Is Roof Membrane Act As An Air And Vapor Barrier

When discussing air barriers roofing membranes and the building envelope another important topic comes to mind.

Is roof membrane act as an air and vapor barrier.

There is often confusion between air barriers and vapors retarders. Vapor retarders do just as they are named they reduce vapor diffusion but not all vapor retarders are equal. Vapor retarders why are they needed. In so much that the roof membrane can often serve as an air barrier it does nothing to prevent this interior air transport.

When to use a vapor retarder so the question arises. That reduction of air and moisture infiltration can then help improve roof longevity. So while air barriers are part of the building envelope vapor retarders are used within the envelope. A vapor retarder can be as simple as a plastic sheet but is not considered an effective air barrier.

The lower the perm rating the less diffusion occurs through a material. Most roof membranes are class i vapor retarders. It should be noted that all vapor retarders are air barriers but not all air barriers are vapor retarders. There are 3 classes of vapor retarder materials as shown in the figure.

Newer self adhered membrane products act both as air and vapor barriers and can act as temporary roofs to dry in the building until the remainder of the roof assembly is installed. It s all about controlling condensation and moisture management within the building envelope. Vapor retarders and air barriers. When is it prudent to use a vapor retarder.