Peter Prodger says " I believe the bees need to have a balance between a thermally good enclosure and adequate ventilation. Excessive ventilation will nullify thermal retention properties. The need for ventilation varies throughout the season. In eastern Ontario, with temperature ranges from -30c to +30c, ventilation and insulation is required mid winter to avoid condensation freezing and then ultimately dripping cold water into the cluster."

A major point of Mitchell's paper is that ventilation is not required (actually detrimental) if the enclosure volume (tall and thin like a tree cavity), bee mass and hive thermal conductance properties are correct (high thermal mass to conductance ratio in his parlance). He accepts there will be some condensation but small enough not to be a problem down to -30/-40 degC and occurring down near the entrance away from stores and bees. By using open mesh floors, you negate to some (significant) degree the effect of the insulation unless there is no air flow...which is the point of ventilating, of course. 

He also states that breaking and forming the cluster is costly on the bees and in the "wild" they would cluster only in extremes not routinely even at those very cold winter temperatures. Did not get a chance to query the cluster breaking/reforming cost with him. Does anyone have info on this?

There is certainly a feeling among my contacts here in the UK (Hertfordshire/Bedfordshire) that poly nucs and hives increase colony winter survival rates and summer productivity but then our climate is not nearly as extreme as yours.

John

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