We are told, with very good reason, that insulation should have a vapour barrier on the warm side. But which is the warm side of the wall of a house? It changes from summer to winter, even from day to night. If it is 20F below zero outside, the inside of an occupied house is certainly the warm side. During the summer months, when the sun is shining, the warm side is the outside.
Sometimes the novice will try to put vapour barriers on both sides of the insulation however vapour barriers on both sides of fiber insulation generally prove to be disastrous. It seems the vapour barriers will stop most of the moisture but not all. Small amounts of moisture will move into the fiber insulation between the two vapour barriers and be trapped. It will accumulate as the temperature swings back and forth. This accumulation can become a huge problem.