I did an overnighter (low 32d) with the scouts this past weekend. I used a 16 oz bottle in a couple of medium weight socks. Had my new BA Yampa and Exped 7 short also. I put the bottle in the bag about 30 min prior to going to sleep. It was warm and toasty getting in, in fact it was too warm. By 2am the bottle had lost enough heat that I had to incorporate my old army wool liner.
So, I need to regulate the output better to enable for a longer "burn". I think next time I'll use my 2 liter Platypus and mayble make a reflectix and/or fleece cozie for it.
To Boldly Hang Where No One Has Hung Before...