I have blown in fiberglass insulation. No vapor barrer. I was
concerned so I did some research, there are those for and against
vapor barrers when the attic is a free air space like I have. since
the attic is a free air space, the idea is that the vapor will migrate
slowly upward and out the house. If I had a vapor barrer some say it
will cause the ceiling rock to collect water and sag over time.

So, this is what I actually did. I bought cans of BIN and painted my
ceilings. Now the rock can't absorb mosture, and I have a vapor
barrer.



I didn't realize that BIN, which is basically shellac, had vapor barrier
properties. Benjamin Moore makes a wallboard primer that is specfically
made to be a vapor barrier.


http://www.benjaminmoore.com/tds/1033/TDS260B.pdf