The following explanation is to assist you to build a model (picture) for winter hive ventilation, which requires Air IN, and water / carbon dioxide OUT. Remember, bees use honey and oxygen (in air) to produce heat (energy), carbon dioxide and water. Glucose (honey) + oxygen => water + carbon dioxide + energy (heat). The water (as vapour) is less dense than air, while carbon dioxide is greater. So, carbon dioxide goes down, while water vapour goes up.

A bit of detail for the ‘data people’: 180 g of glucose + 135 litres of oxygen => 108 ml of water + 135 litres of carbon dioxide (at 0 degrees C). In other words, 1/3 a pot of honey + 7x20 litre (4 gallon) tins oxygen => ½ cup of water + 7x20 litre tins carbon dioxide. Because air is only 20% oxygen, the amount required is 675 litres, or 33x4 gallon tins.

SO for bees to consume about 1/3 pot of honey, 33x20 litre tins of air must enter the hive, followed by ½ a cup of water and 7x20 litre tins of carbon dioxide leaving the hive. The water may leave as a vapour our the top ventilation holes, or as a liquid dripping down the insides and out the entrance, while the carbon dioxide will sink and leave out the entrance or other gaps around the floor.

Summary: A hive needs top ventilation and should slope towards the entrance. If an entrance reducer is fitted, it should be cut away on the outside edges to allow liquid water to flow out the front edges, or the hive should be fitted with a gauze screen floor.

Unless the water vapour all escapes through ventilation holes, the use of insulation will influence where it will condense to form liquid water. Polystyrene foam under the roof will prevent condensation under the top board. Water will condense on the inside wall of the hive if that surface has less insulation (ie is at a lower temperature), than the ceiling. The liquid water will then run down the hive inside wall, rather than drip onto the cluster.

For those who feel water should leave the hive as vapour, the amount of air entering and leaving the hive increases enormously. Converting 18 ml of liquid water (say 1 table-spoon) to a gas at 0^oC produces a volume of 22.4 litre (say a 20 L tin), but to remain a vapour, it must be mixed with another gas (eg air), in the ratio of about 0.6% water vapour to 99.4% air. If the water vapour content gets above 0.6% at 0^oC the water condenses into tiny drops and collects on any surface (eg our breath on cold days). So for 1 tablespoon of water to exit a hive as a gas, about 100x20 litre tins of air is required to ventilate the hive. This amount of air will also itself take a lot of heat out of the hive (as well as the water vapour). Multiplying by 6 to scale up to about 1/3 pot of honey gives a volume of 600x20 litre tins (12 cubic metres or 3 toilet rooms) of air entering the hive. The same volume of moist air must leave, along with 7x20 litre tins of carbon dioxide and some heat energy.

THINK before you make major changes to your normal practice. Try one or two changes each year. Insulate the ‘tops’ and cut away the corners of your entrance reducer this year. Try a few hives with mesh screen floors, either ‘open’ underneath or with some ‘skirt’. I have some 10mm thick polystyrene foam which I have made into ½ thickness insulating frames and intend trying one each side of a 9 frame brood box (in Auckland). They have to be painted or the bees will chew them to pieces. I hope this will keep the outer frames ‘drier’ and stop moulds on the outer emptied frames.

I think the main points are that the ceiling of the hive must be much better insulated than the walls. Condensation, (dripping) must not occur on/from the ceiling, and heavy carbon dioxide must exit the hive. Make sure the ‘wrapping’, insulation, is better on the top than the sides and that the bottom entry is never blocked. What works for you, works for you, consider changes thoughtfully. I hope I haven’t made any bad error/s, please feel free to query the values given. (they are only approximate).

The air in the winter hive can be crudely likened to a floating bubble, a 'warm air balloon'. Forced ventilation, notably wind at the entrance or mesh floor, is a major influence on the turnover of this 'bubble'. Bodily movement of air masses can be a far greater influence on the water regime in the hive than the considerations of gas densities raised by Paul. The temperature of the 'warm air balloon' is, as Paul says, influenced also by that anomalously large quantity, the latent heat of evaporation of water (same thing, with a minus sign, as the heat given out when water condenses).

If a hive is struggling for heat, it will keep warmer if water vapour actually condenses from the air before the air leaves the hive; the problem then becomes to ensure that the liquid runs away harmlessly. If ventilation is inadequate, some fanning may be organised (has anyone studied this?).