Insulation, Radiant and Vapor Barriers - Vapor barrier ceiling?

We are renovating a single room cottage. We put in new windows, door, insulated the walls with fibreglass, vapor barrier, et.
We are about to do the ceiling. The cottage has electric heat. I understand that a vapor barrier goes on the inside warm side but how does the moisture escape out if the walls are airtight and then the ceiling has vapor barrier? Why doesn't the warm heat rise and the moisture get trapped by the vapor barrier and destroy the ceiling drywall? I understand that the space between the ceiling and roof needs to be vented but how does the moisture get through the vapor barrier to that space and then vented?

I'm not fully understanding some element to all this. There is currently no vapor barrier in the old ceiling and why is this a problem (or not).

thanks.

Is there an attic above the ceiling? The idea is to keep warm, moist air out of attic by sealing all gaps where air can enter area above ceiling. The vapor retarder goes down and then the required R-value insulation for your area. If using fiberglass batts, the first layer should have the attached vapor retarder facing down toward ceiling. Additional layers of insulation should not have the attached vapor retarder.

Attic area should have adequate ventilation so that any air that enters the attic can pass out of the attic. Warm, moist air trapped in attic can cause moisture problems, mold, and mildew.

If temperature and humidity are maintained at occupancy levels year round, then humidity should not be an issue. Temperature around 70 degrees and humidity 35-55% (measure with hygrometer which is sold where thermometers are sold).

Vapor retarders do not trap humidity. They tend to minimize the amount of humidity that will pass into areas where you do not want it to go--attics and wall cavities. When warm, moist air makes contact with a cold surface, it condenses and causes moisture problems, mold, and mildew. With adequate insulation and vapor retarder and gaps sealed in attic area, you conserve energy by keeping the heat in the area below where it belongs

http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11810

If there is something you want more information on, please feel free to ask.

Thanks for the answer and the link. I understand better now. I wasn't aware of the actual creation of condensation due to a juxtaposition of a hot and cold temperature surface. It's not just moisture in the air but the release of that moisture from the air that's a variable

Even if we use a paper back insulation up in the rafters. Paper to the room. We still cover the whole ceiling with a 4 mil poly then the drywall.