I keep 24 hives in Washington State, on the wetside of the mountains (as we say here), where it rains a LOT. I am interested in the conversation here regarding ventilation/humidity, as its something I've spent some time with in controlling in my hives (day job is an architect so its sort of what we do). For the past 5 years or so throughout the winter months, I install a quilt box of cedar shavings on top (which is hinged and weatherstripped for access to feeding, and therefore does not break a propolised seal when accessing), without any vented entrance at top. I reduce the front opening, and also (though not always) install mite boards with a rear seal (though these are loose enough for some air flow).
Since having done so, I have found both black mold on the hive body interiors and the mold that occurs in frames to have gone from heavy, to nil, and of course, there is no visible moisture condensation on any surfaces. Hives appear to be bone dry. I would add that other local beekeepers accomplish the same result with insulation and an opening at the top, so by no means to I think the quilt box is essential, but it certainly changed how I do things, and I believe, has significantly increased survivability.
However, although it makes me feel like I've bettered the hive health in doing so, I really have no idea. The primary and unarguable (I assume) improvement is that the colony is no longer "rained" on- though that could probably also be done in a redesign of the top cover, or simply (as many people do) insulating it. Quite possibly that could be the only actual benefit. Possibly in a tree cavity is accomplished purely by the shape of the cavity. Whether or not their health is improved by a lack of condensation otherwise, or a lowered winter humidity (which I don't know if this even happens, see below) , or a higher interior hive temperature, I don't know.
As a possible point of interest, Ed Clark, in his 1918 Constructive Beekeeping (https://archive.org/details/cu31924003100306) suggested that condensation in a hive was actually a desirable trait, and designed a hive to encourage it. He felt that that the propolised inner surfaces of the hives were done intentionally by bees as a way of ensuring that water was condensed as an important source of water (rather than being soaked up by the wood fibers). Maybe anecdotal, but an interesting possibility.
Of the same era, measuring temperature throughout the hive was also the subject of the interesting 1918 study The Temperature of the Bee Colony (https://archive.org/details/temperatureofbee96gate). Its primary finding was the opening at the bottom had no affect on the upper hive temperature, and that no matter to what degree it was closed off, remained at ambient temperature (an interesting study with other observations as well).
In the past two years I have installed very basic RF temp/RH sensors in 7 of my hives near my house, and have manually recorded that data. This year's graphs are at http://mattwaddington.com/hivetemp.htm. These also include a control hive (same build but no bees).
What I have noticed has been noticed I am sure by anyone that has tried this, and not that surprising, that the inner hive top temperature (not the cluster but adjacent to it) tracks the ambient temperature by 5-10 degrees F, that all hives have similar interior air temperatures, but the relative humidity fluctuates wildly from hive to hive, and day to day, without apparent rhyme or reason (or at least, I haven't seen a pattern yet). Otherwise, nothing too ground breaking, and I can't say it makes me rethink what I am doing yet.
I'd be interested to hear of and read any significant studies on this topic, should anyone know of any.
Matthew Waddington/Duvall, WA
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