With a warm enough pad, I can take my 20*F Western Mountaineering Ultralight down to 10*F. That's wearing my non-breathable rain jacket and pants over my base layer as a vapor barrier, and all my insulating clothing on top of the vapor barrier.

I don't think I'd like to sleep out in a bivy, though, unless I also had a tarp. More important, I need a bug-free shelter for my dog!