Western Mountaineering has been making ultralight down sleeping bags for over 30 years with a reputation for well made, but expensive sleeping bags. Western Mountaineering uses highly compressible 850+ fill goose down which much better heat retention with less weight than the 550-700 down fill found in other sleeping bags. The weight difference also becomes significant in a -25F degree winter bag.
I bought a Western Mountaineering Big Horn Super MF rated to -25 with 9″ inches of loft from its 850-plus fill and a breathable microfiber shell. It was pricey but my last cold weather down sleeping bag lasted nearly 30 years. I expect this one to last for my remaining winter camping career. Since I bought this bag it has been merged into the Western Mountaineering Puma series. I feel confident I will never sleep cold again.
The Puma has two large down-filled draft tubes on either side of the zipper to block heat loss and a very comfortable draft collar that wraps around your upper chest and behind your neck.
I had three options to consider.
1. Length. The sleeping bag was offered in three lengths: 5’6″, 6′ and 6’6″ with corresponding increases in the shoulder, hip and foot girths. At 6’4″ I went for the longest length. Usually with a winter sleeping bag it is wise to get a longer length as one is typically stuffing extra items down into the foot area to keep them warm. This long length provided a shoulder girth of 66″ with room to layer, store clothing and still provide a comfortable fit.
2. Extra Down. Western Mountaineering offered an option to have the bag overfilled with an additional 4 oz of down. I went for the extra warmth, although I am not sure how the temperature rating was affected. The total weight for my bag is three pounds, 14 ounces.
3. Shell Fabric. I had a chose of two shell fabrics Microfiber or Gore Windstopper™ 830 Fabric. The Gore Windstopper™ fabric is highly weather resistant and provides superior protection against moisture and condensation for damp or extreme conditions. Microfiber shells are constructed from tightly woven nylon or polyester with high thread counts. The tight weave of Microfiber is water repellent, more breathable, and lighter weight than that of coated or laminated fabrics such as Goretex. Microfiber shells are a good option if you expect limited contact with water such as tent condensation. There is also a considerable (~$100) price difference between the shells. I chose the more breathable Microfiber shell since I usually sleep in dry conditions (e.g. tent or lean-to). TheGore Windstopper™ might be more appropriate if one regularly sleeps in snow shelters.
I also have a Western Mountaineering Caribou which I have slept in temperatures down to 19 degrees. I regularly use a Design Salt silk liner – theoretically rated at 9 degrees – with all my sleeping bags to keep them clean and give me a light covering in case I have to open up the bag due to overheating.