Aaron Nash wrote a review of Black Diamond Twilight Bivy on April 16, 2012
After reading some mixed reviews on this bivy, I decided to give it a shot in an effort to lighten up my kit by about a pound. I was hoping to use it in moderate weather in the alpine where little to no precipitation would be seen. As the weather forecast was looking beautiful, I took it out this weekend for a test run atop a snowy ridge in the Cascades. Ideal conditions, clear skys, light wind, no snow, and this thing managed to sucked hard. I woke up three hours into the night to find the inside of the bivy completely soaked. A few hours later and my bag was soaked through and no longer providing me warmth. I have no idea how the product description can claim bit of breathability, as this bivy was about as breathable as a trash bag. Quite a long and cold night from that point on. A point to note; I slept with the hood open and my face out into the air all night long. My breath did not contribute to the condensation in the bag. The seams were also sealed with Seamgrip prior to spending a night in it.
It was a bummer that this bivy didn't work out as I would have liked. I don't recommend this, or any Black Diamond bivy made of this material to anyone who would use it in an alpine environment. I'm glad I tested it on a mellow climb where the only repercussions were a shiver bivy and a lack of sleep. Anything bigger and things could have been worse.
Do yourself a favor and steer clear of BD bivys.