In reply to puppy:
I regulary use on aconcagua a mountain designs www.mountaindesigns.com.au serac series bag, rated to -20 celcius on 2 closed cell foam, full sized sleeping matts with a therma rest thrown in for comfort and extra insulation. generally get a lot of frost on the inside of the tent from camp 1 and up, just a matter of making sure you dry your bag out during the day when possible. this bag works well for me, im a cold sleeper..
dan