I know this time of year is the worst for condensation in tents, as I have experienced plenty of times, ( and I do know that ventilation is key ) but I saw someone the other day use a small bivi bag over the top top of their down sleeping bag ( in a tent I must add ) to keep the moisture off. Last week it was very cold and wet up in the Peak District where I was camping and even though my Rab Ascent 1100 down sleeping bag has a water repellent coating it still got damp to the touch. Does anyone else use a bivi bag over their sleeping bag and if so, does it work / make a big difference?