Is Roof Membrane Act As An Air And Vapor Barrier

Most roof membranes are class i vapor retarders.

Is roof membrane act as an air and vapor barrier.

There is often confusion between air barriers and vapors retarders. When to use a vapor retarder so the question arises. The lower the perm rating the less diffusion occurs through a material. When is it prudent to use a vapor retarder.

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. There are 3 classes of vapor retarder materials as shown in the figure. Condensation within a roof or wall system can be very damaging to a building s structure attracting mold if it doesn t dry out in a reasonable time and. A vapor retarder can be as simple as a plastic sheet but is not considered an effective air barrier.

A roof design that includes an adhered roof membrane with multiple layers of insulation with board joints offset and staggered over a vapor retarder air barrier helps lower the risk that air and the moisture it carries will infiltrate the roof system. Vapor retarders do just as they are named they reduce vapor diffusion but not all vapor retarders are equal. It should be noted that all vapor retarders are air barriers but not all air barriers are vapor retarders. In so much that the roof membrane can often serve as an air barrier it does nothing to prevent this interior air transport.

It s all about controlling condensation and moisture management within the building envelope. That reduction of air and moisture infiltration can then help improve roof longevity. Vapor retarders why are they needed. So while air barriers are part of the building envelope vapor retarders are used within the envelope.