If the thorax cools below the chill coma temperature of 9-11°C,honeybees are no longer able to activate their flight muscles for heating(Free and Spencer-Booth, 1960; Esch, 1988; Goller and Esch, 1990) and eventually fall off the cluster. Watmough and Camazine(1995) assumed that the outer bees react with thoracic heating to avoid chill coma. Efficiency considerations, however, suggest that the surface bees should avoid endothermy because any heat from the surface bees is immediately lost to the surrounding air. Our data show that endothermy seldom occurs in the outer bees and, if it does occur, is only weak (Figs 1,2,3). Intense endothermy of surface bees was observed only in an emergency case when only one layer of bees was left on the flat side on an outer comb. Before the bees walked to the adjacent beeway between the combs they heated up their thoraces. On the opposite cluster surface, where up to four layers of bees were sitting, intense endothermy was not observed. There, the body temperatures resembled the situation shown in Fig. 1C (our unpublished observations)." ~ Stabentheiner

"Humidity is an important microclimatic variable for honey bees since their eggs require a relative humidity of above 55% to hatch successfully, with the highest survival between 90 and 95% relative humidity (Doull 1976).  There was a significant decline in the number of normal larvae that emerged when eggs were incubated at 100% and 80% relative humidity. At 50% R.H. many eggs shriveled and of the remainder, only 2.9% produced normal larvae.  No eggs hatched at humidity’s below 50%. Abnormal hatching was found to be due to failure of the hatching fluid to dissolve that part of the chorion (egg shell) covering the heads of the larvae." ~ Collison, Bee Culture Magazine

"With good insulation and low ceiling, little condensation will form on the ceiling of the beehive. However, moisture will condense on the floor or leave through the lower entrance." ~Toomemaa 2012

"Operators of indoor wintering facilities set the thermostat around 41F. At this temperature, every bee in the cluster can remain at rest and still avoid chill coma, provided the insulating shell maximizes its density" ~Heinrich, 1981

"CO2 levels rise and oxygen declines, inducing a quiet stillness that puts the colony into a state of semi-hibernation while broodless in early winter." ~Van Nerum, 1997 

“It is widely known that the lower the metabolic rate of wintering bees and the associated food consumption, as well as the temperature of the winter cluster, the deeper is the bees' dormancy status, the smaller is the exhaustion of the organism and accordingly the more successful is the overwintering.”

“It is widely known that the lower the metabolic rate of wintering bees and the associated food consumption, as well as the temperature of the winter cluster, the deeper is the bees' dormancy status, the smaller is the exhaustion of the organism and accordingly the more successful is the overwintering.” ~ Toomema, 2016

Should You Put an Upper Entrance on Your Beehive Over Winter?

If you are new to beekeeping and have never overwintered a hive, then I recommend not putting an upper entrance on your hive.

Instead, use a screened bottom board as a back-up source of ventilation (cover over most of the screen with corrugated plastic but leave 1" gap where the screen is not covered by the plastic on all sides - see video at top of the page where I show you how to do this) and make sure you visit your hive throughout the winter to clear out the lower entrance so bees can exit the hive. 

To prevent condensation from dripping onto your cluster, you should also put insulation directly under your lid and have something to collect the condensation that might drip off of the lid. This can be candy, a box full of dry white sugar, a moisture board, or quilt box. Just make sure there is a way for your bees to access the emergency feed above your frames of honey and that this moisture-absorbing substance gets replaced when it is oversaturated.

If you have overwintered a beehive before, then you can use information from previous years to figure out what to do.

If you found that your bees were eating through their food stores quickly and you had to feed them more than you should for your area, then take off the upper entrance.

If you have trouble with excessive moisture in the hive in the form of moldy comb or your quilt box is soaked with moisture every week or every other week, then try a winter with an upper entrance to see if this allows the excess humidity to leave the hive.

If you lose bees to the cold weather, then definitely take the upper entrance off the beehive and have a mostly blocked screened bottom board in instead.

You you have difficulty getting to your bees so you can clear out the lower entrance, then put an upper entrance on your hive with even more emergency feed than needed and wrap your hive on every side except the south facing side so the sun can heat up the hive.

Very Important

There is no right or wrong answer to this question. What works best for someone in the northern states won't be what's best for those in the coastal states where the temperatures go up and down all winter. 

Talk to other local beekeepers about whether they use an upper entrance. 

Keep tweeking your winter preparation every year. You don't have to do it perfectly your first year. Learn from your mistakes. Inspect your hives come spring to see how they are doing and adjust what you do and how you do it next year.

Some Musts When Overing Your Bees

There are few things all beekeepers can agree on, but there's a few when it comes to preparing your beehive for cold weather.

Ventilation and a Way Out

All bees need oxygen and a way to leave the hive. Your bees need a way for outside air to get in and a way for them to leave the hive on warm days. You should also have a back up source of ventilation either from an upper entrance or a screened bottom that is partially blocked with a piece of plastic.

Cold

There are a few things you can do to help retain the heat your bees produce. It is always a good idea to put insulation under the lid of the beehive. If temperatures in your area go below freezing, then you can also wrap your hive. Howevver, this will prevent the hive from warming up on those sunny days, so either don't wrap the south-facing side or only put the wrap on when you're getting really bad weather for a few days in a row.

Moisture

You don't want the hive to be dry. It is very important for your bees to access the condensation in the hive. You just want to make sure that the condensation on the lid is getting abosrbed by something above the cluster so it doens't drip onto the cluster. This can be a moisture board under the lid, candy or dry white sugar or a quilt box. If you are using candy or sugar to absorb moisture, you will have to continue to check on it and replensih it when needed. The bees will eat it. If there's not much left, it won't be able to protect the bees!

Food!

Of course every beehive will need adequate food or else they will starve. This is not just in the form of honey, but also emergency feed up above the boxes. Your emergency feed can be dry white sugar or a candy you make. You should also check on the feed every week to make sure there's enough food. You can just tilt the hive slightly to get a feel for the weight (when full) and use this to continue to gauge how much food is left. On warm, sunny days, you can peak under the lid to see how much food is in there. Do not bother the brood or expose the bees to the cold air.

Change the Hive Configuartion

The hive should be consolidated so that the space isn't too big for the bees to warm up. For the average colony, this would be two deep boxes. For small colonies, this would be two deep nuc boxes. Take off the queen excluder, put on a mouse guard over the lower entrance and if you have a slatted rack, put that under the lower most box of the hive. The cluster of bees should be in the center of the lowest box and honey frames are put on either side of the cluster as well as fill up the box above it. How much honey your bees will need depends on your winters. Please talk with other local beekeepers to see how much food they leave their bees. Many association have online meetings and facebook groups if you can't make an in-person meeting. On average, most states in the U.S. that get freezing temperatures will leave their bees 35 pounds of honey. In the colder states, beekeepers will leave 50+ pounds of honey.