Vapor Barriers and Air Barriers

Moisture is transported into building cavities by two methods; the first and most significant method is via air movement, the second is by vapor diffusion. A vapor barrier does not control moisture movement caused by air movement, nor does an air barrier stop vapor diffusion into the building.
Due to convection, warmer air always moves towards colder air. Warm air holds more water vapor than cold air. As warm air moves through the building envelope towards the cooler outside air, the air cools, reducing its ability to hold moisture. This causes the water vapor to find a dew point and to condense into water. Once condensation has occurred, the insulation gets wet, thus reducing its R value. This moisture then compounds the problem by increasing the convection rate between the interior and exterior wall surfaces.
Air leakage through a building assembly carries with it moisture. The higher the volume of air leakage, the more moisture that is transported into the building cavity where it can form condensation. Fibrous insulations such as cellulose and cotton batts allow this moisture to pass through and reach the cold surface where condensation can occur. These insulations are more susceptible to moisture gain via vapor diffusion as they tend to wick moisture from the air.
Resistance to moisture transfer is measured by Perms. The lower the perm rating of the material, the better it is at stopping moisture flow. Therefore, using an insulation that is both an air barrier and a vapor retarder will help to solve many of the problems associated with water vapor movement.
The amount of water vapor in the air is measured by its relative humidity, or RH. At 100% RH, water vapor condenses into water. The temperature at which water vapor condenses is its dew point. The dew point of the air depends on its temperature and relative humidity. The exact dew point is most easily determined by using a psychometric chart or, more simply, by applying the following formula:

RH = Amount of water vapor in air at a given temperature/
Maximum amount of water vapor that air can hold at that temperature

Standard vapor barriers generally make poor air barriers. With conventional fibrous insulations, the use of both a vapor barrier and an air barrier is needed to control air movement and vapor diffusion. Preventing condensation involves reducing the RH of air, stopping the movement of water vapor, and increasing the temperature of the interior insulation surface in order to keep the surface temperature above the dew point.