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 was no longer using lang hives, just warré hives with quilt boxes placed on top of propolised top-bar cloths. I checked the bottom of a few quilt boxes serveral times over the course of the winter. Though I didn't find any moisure 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.
2014/2015 Another brutaly cold winter. One colony had been blown over during an intense wind storm in early November. In the frantic process of putting the hive back together again, the proplised top-bar cloth was removed. The hive overwintered with only the quilt directly on the top bars. In the early spring the upper 3/4 of saw dust and straw was a little damp as was the roof. This was replaced with fresh dry material.
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. Information on design variation and precursors to the warré quilt can be found here.