Originally Posted by
BillyBob58
There are so many variables. In addition to the normal loft to be considered, your UQ is also going to be (likely) more exposed to the wind and wind blown snow or rain than your top quilt. But the main thing is considering a way to get your quilt to just barely contact your back without A: compressing the down or B: without any gaps or openings at the sides and foot end or under your back.
There can be a learning curve to this, and until you master it I would say consider your quilt is going to be good to a much lower temp on top than it will be on bottom. In fact, unless you can successfully solve any gap or foot end/head end/side opening problems, you will likely be cold no matter how thick the quilt, IMO. Not that you can't solve these problems, just that you must be ready for them.
I speak as some one with no experience with traditional down UQs yet. But as some one with mucho experience with Super Shelters, short synthetic UQs ( Warbonnet) and PeaPods, which is an UQ/top quilt combo. Similar to an UQ, but with some significant difs. But in all of these cases, you can't leave the doors open with gaps or openings or you will be cold. And ( except for the WB) if you pull them to tight, you will have no gaps but will compress the insulation, and be cold.