A vapor barrier is a vital component of exterior walls. It controls vapor transmission and helps prevent condensation from forming inside walls and ceilings. Warm air inside your home has a higher pressure than the cold air outside and contains moisture.
To equalize the difference in pressure, the warm air tries to escape through your walls and ceiling. The vapor barrier is what prevents that from happening. Without it, the escaping warm and humid air would pass through the insulation and contact the cold outer wall surface, where it could condense and collect. The resulting buildup of water inside the wall has the potential to damage wall framing and promote the development of mole.