What I've done is go with a 10 degree bag and a polypro liner/warm-waether bag that I made myself (if you can use a sewing machine, it's a quarter the cost- buy your polypro, fold it, then stitch another layer on top of that for added airspace), and an air pad. It's nice down to about 0 in a well built snow cave, and tolerable to about -15 in a well wind-breaked tent. I've never been out any colder than that, so it may not be it's lowest limit.