A condensing colony has two fundamental characteristics. The first is lots of insulation. The insulation can be from very thick wood, as with a tree cavity, or in the form of a fabricated sleeve that covers all the outside surfaces of a conventional box. Also growing in popularity are prefabricated insulated boxes, many of which are currently being supplied by companies worldwide. The second characteristic is allowing bees to control ventilation in all seasons under all circumstances, meaning no beekeeper intervention beyond providing an appropriately sized entrance approximating those found in a natural tree colony.
Winter
In winter, the rising hot air off the cluster meets the top surface of the tree cavity, or box, and spreads out along the top surface. If the top surface is above the dew point temperature, the moist air continues to move toward the side walls until it meets a colder surface. Condensation then begins on the outer edge of the top surface or farther down on the hive's outer comb or wall surfaces. This natural convection loop allows condensation droplets to form away from the warm cluster. When the droplets form, latent heat is released so that a condensing colony's convective loop provides a degree of natural heat recovery in winter.
Traditional practice is to add ventilation to vent moisture and keep things dry. It's an understandable reaction since beekeepers noticed the consequence of hot, moist air hitting a cold surface directly above the cluster, causing condensation to fall back on the bees. We know that a cold, wet cluster dies almost immediately. However, the consequence is that the natural convective loop is compromised, and the accompanying energy losses stress the colony. Since bees can precisely control the hive gases independently, added ventilation is unnecessary.
Thinking in terms of eliminating moisture is missing the value of keeping it. Maintaining temperatures above the dewpoint on surfaces directly over the cluster in a condensing colony is possible even when outside temperatures are as low as -40 Fahrenheit. In a ventilated colony, modeling shows that maintaining temperatures above the dewpoint is impossible even at moderately low outside temperatures. We know from tree colonies that conserving colony moisture raises the relative humidity to a point where essential bee biology benefits. Available moisture, in droplet form, provides bees a needed source to thin honey and hydrate bees that lost critical metabolic moisture on the outer mantel of the cluster.
As the body temperatures of the mantel bees fall, they generate heat by using their indirect flight muscles to shiver. While shivering, bees metabolize carbohydrates and exhale carbon dioxide. Their respiration creates an environment with increased carbon dioxide and reduced oxygen levels. Both conditions would be toxic to humans, but these alterations are intentional with bees. The changed environment around the cluster causes their regular metabolic rate to slow to ultra-low, which conserves energy and traps some needed humidity. Beekeeper-added ventilation eliminates the possibility of this natural survival adaptation to occur.
The various microclimates and convection currents described above evolved over millennia in natural condensing environments. Their importance to brood rearing and their role in the hive's overall health merits consideration, and since our bees revert to a condensing colony naturally encouragement to do so should be part of our beekeeping practice.
Bill Hesbach is a Connecticut native with a background in engineering. He's an Eastern Apicultural Society Certified Master Beekeeper and the President of the Connecticut Beekeepers Association.