Damp

Everyone accepts that damp kills bees, but very few deal with the cause. Wooden hives get damp in winter and this must be one reason that beekeepers using these hives suffer 10% - 30% winter losses, (UK and Canada West Coast respectively). Usually, there is no obvious reason for these winter losses. Whereas beekeepers using poly hives, in the same environment, barely ever lose a colony. The contrast is stark.

The science of damp for the non-scientist

You will be familiar with putting salt in a hot saucepan of water. The salt disappears — it dissolves in the water. If you add more salt, there comes a time when no more salt dissolves. But if you increase the water temperature, it will. Similarly, in the world of damp, the water represents the air and the salt represents the water in the air.

The amount of water air can hold depends on the temperature and pressure. This makes sense if you think about weather forecasts. The higher the temperature of the air, the more water it can hold. So when air cools to a certain temperature, the air can no longer hold on to it. The water vapour turns into liquid. The temperature at which vapour starts to turns to liquid is called the dew point; at that point the air is saturated (it holds as much as it can) its relative humidity is 100%. The release of some water (condensation) is known as precipitation (e.g. rain). When air has not reached its dew point, it is called unsaturated.

Humidity is a measure of how saturated the air is. It expresses how much vapour (moisture) is in the air at a certain temperature; Relative humidity is an indication of how unsaturated the air is compared to when it is saturated. It is the amount of moisture the air holds compared with the maximum it can hold at the same temperature and pressure. 100% is the tops.

Here is an example for water: a temperature of 16 °C with a relative humidity of 60% results in a dew point of 8 °C.

Evaporation has a cooling effect: water molecules are loosely bonded together, breaking the bonds requires energy. Heat is energy. So we cool when we sweat.

Condensation releases heat. This dynamic is not large enough to toast bread, but it influences the weather.

Humid air is lighter than less humid air at the same pressure and temperature (because nitrogen in the air is “heavier” than water). If it went down rather than up, we wouldn't have any rain.

The air temperature that can hold the maximal amount of water vapour is 35 °C (95 °F).

Humidity and temperature levels usually change in tandem. However, nectar processing areas in the hive need to be at a lower humidity (20%) than brood rearing (50-60%).

The temperature in the brood nest / winter cluster varies between 18 and 35 °C. The temperature of the mantle is 9 °C.

Humidity greater than 79% renders varroa virtually unable to breed.

High ambient humidity could, at least theoretically, could result in condensation on the edges of the cluster. The only way the cluster can prevent this is to raise its temperature, or to fan the moisture away to make it evaporate.

Every beekeeper knows that damp not cold kills bees and that hive ventilation helps with damp. If air outside the hive has a lower relative humidity or higher temperature than that inside, terrific, job done! What about if the outside air is cool and almost saturated? I’m not sure.

The inputs affecting the milieux of the hive can be computed using a long list of variables and a long list of mathematical equations, which are quite beyond my understanding.

Dealing with damp

Not all humidity is unwelcome.

In the winter, the bees need water to dilute honey. Brood only survives when the humidity is greater than 55%.

Causes of condensation:

  • Wood is hygroscopic. The walls of a wooden hive equilibrates with whatever surrounds it. Whereas Polystyrene:

    • Conducts heat poorly, so the surfaces stay at ambient temperature.

    • Waterproof: so there is no water to evaporate and cause cooling.

  • Anything in the hive that causes cooling by conduction.

  • High humidity: i.e., the air outside is almost saturated. The slightest cooling and precipitation occurs.

  • Dead space, particularly at the top of the hive, that accumulates cool air.

  • Reduction in air circulation. This is a tricky one. Bees block holes they don’t want. They accept varroa mesh/screens floors, but propolise top vents.

Solutions

  • Maximise insulation — remove the inner cover in cool damp weather (e.g. all winter). In wooden hives, put 50 mm of insulation under the roof. It dramatically reduces conductive and convective heat losses.

  • Wrap wooden hives with a material that provides insulation and water protection.

  • Put mats above the crown board to mop up water - I don’t know about this.

  • Keep the hive walls dry.

  • Warm the inside of the hive.

  • Adequate airflow — leave the varroa board out and have the entrance fully open. Airflow under the hive would theoretically cause vortices if the frames did not baffle the current. When they are not clustered, bees can move around and adjust the flow. Feral bees in bee trees sometimes reduce the size of the entrance with propolis, but that never happens in hives.

    Top ventilation has a beneficial effect. Under normal circumstances, it is unnecessary in poly hives. Ventilation can be lifesaving, particularly if you use wooden hives. Ventilation flushes humid air out of the hive and so potentially causes cooling by increasing evaporation.

    • Wooden hives: place matchsticks under the crown board.

    • Poly hives: use a slow ventilating “blanket” under the roof, as below:

Quilt under the roof : useful to vent air. It consists of upholstery wadding with fly netting on its undersurface. To prevent the bees playing with the wadding, it is enclosed in a sleeve of garden cloth.

@ truewood.ca

This warming mat provides gentle heat when it is placed on the floor of the hive. The mat is usually used to warm, pet amphibians. It is excellent for stimulating spring build-up, used one month before the weather becomes favourable.

@ truewood.ca

These devices, when used with poly hives, are associated with an absence of winter losses in extremely humid, cool winters and spring climates, where temperatures barely reach 15 °C in May.

Their construction increases airflow, whilst one assumes, not causing a howling gale. Whatever, they are effective and reduce relative humidity in the hives from 90%, to 80% (West Canada).

H8