G3 - Until last year I wrapped with bee cosies. The year prior I expanded to 19 hives and only had 10 bee cosies so 9 got wrapped with tar paper only.
This yard (above) was all cosies (7) except the 3 deep hive which was a cutout I did in August (it survived the winter using tarpaper)
For whatever reasons, I lost 8 of the 19 hives over winter (5 in this yard alone), and all of them had the cosies on. I mentioned in another thread that the tar paper seemed to allow the hives to warm slghtly on cold but sunny days where as the cosied hives were cold when I slid my hand between the cosy and the hive body. This winter I had a hive survive that only had a frame and a half of bees (with queen) that was hard over in frame # 10 position on the sunny side, taking advantage of the heat provided by the tarpaper in the sun.
This past winter I pitched the cosies and went with tarpaper only and had much better results. I cannot say for certain if that is what made the difference, but I have my suspicions. I do not want to slag the cosies, they may work well for some, I merely am offering a personal observation.
I know Perry that the tar paper should breathe somewhat. I don't know squat about the construction of cosies, but would their construction allow them to 'breathe'? It would be interesting to get some detail of what the inside of the hives that perished looked like. In a worst case situation CO2 and water vapor could be a lethal combination.