I got out yesterday for a quick overnight. A tad chilly but fun. Tried out a used bivvy I bought. Bivvy is too tight for a puffy bag and full size Thermarest. I used a summer bag (32F rating), but I wore a down sweater and down pants and was fine. Feet were slightly cold, but not so cold that I couldn't sleep.
Well, I got 'em on eBay for substantially less than I would pay anywhere else -- but still expensive. 800 fill always is.
BUT my old warm fleece pants are about half a pound heavier and probably not quite as warm.
The purpose of this trip, other than to just get out, was to test if I could be comfortable in a 32F bag in mid 20's F weather. I was comfortable, and I'm a cold sleeper.
I figure that since I am going to need insulation for my legs anyway, why not do double duty and use that leg insulation to allow me to carry a lighter sleeping bag? That's the idea anyway which works as long as you don't get your leg insulation wet. I figure on a day where it's warm enough to get wet that I probably won't be hiking in down pants.
No, that's an ancient REI Gore-Tex bivvy. It's at least 10 years old, probably closer to 20. It's light. That's about it's only redeeming quality. It's a tight squeeze with a 1.5" thick Thermarest and a puffy sleep set up.
Tried out a used bivvy I bought. Bivvy is too tight for a puffy bag and full size Thermarest. 
I used a summer bag (32F rating), but I wore a down sweater and down pants and was fine. Feet were slightly cold, but not so cold that I couldn't sleep.
