I work as a Heli pilot in Alaska year round and have to camp out at job sites sometimes and it's always good to have something warm incase of a breakdown especially in the winter. I have not slept in this outside in colder then -10F but how I felt at that temp, I'am sure it would work great in -40F with maybe a thin layer on at worse. What I like about the bag also is that I have used it in 60 degree weather and still been very comfortable with it unzipped I was worried I would cook myself ou...t overnight. I also use a aftermarket stuff sack and can really get this thing packed down I wouldn't recommend leaving it like that too long. Although it has lofted very well 30 minutes after unpacking when it was packed down like this. I would defiantly recommend it, it is well worth the money and quality seems excellent too
Fantastic warmth at full winter conditions, no liner or extra clothes needed.
If you're planning a winter expedition in the most frigid temperatures, The North Face Inferno -40F Down Bag has all the features you need to keep you comfy and warm while you sleep in the cold outdoors. Engineered with 800 fill ProDown, the Inferno -40F Down Bag has a generous cut that lets you wear more layers to bed and a full draft collar and integrated draft overlap to prevent heat loss and keep you ultra comfortable. Water resistant Neovent Air fabric on the hood, footbox, and back protects those moisture prone areas, and the trapezoidal side baffling prevents the down from moving. A center zipper gives you the easiest access, and an internal pocket provides insulated storage.