You definitely don't want to use that vapor barrier in relatively warm weather; it will be a sauna inside! But once the thermometer gets below freezing, it can be a life-saver, keeping your sleeping bag insulation dry. Although I'm a heavy sweater, I didn't sweat at all in my vapor barrier on below-freezing nights on my recent Wind Rivers trip.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey