Over a British winter, condensation kills more colonies than cold does. The cluster of bees generates heat and, through respiration, a surprising amount of water vapour. If that moist air cannot escape, it rises to the cold roof, cools below its dew point and condenses into water that drips straight back down onto the cluster — a much bigger threat than low temperature alone, since bees can survive deep cold but chilled, wet bees die quickly.
A single overwintering colony can produce several litres of water vapour over a winter through respiration alone — roughly one molecule of water for every molecule of stored honey they metabolise for heat. Good ventilation design is about removing that water without stripping away the warmth the cluster works so hard to make.
A cutaway hive shows warm, moist air rising from the winter cluster to meet a cold roof — adjust top-vent gap, entrance width, floor type and outside temperature to see whether that moisture escapes or condenses into dripping water above the bees.
Moisture-laden air rises through the hive and either escapes through the top vent and entrance draught, or cools past its dew point on the roof and condenses into droplets that grow and drip back onto the cluster.
Lower the outside temperature and close the top vent to watch humidity and condensation risk climb. Open the top vent, widen the entrance, or switch to an open mesh floor to clear the moisture — while watching cluster heat loss rise in turn.
Bees survive deep winter cold remarkably well by clustering and shivering — it is chilling by condensation dripping onto wet bees, not cold air itself, that kills most overwintering colonies in the UK.