Conventional wisdom has been that beekeepers should avoid the accumulation of moisture at the colony top during winter. We need prevent droplets of condensate moisture accumulating at top of the colony. This can rain back down on the bee cluster creating a cycle of bees getting wet, the bees then needing to move to dry off, resulting in yet more warm air rising, more droplets forming at top and even more moisture raining back on the bees. Like bees in bee tress or us in our homes, overwinter we wish to keep warm and dry.
Many Oregon beekeepers add openings at the top of the colony so moisture-laden air is vented outside the colony. In a ventilating hive we use an inner cover with a notch to provide moisture escape or remove the covers to scrape the propolis sealing the top box to the inner cover or perhaps add a wooden shim, carpet tack or small stone/stick to vent top of the hive. Some beekeepers prefer to drill holes in hive bodies above, at or to the side of the handhold.
Another popular winter ventilation management is to add a quilt box or ventilating (Vivaldi) board at the colony top to capture excess moisture. Wood shavings, straw or burlap/old terry towels/athletic socks are added to absorb the moisture and screen side vents shunt the moisture to the outside. If colonies are populous enough and the top venting of moisture does not create constant drafts to allow most of the heat to escape, our bees will survive until spring.
Providing an upper bee entry/exit is recommended in areas of heavy snow cover if the lower entrance is likely to become blocked with snow or ice. In addition to venting moisture-laden air, the bees can use upper entrances for mid-winter cleansing flights to void accumulated feces. But an upper entrance also vents heat and moisture from the hive and it can create a constant cold air draft movement though the boxes. Bees need to replace the heat and honey is needed to run the muscles that produce that heat. The greater the heat lost, the more heat that needs to be generated.
Bee trees/our homes
For comparison bees in a bee tree have one single entrance, usually near the bottom of the comb area. They line the interior surface with propolis. There is no upper entrance or provision for moisture escape. Wintering populations are usually small since there is less comb space.
In our homes, we use thin wall insulation with heavier attic insulation below a vent in the air space under the roof. We keep doors and windows shut as much as possible to reduce draftiness and may add insulation around windows/doors to avoid cold air entry. The less attic insulation or more opening of doors or windows the more we have to heat our home (or think of reverse in summer when we air condition). Heat and humidity accumulate in bedrooms on an upper level.
A condensing hive has three distinctions over a ventilating colony: (1) the condensing hive has lots of top insulation, (2) the bees control ventilation, circulating fresh air to avoid CO2 buildup while conserving heat and humidity (versus the beekeeper doing so) and (3) the beekeeper only provides a single colony entrance near or at bottom of the comb (no top venting or moisture trapping device). Top insulation means moisture-laden air shunts to the sides rather than forming as moisture droplets at the colony top.
The condensing colony has added insulation at the top of the colony and no upper vents to avoid cold air drafts and loss of heat. The idea is to hold the “heat pool” at the top of the colony so the bees don’t need to generate as much heat to replace that being lost. The moisture does not condense on an inner surface at the top of the hive. It is distributed to the sides and circulated downwards. On the coldest days it might appear as frost at the side of the box.
A minimum of R 5 (R is a measure of insulation value), or R 10 at the top of a colony is sufficient to reduce heat loss, allowing bees to recapture the heat rather than vent it to the outside of the hive. A one-inch-thick extruded polystyrene (XPS) foam board provides R-5 insulation. Cut the insulation to fit inside the telescoping cover or place it within the quilt box (covering the screen sides). Do not insulate above a moisture accumulating area – the trapped moisture negates insulation above it
Some reading about Condensing vs ventilating
For an interesting discussion of when ventilation at the colony top is of benefit I recommend article by Derek Mitchell in August 2017 ABJ. If there is upper venting within the “heat pool” at the top of the colony, then considerable heat can be lost but if the vent is below this region, a top vent means “not much heat is lost through the vent and any draft is minimal.” Bill Hesbach, EAS Master beekeeper and President of the Connecticut beekeepers focuses on the condensing colony (Hesbach Feb 2020 ABJ). Bill says a ventilated bee hive must “live in a constant state of stress,… excessive ventilation causes forced survival behaviors … and extended intervals of thermoregulation.” For more details I discuss the two types of winter hive preparations in my November American Bee Journal Beekeeping Basics column Winter Success.
Wrapping the winter colony
At one time beekeepers wrapped colonies for protection of the bees during winter. Some still do. Wraps to cover the outside of the hive include tar paper, wood, plastic or Styrofoam. Beekeepers at one time placed a wooden outer box over their hives in the winter, storing them the rest of the year. The original Langstroth was a double walled hive. Wind protection can be of benefit if colonies are in an exposed location.
Today plastic hive boxes are increasingly common. One of the advantages is they provide for a better insulted hive. However, the majority of the heat potentially lost by clustering bees in winter is at the top of the hive, not the sides. Seeking to better insulate the top portion of the hive will pay the biggest dividends in better survival. As for the bottom close screen bottoms if winds can whistle beneath your colony. A slatted rack between bottom board and the lowest brood box will help insure bees set up the initial cluster within the lowest box.
A condensing hive more closely resembles the natural bee nest in a tree and what we do in our homes. Consider top insulating to insure warmer and cozier overwintering of bees. Good luck with wintering.