I went for a mini tour this weekend, 3 days in upstate new york. It was fairly chilly at night, probably in the low fifties. I was using a Mountain Hardware 20 degree synthetic bag, and I was a bit hot. I didn't like how I would sort of stick to the nylon of the bag, or the nylon of the pad.

I'm going on an 8 month trip through South America and I was thinking of picking up a bag liner. I figure in the hot weather I can use the liner only, and when I'm up in the andes I can combine the two bags for super cold weather.

What have peoples experiences been with bag liners, and in what temperature ranges have they been useful? Does the layering idea work?

Also, I'm using a neoair pad underneath.