The local Wharfedale Beekeepers Association has just published the following interesting article on the Bee Cosy in its April newsletter - Combings
At last year's honey show i collected my new "Bee Cosy". The day after I placed it on my hive, then let nature do its thing. During the winter which has been very wet I looked in my apiary and the two hives without a "Cosy" were wet, (to state the obvious) but the hive with the "Cosy" on looked dry, the water seemed to run off like water off the proverbial duck's back.
Then the snow came, and when I went to check my apiary the hives were covered in snow but the hive with the "Bee Cosy" on had around four inches of snow on top and the the two hives without a "Cosy" on only had around an inch of snow covering them, and something else I noticed, on one part of the roof the snow had melted more and I wondered if that was where the cluster of bees were and the heat they were generating had melted the snow more in that spot, no such melting spot on the hive with the "Cosy" on and so much more snow on top, the only other question I had to ask myself, was there any heat being generated at all by the bees to melt the snow, or had the harsh winter killed my bees? The snow was still lying around there was no way of knowing, I just had to wait for a warmer day. A week or so later I saw bees flying around all three hives.
On the 12th February I had a quick peek under the roofs of all three hives to check the fondant levels. The two hives without the "Cosy" on had definitely been using the fondant, and the hive with the Cosy on had not touched it. Also I noticed that they seemed more active, not as sluggish as the other two hives. the day after, more snow fell. This time I took some pictures, you can see the difference in the depth of snow and the spot where the snow has melted more. Hope this has been some help to you.