At issue is heated air that escapes the cluster and rises to the top of the hive. Upon contact with the cold hive ceiling the air cools and releases moisture. Condensed water now threatens to drip back down and chill the bees. Practically without exception, the solution given is to ventilate the upper hive to allow humidity to escape. A very excellent article published in Bee Culture magazine last year expressed concern with that technique:
"...beekeepers have devised ways to use the inner cover’s escape hole to ventilate all that warm moist air, without regard for the consequences of lost heat... we know how to keep condensation levels down by adding lots of ventilation, but do we know enough to understand how to balance ventilation with the needs of wintering bees? ...how much of our current practice of provisioning 60-100 lbs of honey per wintering colony is being driven by removing lots of heat the bees must replace?" http://www.beeculture.com/winter-management
If there's a positive associated with the plight of honey bees, it's the millions of dollars of research money spent every year trying to help them survive. Some of the things we have learned contradict long standing theories, but the traditional nature of beekeeping makes old habits hard to break. In the case of upper ventilation, two things bees can't live without are heat and water. Methods aimed at eliminating them should be met with a healthy dose of skepticism, but that hasn't been the case. Myths that seem to make sense can trump the research. And if the colony happens to survive the winter for reasons unrecognized by the beekeeper, more power to the myth.
Myth: Moisture in the hive is bad.
Beekeepers go to length to ensure water is available to their bees all year, then the cold weather arrives and every effort is made to deprive them. Bees need water in winter to dilute honey, supply digestive secretions, flush waste and produce royal jelly (Johansson 1979). It's sourced from the honey they eat to fuel their heating efforts, which then causes the water to be released again (think bee sweat). The colony must reabsorb and recycle the lost water because there isn't anywhere else to get it. The cluster core is already at high risk of dehydration (Omholt 1987). Watch for low level foraging on warm winter days when bees fly out to cleanse. They are in a desperate search for water.. Also, humidity in the hive disrupts mite reproduction and is essential for eggs to hatch. It also may produce some mold. Having evolved in the warm, wet environment of a tree cavity not served by upper ventilation, bees are capable managers of such common microorganisms.
Myth: Upper ventilation rids the hive of moisture.
If upper ventilation succeeded in removing moisture from the hive, there'd be no need for a quilt. Humid air rises to the top of the hive and meets cold air coming in through the vent holes, causing condensation. A barrier made of wood shavings (or similar absorbent material) is positioned between the cluster and the cover to absorb this condensed water as it comes back down, thus keeping the cluster dry. The quilt's damp upper surface is clear evidence that condensation is taking place inside the hive and above the cluster, exactly what the system is meant to prevent. The vent holes may facilitate evaporation to dry out the quilt, but they're also largely responsible for the quilt becoming so wet (Toomemaa 2013).
Myth: A quilt helps to insulate the hive.
Humid air would hardly reach the vent holes if the quilt insulated. It must remain permeable enough to allow humidity to flow through it. Only then can water condense above it to be reabsorbed as it comes back down.
Myth: Moisture not cold kills honey bees.
It should be well understood that cold kills honey bees. That is the reason they cluster together in winter to trap heat. In no small amount, they need heat to survive. When thorax temperature drops below about 40F the bee enters a chill coma, causing her to fall from the cluster and die on the hive floor (Free 1960) . If moisture killed bees, we'd have colonies collapsing all over the valley every time it rained on a warm spring day. It is only true that a wet bee loses heat faster than a dry bee, because water is a better conductor of heat than air. But it is the loss of heat that kills.
Myth: The hive can only be ventilated through the top.
During winter, air circulates the hive automatically due to convection. This refers to the behavior of warm air to flow toward cooler air in an attempt to equalize. As long as cold outside air is entering the hive and warm air is escaping the cluster, temperatures won't equalize so the air remains in motion. Provided the cover is well insulated, the absence of upper vent holes allows rising warm air to deflect back downward where it loses heat to the incoming flow. In this way, fresh cold air gets preheated before reaching the cluster, reducing the metabolic load of the colony. Moisture either exits the hive with the flow or condensates in the coldest areas beneath the cluster, with air exchange finally taking place at the entrance (Sudarsan 2012).
Myth: Bees need lots of ventilation.
It's easy to consider the needs of the colony in human terms, but we could not survive in the toxic environment of the winter cluster. This is important when deciding how much ventilation is actually needed. Constantly flushing large volumes of air from the hive works against colony efforts to lower their metabolic rate. The accumulation of CO2 and reduction of O2 in the winter cluster is a survival tactic, inducing a stillness that promotes the longevity and vitality of the colony (Zherebkin 1979). There is a considerable amount of research documenting the success of colonies overwintered with minimal ventilation. Colonies with closed entrances consumed less food, lost less bees and reared more brood in spring. (Naumovich 1891; Ivanov 1893; Lukin-Jerzhov 1889; Tsvetkov 1945; Perepelova 1947,Gurijev 1949; Starkov 1956, Michailov 1963 - as referenced by Toomemaa 2016).
Myth: Insulation causes moisture problems.
If bees truly know best, the hive should be sealed and insulated ahead of winter. The colony gathers large amounts of propolis in the fall making every effort to close gaps in the hive. Their preference for nesting in trees and house walls demonstrates the importance of an insulated cavity. Condensation happens when relatively warm air contacts a cold surface. Insulation helps keep the upper surfaces warm enough to delay water from condensing until clear of the cluster. Condensation below the cluster does not pose a threat. The colony makes no attempt to raise the temperature of the hive interior, but heat does escape and insulation helps retain it. This results in a slightly warmer ceiling that discourages condensation without ever having to increase ventilation. Control moisture by preserving heat, not removing it (Bornus 1974). Unfortunately, you can't have it both ways. Attempts to insulate a hive that is top ventilated are futile. It has been shown that air flow through the roof negates the value of insulation (Bielby 1974).
Mites aside, successful overwintering is a matter of reducing the metabolic load of the winter cluster. Consider aligning beekeeping technique with the will of the colony. Manage moisture by imitating the tree hive, which is characterized by a well-insulated cavity and a small bottom entrance (Seeley 1976-8). The consequences of upper ventilation may go unnoticed when expressed in the form of brood reduction, higher food consumption and shortened lifespan, all the result of the high metabolism required to replace heat loss. Mere survival of the colony is not the indication of an ideal strategy. Less than half our hives survive winter, yet our techniques remain basically the same. It's time we find a better way.
Bielby, V.B. 1974. Influence of type of hive on wintering conditions (as referenced by Toomemaa 2016)
Bornus, 1974. Wintering of bees in one-wall hives in cold climate (as referenced by Toomemaa 2016)
Free, 1960. Chill-coma and cold death temperatures of Apis mellifera
Johansson, 1979. The Honey bee colony in winter
Omholt, 1987. Why honeybees rear brood in winter. A theoretical study of the water conditions in the winter cluster of the honeybee
Seeley, 1976. The nest of the honey bee (Apis mellifera L.)
Seeley, 1978. Nest site selection by the honey bee, Apis mellifera
Sudarsan, 2012. Flow currents and ventilation in Langstroth beehives due to brood thermoregulation efforts of honeybees
Toomemaa, 2013. Determining the amount of water condensed above and below the winter cluster
Toomemaa, 2016. Winter mortality of honey bee colonies: reducing the impact of different factors
Zherebkin, 1979. Wintering of bees (as referenced by Toomemaa 2016)