You could also supplement your bag by using a down coat and hat and underwear. I use one or two hot water bottles (Gatoraid) in a large ziplock baggie wrapped in insulation and they can really really extend your comfort zone. This only works if you can use reliable wood fire, however. I use the TNF Beeline bag with a 1/3 zipper. I don't want to carry a 3 lb. 15 or 20 degree bag when it is 30 or 40 outside. A full zipper bag can be opened as a quilt on hot nights. Consider a synthetic if it is damp where you are.