Understanding the Critical Elements of Air & Vapor Barriers

The science behind wall systems and the tangible benefits of incorporating this technology into building structures
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Sponsored by SOPREMA, Inc.
By Jack Garnett
This test is no longer available for credit

Effects of Air Leakage

An air barrier’s influence on energy costs is one benefit; its ability to influence air movement—which can affect the structure’s performance and longevity—is another.

As a general disclaimer, all components within a wall assembly can tolerate some level of moisture exposure. That said, issues occur when moisture levels are elevated for an extended period of time. When moisture collects within a wall assembly and is absorbed by wall components, eventually problems emerge.

Diagram showing how condensation occurs in a wall system.

Condensation forms within insulation layers when hot, humid air contacts a cool, dry surface.

When cool, dry air meets hot, humid air, condensation occurs. Condensation can simply be defined as the change in the state of water from vapor to liquid water, causing water to collect as droplets on a cold surface that humid air has contacted. During hot, humid weather, moisture present in the warm air collects in the batt insulation, which is not designed to absorb elevated levels of moisture. During cold, dry weather, the warm air inside a building can also exit the structure through the wall assembly, creating condensation in the exterior wall cavity that can lead to degradation on the exterior of the structure.

It is worth noting that while the images you have seen so far show air and vapor traveling straight through wall systems, in reality, this is not always the case. As illustrated here, air movement can cause indirect degradation to a structure when vapor drive issues emanate from one location, then travel within the wall and cause damage elsewhere. As a result it is sometimes easier to simply observe that a problem exists without knowing the exact origin of moisture intrusion.

Diagram showing indirect air movement through a wall system.

When structural degradation occurs, finding the source of the problem is not always as easy as looking on the opposite side of the wall.

Unimpeded vapor drive via air movement can compromise air quality within a structure. Warm, moist air from inside or outside a building may accumulate in vulnerable wall components, which can cause mold growth and compromise the quality of air. Typical growth occurs behind interior wall coverings, behind EIFS veneer or stucco systems, or within a wall assembly that isn’t properly managing water and/or vapor drive. Many times people do not realize that simply applying a non-permeable paint or wall covering can negatively affect the performance of the entire wall.

Structural Durability

When buildings are designed, they are likely done so to provide service lives of 50 years, 100 years and perhaps longer. Often, however, mistakes are made in design and/or construction that cause building structures to last no more than 10–15 years. Structures are sometimes built that will require a complete exterior renovation project in the first five years after completion because the walls were incorrectly designed or installed. It is imperative that designers, manufacturers, and installers work together to ensure that all the wall components—including air and vapor barriers—are designed and installed correctly if our buildings are to perform to the desired service life.

Diagram of the layers and barriers of a building.

It is critical that there be a physical transition from waterproofing to wall systems to the roof to protect a building.

Perhaps the most important thing to take away from this course is the idea that it is critical that there be a physical and permanent tie-in transition from the waterproofing to the wall system, and from the wall system to the roof membrane to prevent unwanted moisture intrusion so that the structure’s durability is not compromised. It is important to work with a manufacturer that offers roof membrane systems, wall barrier and foundation/under slab waterproofing and barrier products with full assemblies that have been tested to ensure transitions from one system to the next are dependable. Materials must be compatible and adhere tenaciously to one another for the duration of the expected service life. The more thought that has been put into these transitions from system to system, the better. Do not rely upon contractors or multiple system manufacturers to vet these transitions—as the designer, you should ensure that transition conditions are detailed correctly.

It is important to ensure that vertical transitions from wall to wall are designed and installed correctly, as well as horizontal transitions such as roof to wall systems. Besides protecting against vapor drive, air barrier membranes also serve the important job of keeping rain out of buildings during construction. This is why they are typically the first things installed on a building once the walls and roof are built—water molecules are too large to penetrate even permeable membranes. These barriers can also act as drainable planes for any water that happens to get past exterior cladding once it has been installed.

Before we move on to compare how different wall system assemblies perform in varying environments, let’s review what we know about non-permeable vapor barriers and permeable air barriers:

  • Air barriers…
    • Resist air leakage and rain penetration while allowing the diffusion of moisture in the form of vapor
    • Allow the walls of a structure to “breathe”
    • Offer designers more flexibility in positioning of the barrier within wall assembly
  • Vapor barriers…
    • Resist air leakage, effectively acting as an air barrier
    • Resist rain penetration, acting as a precipitation barrier
    • Resist vapor diffusion, thus serving as a vapor barrier


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Originally published in Building Enclosure
Originally published in April 2017