Low-Slope Roofing—Air Barriers and Vapor Retarders

Roof assemblies have more roles than simply keeping water out. Understanding control layers is key to successful design of modern roofs.
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Sponsored by GAF
By James R. Kirby, AIA, and Thomas Taylor, PhD

Flashing and Termination of Vapor Retarders

Vapor retarders should be treated like air barriers; they need to be continuously installed, sealed, and flashed around penetrations and terminated to wall systems such that interior air cannot migrate up into the roof assembly. Always consult the manufacturer’s recommendations for the specific vapor retarder being used. An example of how a vapor retarder is flashed to a pipe penetration is shown below:

Vapor Retarders and Structural Concrete Decks

A commonly used method to help address moisture issues in structural concrete decks is the use of a vapor retarder. Concrete roof decks that retain moisture continue to be a concern in the roofing industry. There have been changes in the materials and methods used in the construction of structural concrete decks over the past 30 years. Two significant changes to note are the use of lightweight aggregate in structural concrete and the use of non-removable forms. The use of lightweight aggregate that can hold more initial water than traditional “hard rock” aggregate, combined with the use of metal forms that are left in place, can increase the likelihood of the presence of moisture in the deck. And recent work indicates that even normal weight structural concrete poured over non-removable forms retains a significant amount of water within the concrete.

The use of a vapor retarder to restrain movement of moisture from the deck into the roofing system is a commonly used method. Allowing concrete to thoroughly dry is most appropriate; however, it is reasonably impractical. In addition, precipitation during construction can add more water to a concrete deck making drying more difficult to achieve. It is common to install a vapor retarder on the top surface of a concrete deck. The Midwest Roofing Contractors Association, MRCA, is advising that a vapor retarder of less than 0.01 perm is necessary over new concrete roof decks. From a practical standpoint, a vapor retarder of less than 0.01 perm is effectively a vapor barrier—almost no moisture passes through. As shown earlier, a Class I vapor retarder has a rating of 0.1 perm or less.

Why are Air Barriers and Vapor Retarders Sometimes Confused?

As discussed earlier, air barriers are required to prevent the loss of conditioned air from within a building. Using the concept of a complete roof assembly, it is important to have an air control layer that is as complete and unbroken throughout the building enclosure. The materials that are part of that control layer should stop air movement and all tie together so that leaks are minimized.

When penetrations and transitions are detailed vapor retarders also block air flow and it is this fact that can cause some confusion. However unlike for example, roof membranes, vapor retarders are frequently materials that can have some limited permeability. As discussed earlier, this can help minimize risks associated with trapped water.

Since vapor retarders block air movement, they can be used as the air control layer within a roof assembly.

The designation of the roof membrane as part of the air control layer (i.e. air barrier) requires that it be terminated correctly to the wall air barrier. As discussed earlier, this is important for the various trades to understand during construction. However, it could be that the vapor retarder be designated as the air control layer. It would therefore serve two functions; air and vapor control. Again, to be successful, it should be tied into the wall air barrier. Two approaches to controlling air flow were shown earlier.


When considering air barriers and/or vapor retarders, it can be beneficial to think in terms of control layers. It is important to determine which layers within the roof assembly are being designated as control layers for air and for moisture.

  • Air barriers are often code required. They are a system of interconnected materials that must be sealed together. The roof material that has been designated as the air barrier must be terminated and sealed to the air barrier layer within the wall assembly.
  • The simplest approach for the roof designer is to designate the roof membrane as the air barrier and carry it up and over a parapet wall or over the edge if a parapet wall is absent. Once overlapping the wall, it must be terminated and sealed to the wall air barrier material so that air and moisture leaks are prevented.
  • While more complex to design and install, it may be prudent to have the air control layer extend through the base of a parapet to the wall. At that point it must be terminated and sealed correctly to the wall air barrier. This approach minimizes air flow into the parapet and lowers the risk of condensation within the parapet.
  • Vapor retarders are used within roofing assemblies to control moisture migration. They also block air flow providing that they are tightly sealed around penetrations and the perimeter.
  • If the risk of condensation is deemed to be reasonably low, two layers of insulation with staggered joints might provide for sufficient vapor control against air intrusion.
  • At higher levels of risk, condensation mitigation from air intrusion could be achieved by adhering the upper layer(s) of insulation and the membrane.
  • For buildings with significant risk of condensation issues, a vapor retarder should be installed below the dew point. It is important to ensure that air intrusion is first minimized and any vapor retarder installed is self-sealing so that insulation and membrane fasteners, if used, don’t degrade the performance.
  • Because vapor retarders block air, they can be used as both the vapor and air control layer. However, such an approach should be used with care since it is dependent on careful design and installation.
  • To be successful, the designated air and vapor control layers must be understood by everyone involved in the installation and construction. Pre-construction meetings that review the control layers are essential.

Moving Forward

Roof system design is always the responsibility of the designer, but perhaps the designers in the roofing industry can find some takeaways from the wall industry. There is always more to learn and understand about the building science of our roof systems.


GAF GAF is North America’s largest and most advanced roofing and waterproofing manufacturer. With customer-driven innovation, the company has protected homes, businesses, families and communities for over 130 years. With plants strategically located across the country, more homes and businesses in the U.S. are protected by a GAF roof than any other product. GAF protects what matters most.


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Originally published in September 2021