I had a chance to test my 20F down sleeping bag in a shelter last night in temperatures below 20F while wearing most all of the hiking clothes that I plan to take this March 20..in the Smokys...
When I got to the shelter it was 10pm and -11C with a north wind(6mph) coming into the open side of the shelter.
In the morning when I got up at 7am it was -14C with little wind.
The bag was a Moonstone Lucid 800.
The clothes I wore were:
Dry smartwool med weight socks,
Merino wool cap/touque,
Icebreaker 140 wool T shirt,
Long sleeved coolmax shirt,
Patagonia down sweater,
Precip type jacket with hood,
NFace hiking shorts,
Sleeping pad was thermarest equal to prolite 4
I was warm enough..feet were very good,skirt helped with legs,head always warm,torso just right and only cool when cold air from breathing hole came in too much from time to time,nose was cold at first so I closed the hole to about 11/2 inches in diameter.
Some snow blew in and melted along one side of sleeping bag so by morning it was cooler in that area.
At no time was I cool enough to shiver.
No gloves or balaclava were used last night..my hands stayed warm, my nose was not always toasty.My bandana would have kept my nose warm.
The coldest night last spring (when I hiked from March 19 to May 2) for me on the AT was about April 11 with temps at night down to 13F. This was at Cable Gap shelter(elev 2880 at mile 156 just south of Fontana Dam and the Smokys)
I was very good in my 10F rated down bag with less clothes on.
I am not a hot sleeper or a cold sleeper..about medium.
I hope this info will help someone deciding what he/she needs to bring.