I agree with Chris. I haven't had -20 yet, but did have below -10 a couple of years back. No problem with the semps that had snow cover. I would cover the blankets (if you use them) with a tight weave-count sheet, to cut down on wind penetration. Plastic is a bad idea, do not use it, as it will cause solar heating, which is undesirable with cold nighttime temps. You could also put a few jugs of cold water under any wraps. The cold water will moderate temps a bit by slowly giving off heat during the night and absorbing heat during the days.
