Vapor Barrier

Below the insulation, above the ceiling. It keeps the insulation dry. Don't know how obligatory, but if you're insulating, the vapor barrier will keep moisture from condensing in the insulation. Wet insulation is a good conductor of heat.

Thanks Roger that's what I thought.

I would have to say "that depends".
Insulation manufacturers follow the old timeworn rule of the vapor barrier facing toward the "warm in winter" side. That means facing into the room if insulating walls.
This rule is fine for most of the country. If you live in the Gulf south though it can be a problem.
The theory behind placing the vapor barrier towards the warm in winter side is that the warm humid air of the heated space will condense when meeting the cool dry air (outside) and moisture will form. By placing the barrier on the inside the condensation will form on the inside and not get the insulation wet (on the other side of the barrier) thus reducing it's effectiveness. Eventually it will evaporate.

There has been some controversy in the past regarding applying that reasoning to the Gulf south.
The problem is that if you live in a very hot and humid climate where heating in the winter is not much of an issue but air conditioning is needed for 9-10 months out of the year the theory fails.
The warm humid air is outside not inside and the cool dry air is inside not outside..thus things are reversed.
The manufacturers do not address this issue perhaps worrying about potential liability but give the same advice no matter where you live.
When remodelling and expanding my house I reversed the insulation from what the manufacturers recommend.
I will report back if things go badly

Interesting Grieg, but still, air conditioning puts the humidity much lower than ambient.

Yes, do let us know how it works.

Yes the humidity inside is low however the humidity outside (in Baton Rouge) is extremely high. This cause the condensation to form on the warm humid side of the vapor barrier. You do not want to have insulation exposed on the side it forms on thus if the barrier is facing out rather than in the condensation forms on the "other" side from the insulation..somewhat confusing is it not ?

Well in my particular situation the house is in cloud forest and the outside temp is almost always sixty to sixty five degrees with humidity in the ninety to a hundred percent range. The ceiling consists of a quarter inch thick asbestos hardboard called "eternit" here and there's an attic above about six feet high at the peak. There is a fireplace in the living room and the idea is to try to retain as much heat from that as possible. I did find and install two and a half inch thick un faced fiberglass and it seems to help a little. By the way with the fireplace and an oil filled space heater I'm able to keep the inside humidity down to seventy five to eighty percent.