I was on a canyon trip west of Delta two weeks ago. The days were warm and the nights were mildly cold. I woke up under my SMD Haven tarp with frost all over my WM bag. The doors were open so there was some exposure to the clear night sky. I slept in my base layer. As I moved around in my bag in the a.m. it seemed that the frost began to melt and my bag got wet. I suspect it was the stratified warm air in the insulation hitting the outter shell of the bag causing the frost to melt. I would guess it was in the 20s that clear night. I was fortunate enough to have a clear day and so I spread my bag out to dry before packing up for the day's walk. My boots were frozen that a.m.
This last week I was up the Cochetope. I slept under the SMD Haven and I also used a Borah bivy. The Borah's top fabric is M90. It was a long clear 9 degree night and I kept the doors closed. I was surrounded by noisey elk. I slept in my down jacket, pants, booties, and base layer. My boots were in a plastic bag in the bottom of my WM bag to keep them from freezing. The top fabric of the long Borah Bivy isn't nearly long with boots being stored and the top edge only came up to my armpits. I woke up multiple times through the night very concerned about the frost build up inside the bivy bag. I was fortunate enough to have a clear day to spread my sleeping and bivy bag and tarp out to dry before packing up for the day's walk.
About 4 weeks ago I was up by the Butte. I slept under the SMD Haven with doors open and just my WM bag. I slept in my base layer, a mid weight layer, and my rain gear. It was a moderate high 20s-low 30s night with a mild wind. I had no condensation issues and packed up and started walking right after breakfast and glassing.
I have no conclusions yet, but I do want to experiment more with vapor barriers to see if that will reduce the condensation issues or if it is strictly a matter of temperature differentials. The answer is probably already here in this forum and I just have to search it out. I don't remember any condensation issues years ago when I was in a unit that issued Feather Friends bags that included vapor barrier bags and bivy, but maybe we didn't sleep much.