions and illustrations tp keep you enlightened and entertained.