When we hear the term “ventilation” we often think of a source of fresh air. But ventilation is also a means of ridding the internal environment of excess heat, moisture, carbon dioxide, and airborne toxins and pathogens.
Like any other animal, bees use oxygen and emit carbon dioxide and water vapor. The air containing these substances is warmed by the bees’ bodies and rises to the top of the hive. If there is no way for this air to leave, water vapor condenses inside the roof or under the inner cover. When enough water accumulates it forms drops that fall back on the cluster and chill the bees.
If the stale air cannot get out, fresh air cannot get in. In a stagnant environment such as this, the concentration of oxygen goes down while the concentration of carbon dioxide goes up. In addition, airborne spores from fungus and mold can build to extraordinary levels, and so can other airborne pathogens such as viruses and bacteria. In my opinion, ventilation is one of the most overlooked elements of beekeeping.
Of the hive types I am familiar with, the Warré hive seems to have the best built-in ventilation. The Warré hive is essentially a top-bar style (meaning frameless) hive in boxes that are stacked like a Langstroth. However, above the topmost set of bars, a piece of starched burlap covers the entire box. Above this is a “quilt.” The quilt is a shallow super with a burlap bottom that contains materials such as sawdust, dry leaves, straw, or woodchips. These materials absorb moisture from the air so that it does not drop onto the bees below.
Above the quilt is a mouse-proof board and on top of that is a gabled roof with ridge vents and eave vents. These vents dissipate solar heat much like the attic of your home.
The only change I would make to the standard Warré hive would be to put vents in the sides of the quilt box as well. These could be drilled through the sides of the box and covered from the inside with hardware cloth much like the holes in a ventilated feeder frame. Air would diffuse through the material, allowing it to dry, without causing an excessive draft in the brood boxes. At the same time it would provide a pathway for carbon dioxide-laden air to leave and fresh, oxygen-rich air to come in.
In any case, studying the plans for a Warré hive can give you some good ideas about creating ventilation for your bees no matter what style of hive you are using.
 Evaporation is a cooling process, so as the water evaporates from the bees they become cold. It’s much like stepping out of a bath into a cold room.