Jackalope wrote an answer about Black Diamond Tripod Bivy Bag on April 4, 2011
In '06 I used the untaped seam version (but I sealed the seams) for about a month on the PCT/JMT in the Sierra Mtns. Spent about 7 nights on snow/ice at freezing temps, one afternoon for about an hour during a hail storm, and the remaining nights either on top of the bag or inside it. Never really had a condensation problem. It does seem to add about 10 degrees warmth to a sleeping bag! Highly recommended for areas like the Sierras. I went back to a Tarp Tent (1.5 lbs) for lower elevations.