For the user of a Gore-Tex bivy sack this means that, on occasion, condensation will occur on the inside walls of the sack. The disadvantages to adding openings to a bivy for ventilation lies in the reduced waterproofness, increased weight and bulk, and higher cost that this creates. OR bivy sacks offer a carefully designed front opening zipper covered by a wide flap which allows ventilation in nearly any condition. Most OR bivies also provide pole supported front sections to increase comfort and assist with ventilation. To keep blowing snow or spindrift from entering through the vent, position the open section downwind.
Condensation can and does occur in tents on a regular basis and the same factors apply.
Tent walls are farther from the occupants than in a bivy and a tent can be ventilated more easily in a wide range of conditions. When considering a bivy sack it is important to be aware of the down sides of its use. Not so well documented are the disadvantages of bivy sacks. Claustrophobia Some folks simply do not do well in bivy sacks due to the tight quarters they present. The difference between spending a full 12 hours in a bivy and a tent are significant.