Bivy vs Underquilt
I was in recent conversation with another hammock camper and he suggested
bivy sack with a regular sleeping bag instead of an underquilt. To me this does t make sense but I was wanting to get y'all's opinions. His reasoning was that the bivy is made of gorges therefor will block wind and other elements.
I don't know just seems like it wouldn't work. I'd be interested to hear if any fellow hammock campers have tried this method and what your results were.
The reason for the UQ is to avoid compression of the insulation, a bivy doesn't fix that problem. I've used my military sleep system quite a few times in my hammock with the bivy. In nice weather it doesn't seem to matter. However in cold or stormy weather it can make you a little more comfortable and compensate for lack of a winter tarp and/or not being able to pitch your tarp to block all the wind and such.
I am assuming you are talking about being in the sleeping bag in the hammock. If you are talking about suspending the bivy/bag under as a UQ then pretty much anything that will shape to the hammock nicely will work.