I use a LaFuma 30 degree 800 Fill Down bag, and I use a small poncho tarp, so recently I got a bivy sack (Equinox) to complement the bag in colder and wetter weather. For warmth and comfort, this combination is terrific (I move around a lot at night, and having my Gossamer Gear pads and bag held in place by being inside the bivy sack is great help).
However, when it drops below 40 F at night, I've noticed lots of condensation inside the bivy sack (both on top of the sleeping bag and on the underside of the top fabric of the bivy sack). I've experienced this in dry Eastern Oregon, wet Cascades, and also the southern Appalachians. So I dry the bag out during the day.
I have two questions:
1. How can I measure loss of loft in the field? In other words, not having a postal scale, I can't measure the weight of the water that now resides in my bag. And even then, how can I know what kind of effect this will have on my sleeping warmth?
2. Should I just expect to have condensation on my bag after a night below 40F, and learn to love it?
I've often wondered if it would be an advantage to have a WPB material around my breathing hole at night, and, I suppose, this review of the Golite bag answers that question! Thanks also to Mr. Clelland for his comments- I might try some of those socks.