Howdy, there was a lot of discussion here on that very thing a while back and the consensus was.....it actually inhibits reflux!!!!

My opinion is don't do it.

"I agree with heeler & diesel. I also tried insulating the column and found that I had trouble with stabilizing and controlling my collection rate. So I only insulate the boiler since it helps reduce the time it takes to reach a boil. I just use a couple of old cotton towels & some string (to keep the towels in place).

That said, I have read posts about some insulation helping when outside ... especially in the presence of a breeze or wind. So give it a try and see if it works for you. If you don't like the results, remove the insulation, toss everything back in the boiler and run it again."