Duane

That night that we spent on Broken Top it got down to about 15 degrees and I had a Marmot Pinnacle 15 degree bag. My silnet bivy tent slid off the hill and I ended up sleeping under the stars in just the sleeping bag with a down coat and down pants on inside. Since the shell is made of dryloft, I do not need a bivy sack. You generally would use a bivy bag for emergency and get in it with your climbing partner, it was not considered an accesory to a sleeping bag. I think the bivy/tarp concept is because modern bags save weight in the shells thus you use an external shell - a bivy. Kind of like a tent footprint is only needed because they use too thin of material in the floor so you add an extra one - a footprint.

A bivy does add warmth though and the combination of a cut down old bibler bivy sack and a UL sleeping bag with a non-water proof shell works for me on nights that go from sort of warm to sort of cold.
Jim