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

Air Movement and the Building Enclosure

The stack effect, in layman's terms, refers to the fact that warm air rises. In a tall narrow column, such as a skyscraper, this effect can be very pronounced. When warm air rises, it creates a higher pressure in the upper interior portion of a building. That increase in pressure also means the warm, moist air will escape through any pathways that are available. It will escape ‘into’ the roof or any air passage that goes to the exterior.

In addition to the increased air pressure due to temperature, there are several other factors that can lead to air being “driven” into roof assemblies:

  • When doors, whether for people or for vehicles, are opened on the ground floor, outside air can enter to replace interior air that is being thermally driven to rise.
  • Wind creates a negative pressure above a roof system and at higher velocities can lift a single-ply membrane between the rows of fasteners in the seams. This billowing membrane can pull interior air into the roof system regardless of temperature or moisture levels.
  • Air conditioning and heating equipment force air through ductwork and into the interior of a building. By forcing conditioned air into a space, the space can become somewhat pressurized. Not to a great extent, but enough to create an imbalance between the interior and the exterior, forcing interior air into the roof assembly.

These three effects are illustrated below:

Building Science Digests, BSD-014: Air Flow Control in Buildings, John Straube, October 15, 2007

Figure 3: Processes that create air-flow across the building envelope.

Roof to Wall Termination of Air Barriers

As described, to be designated as an air barrier, the roof material must be tied into the corresponding material in the wall. One example of how that can be achieved is shown here:

Another example, in a situation without a parapet wall is shown here:

In each case, regardless of the situation, the designer needs to ensure that the air barrier is continuous around or within the building enclosure. All the materials designated to be part of the air barrier system must be securely connected and sealed to each other.

Air Barriers and Roof Penetrations

An air barrier’s effectiveness can be greatly reduced by openings and penetrations, even small ones. These openings can be caused by poor design, poor workmanship, damage by other trades, improper sealing and flashing, mechanical forces, aging and other forms of degradation.

The National Research Council Canada collected research data that illustrated how even small openings can affect overall air leakage performance. For example, only about 1/3 of a quart of water will diffuse through a continuous 4 feet by 8 feet sheet of gypsum during a one-month period even though gypsum board has a very high permeance.

However, if there is a 1-square-inch hole in this same sheet of gypsum, about 30 quarts of water can pass through the opening as a result of air leakage. This relationship is illustrated in the figure below. The example illustrates that air leakage can cause more moisture-related problems than vapor diffusion.

As buildings become tighter, small air leaks can cause significant issues during winter months for buildings in the north. In such a case, a small leak can lead to significant condensation within the enclosure, leading to long term damage. An example of air leakage at a window to wall connection is indicated in the following infrared picture.

Image courtesy of FLIR with GAF modifications

Can roof membranes always be part of an air barrier system?

Roof membranes that are automatically considered by Code to be suitable for use in an air barrier system are:

  • Built-up roofing membrane.
  • Modified bituminous roof membrane.
  • Adhered single-ply roof membrane.

Single-ply membranes that are mechanically attached are also considered to be included so long as the manufacturer can provide a data certificate confirming that the material has an air permeability of no greater than 0.004 cfm/ft2 (0.02 L/s · m2) under a pressure differential of 0.3 inches water gauge (75 Pa) when tested in accordance with ASTM E 2178. In practice, most manufacturers have this readily available on request. But note that the IECC states an important caveat, that materials shall be deemed to comply provided joints are sealed and materials are installed as air barriers in accordance with the manufacturer’s instructions.

So, the answer is that, yes, properly installed roof membranes can be used as part of an air barrier system. To return to the concept of the complete roof, membranes can be considered as both the water and air control layer in a roof assembly.

Air Barrier Code Compliance

The roof membranes previously discussed can be part of an air barrier system and their use will be considered code compliant so long as they are correctly installed and tied into the wall air barrier layer. However, two additional means of achieving compliance are acceptable.

  • Assemblies of materials and components (sealants, tapes, etc.) that have an average air leakage not to exceed 0.04 cfm/ft2 under a pressure differential of 0.3 in H2O (1.57 psf) when tested in accordance with ASTM E2357, ASTM E1677, ASTM E1680, or ASTM E283; The following assemblies meet these requirements:
    • Concrete masonry walls that are
      1. Fully grouted, or
      2. Painted to fill the pores.
  • Whole building testing—air leakage rate of completed building can be tested and confirmed to be ≤ 0.40 cfm/ft2 at a pressure differential of 0.3 inches water per ASTM E779, ASTM E3158, or equivalent method approved by a code official.


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Originally published in September 2021