tar paper weight does not matter. it is the COLOR that makes the difference. You can wrap a hive with 30 inches of insulation, it will still be below freezing inside. The cluster is where it is warm, and the bees work from the outside in when they get cold and continue to rotate.. The tar paper is for solar gain on those days that they would not normally be able to MOVE the cluster...
Bees will starve to death two inches from the life saving HONEY if they cannot move to get to it. The tar paper gives them a few extra days each winter to make that move in comparison to a hive that has no wrap.
On the days that the cold is marginal, when they would not have been able to move, the tar paper gives them that ability because of the added warmth absorbed by the black wrap. If you wrap with anything that is not black, then you may as well not wrap them with anything because it will not make that difference in solar gain.
Obviously (I hope) there will be exceptions due to humidity and the length of COLD your bees have to endure. I have spent a lot of time and energy testing the differences, and reading the results of others who have also tested the differences. There is a huge difference in location. Obviously north vs south would be different, but I have also found a compelling percentage of differences in areas that have severe COLD and WIND in comparison to mountain valleys that have severe COLD with NO wind...
I have come to believe that those places with little winds to worry about are better off than places like here in the midwest plains that often have NASTY winds.
In reading that silver insulated bubble wrap was as effective as ANY tar paper I was a little startled, until i found that the people doing this test lived in a forested valley region... They were adamant about their findings and when i asked what the average wind chill was I got a nasty reply... I don't need to deal with people like that, but it goes to show that some people ignore the fact that they live in ONE place in this country... They are giving advice and recommendations based solely on their own location.
Black tar paper works well where I live. I use the small upper entrance. insulate the top of the hive with 2" pink foam and put sugar cakes on the top bars for insurance. I have realized about a fifteen percent increase in survive ability using the tar paper, and about a 5% decrease using the plastic bubble wrap... I can only assume that it is because the plastic traps moisture??
As I said.. it is a regional thing... Talk to folks near where you live that keep bees. Take notes, compare the notes.. throw out the oddballs and pay attention to the methods used by the majority where you live.
Scott
