We don't want to bore you too much technical jargon but it is important to understand that there is a difference between air barriers and vapor barriers.
A vapor barrier is designed to restrict the flow of water vapor through a material. The air barrier, on the other hand, restricts the flow of air through a material.
Since they are intended to control the rate of diffusion into a building assembly, water vapor material will control the rate of moisture flow where they are placed. They don't have to overlap or be continuous.
Then there are water resistive barriers, which are systems using materials primarily designed to keep liquid water from entering the building enclosure. They are designed not be be a vapor barrier and are designed to be installed on the cold side of the insulation.
To make it even more confusing, combined air barriers, vapor barriers and water resistive barriers can be provided in a single material.
It's important to know when you are dealing with the building envelope which material you choose to perform each different function.
But, that is what we are for, so when you are looking to design your air barrier system for your building, give us a call and we'll worry about all of that for you.