Originally Posted by
Tipi Walter
...Plus, a down sleeping bag is never as dry and as lofted and as warm as it is at home unfurled before a trip. This is important. Such a bag may very well be a true 0F rated bag when taken out of the house and thrown in the backyard tent tonight---but take that same bag out on a long backpacking trip and it's at-home warmth and loft won't be as strong. This is due to trip length, air humidity, your body's condensation and other factors.
For this reason it's important for winter backpackers who want to carry down bags to GO OVERKILL. If you expect 0F, get a -20F bag.