In the spring we often find that our hives have had to cope with excess moisture building up over the winter months. When it's bad, winter moisture problems will lead to mouldy comb and cold water dripping onto and chilling the bees.
The typical cause?
The bees themselves will produce a great quantity of moisture just by breathing. Warm moist air rises, spreads along the top of the hive, comes into contact with the colder side walls and inner cover, condensation forms and drips back down into the hive.
The simple and oft suggested remedy to this problem is to provide some form of upper ventilation. Some swear by the top entrance, but so far that option still seems a little drastic to me as so much upward moving heat is lost at the same time as the moisture.
So how do we reduce the moisture without losing the heat?
The short answer is insulate and absorb. You want to give the moisture somewhere to go and try to minimize the temperature difference between the hive walls and the cluster. It's been suggested this problem might be less likely to occur in a wild hive where the tree wall is thicker, there is much more than a thin layer of wood above the hive and that a tree might better absorb moisture than a plywood inner cover. The moisture absorbing qualities of a tree cavity hive are largely speculations as far as I know, and likely not true if we accept the research out there that suggests bees propolise the inner surfaces of trees.
So how can we attempt to emulate those conditions with modern day man-made hives?
Aside from insulating our hives, we have started adding ventilation boxes on top of our hives. Essentially a short box filled with straw, wood shavings or other absorbent material. The side has screened holes(keep the mice out) and burlap is stapled to the bottom.
In the photos you can see we placed it above an inner cover with a vent hole, but it could be placed directly above a brood box.
We used these during the 2011/2012 and 2012/2013 winters and had dry hives in the spring. In a few cases there was a bit of dampness in the ventilation box.
2013/2014 was a particularly cold winter. I checked the bottom of a few quilt boxes placed directly on the top bars of my hives with no inner cover serveral times over the course of the winter. Though I didn't find any moisute build up, it was interesting to discover that on warmer sunny days, the temprature at the bottom of the quilt box would be a few degrees below ambient. At this point I'm starting to suspect that in colder situations, the bees are not producing losing moisture, and the air is not warm enough to carry much moisture to the top of the hive from a cluster further down in the boxes.
More on vent boxes or quilts
Those familiar with the warré hive will without doubt find this very similar to the quilt which is traditionally used year round to help stabilize the climate within the hive.