clindley

Which is warmer for a cold night in January at 11000 feet? A zero degree down bag inside of a tent or wearing a down jacket and pants and sleeping in a bivy sack in side of a tent? My goal here is to possibly drop the usage of a down sleeping bag for down clothing. Is this an absurd notion? Would it be pretty stupid to use a bivy sack in combination with down clothing? I absolutely love my bivy sack because it keeps me on my pad.