Michael from Edmonton added a second story to his house and has a question about the vapour barrier in one particular place. But first of all you have to see the video he sent us showing how he built and installed the roof! It was built, and even shingled, on the ground and then he used a crane to lift it into place. In fact this construction technique is becoming more and more common as contractors who are used to commercial construction start building single family houses. They are used to using cranes -- and roofers can't fall off a roof that is sitting on the ground.
Back to the vapour barrier question. Michael added a second story and there was already insulation between the first story ceiling and the attic floor. He knew that he didn't need a vapour barrier between two warm rooms -- the existing first story and the new second story -- but he was worried about the area around the rim of the house between the ceiling and the floor. He is right -- theoretically there should be a vapour barrier to keep the moisture from moving out to the cold side of the wall. But the reality is that stopping air leaks is 100 times more important as far as moisture movement goes than vapour barriers. So if he goes a bit crazy sealing all the air leaks into this section from both the top and the bottom, the paint on the ceiling and the finish on the floor will be enough of a vapour barrier to do the job. So he really doesn't need to tear out either the ceiling or the floor just to install a vapour barrier.
In many renovations, there is enough paint already on the old wall (two coats of oil paint is enough) to qualify as a good vapour barrier. But you must take sealing up all the air leaks very seriously.