The dreaded double vapour barrier syndrome has been a topic of building construction conversation for decades. The theory is; when two vapour barriers are part of the same wall assembly (one on the outside of the wall and one on the inside of the wall), moisture may be trapped between the inside and outside vapour barriers; thus reducing the wall's ability to dry. We will attempt to make this explanation simple but first it is important to understand some basic physics. We refer to this as Thermodynamics 101.
Migration - Warm air migrates to cold air and high pressure areas move to lower pressure areas. In the winter, the inside air of a building is warmer and its pressure is higher than outdoor air.
Diffusion - Diffusion is moisture moving through (penetrating) building materials. Vapour barriers slow moisture transfer/diffusion down but never really stop it.
Air Pressure - Warmer higher pressure air moves to colder lower pressure areas by the easiest path possible. This happens through breaches (tiny holes and crevices) in building envelopes.
Condensation - As air warms, its ability to hold water vapour increases; as it cools it decreases. Relative humidity refers to the amount of moisture that air can hold at a specific temperature. When warm moist air meets cold air or objects within the wall cavity, it condenses and turns to water, frost or ice. This is referred to as the "dew point".
What happens: in the heating season the inside of buildings are warmer than the ambient temperature outdoors. Consequently, through crevices and tiny holes, the warmer higher pressure air migrates towards the colder lower pressure air. When the warm moist air meets the cold air (or objects) within the wall cavity, it condenses and turns into water, frost or ice. The results are wet insulation, framing and building materials. Long term consequences are mildew, mold, wood-rot and potentially serious health problems.
Try this: Blow into an empty Coke bottle. Because there are no holes other than the one you ary trying to blow in - the warm pressurized air from your lungs is unable to replace the cooler stable air in the Coke bottle. In other words, because there is no leakage (holes), the air from your lungs cannot replace the air in the bottle.
Conclusion: High performance impermeable insulation (minimum 2" thickness) installed on the exterior of a wall assembly stops air leakage and keeps wall cavities above the dew point temperature. If exterior continuous insulation is less than 2" thick, the potential for significant condensation exists.