After reading this article, you should be able to:

  1. Define wall system terms—specifically the difference between an air barrier and a vapor barrier.
  2. Explain the science of wall systems, including the potential impact on energy savings and the beneficial health effects of controlling moisture migration into living environments.
  3. Describe the detrimental impact of moisture infiltration into building materials and the safety concerns inherent in unsafe structures.
  4. Determine the proper wall system to use for a given climate in order to limit air and moisture movement and ensure the comfort and safety of building occupants. 

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An Introduction to Wall Systems

Wall systems are quite literally all around us, and they are more complex and dynamic than you might think. Despite their ubiquitous and seemingly mundane nature, an in-depth analysis of the science of wall systems and building envelopes could easily fill entire books—let alone this course. In this unit, you will learn the basics of wall system construction as they relate to air and waterproofing, the differences between air and vapor barriers, and how to determine the proper wall system for use in a given environment to maximize the comfort and safety of building occupants.

The first thing to realize about wall systems is that there is no “magic” solution to perfect wall system waterproofing that fits every condition. Items that influence the overall performance of a building enclosure include the wall components utilized, project location, insulation type and location, and type of occupancy, for example. Given this level of variability, there is no one “ideal construction” to employ in every situation. Still, there are some general rules to help you choose the right system for your specific application.

While you may be familiar with some of the wall examples that will be discussed in this course, there will likely be some unfamiliar examples, too. To make the most of your time, the course content will focus mainly on overall wall construction concepts and themes, rather than focusing on specific wall components, to help you grasp the principles of air and vapor barrier choice and design.

Topics will include:

  • Important terminology
  • The science of wall systems   
  • Energy savings   
  • Effects of air leakage
  • Durability  
  • Construction types
  • Wall system options

Understanding Wall System Terminology

While we often look at the walls around us as solid barriers between us and the outside world, when it comes to air and water infiltration, they can be far less impenetrable than they seem. Installation mistakes during construction, poor wall system material choices, structural damage and other factors can contribute to air or moisture finding its way into or through walls, and the results can be ugly, costly or downright catastrophic.

Air leakage is something that we never want to see in wall systems. Air leakage issues typically occur in areas of penetration in walls, window or door openings, foundation-to-wall transitions and wall-to-roof transitions—i.e., at any transition point or tie-in where detailing and material compatibility are vital. Not only can air leakage negatively impact building energy efficiency and occupant comfort by facilitating heating and cooling loss; it also exacerbates problems related to water infiltration.

Importance of Air Barriers

"Air movement is the dominant factor in the transport of moisture through building envelope assemblies. It is also an important component of heat transfer. Many problems concerning building envelope deterioration can be attributed to inadequate or failed air barriers." – National Research Council of Canada

Water vapor transmission tends to occur even in properly assembled walls. One way moisture can enter walls is via a process called vapor diffusion in which water molecules move through porous materials from regions of high concentration to regions of low concentration by means of random molecular motion. As long as moisture transmission is controlled, wet materials in a wall assembly will naturally dry via vapor diffusion. If moisture buildup is unmitigated, however, moisture-sensitive wall components may degrade or pose health risks.

Air movement can vastly expedite the rate of vapor diffusion with a wall, causing problems within the wall assembly and potentially shortening the life of the structure. In fact, air leakage transports 100 times more water through a small void in a wall system than if vapor was diffused through a 4 feet by 8 feet sheet of barrier material. Next to bulk water leaks, condensation resulting from air leakage is the main cause of moisture problems within buildings.

Because of the deleterious effects of air leakage and excessive moisture infiltration, wall systems are typically equipped with vapor control or “barrier” materials to protect the building during construction, then prevent air leaks and control vapor diffusion levels once the building is ready for use. Prevention of moisture problems is the most important step to ensure the long-term performance of wall assemblies, but it is often difficult to eliminate all sources of moisture during the lifespan of a building. With proper installation of barrier materials throughout wall systems, however, moisture accumulation can be restricted to a level that will dry naturally.

An air barrier is a material that is placed on the exterior back-up wall of a structure and is designed to control the movement of air within the structure. The air barrier is illustrated by the dotted line in the wall visual. An air barrier stops air, but it allows vapor to move within the wall assembly, thus making it a permeable membrane. Air barriers are categorized according to their water-vapor permeance, and the degree to which water vapor passes through a wall is specific to each type of air barrier. Air barrier membranes are assigned permeance ratings from 0 to above 75 U.S. perms, and the industry recognizes materials that have a vapor permeance of 10 U.S. perms or higher as being “permeable.”

Permeable membranes help expedite the drying rate of walls, mitigating moisture-related concerns such as mold formation, wood decay and corrosion. In general, a more permeable membrane will enable more drying than a less permeable one, but it will also allow more water vapor to enter the wall assembly from the outside. A highly permeable membrane can help with drying, but it will be less effective if vapor diffusion is being restricted by other layers in an assembly.

There is a general conception in the industry that the higher an air barrier’s permeance rating, the more it will facilitate wall component drying, but this is not always the case. The graph above shows that replacing a 10 U.S. perm membrane (the red line) by a 50 U.S. perm membrane (the blue line) improves the normalized drying rate by only 8 percent, which is negligible. Increasing from a 50 to a 100 U.S. perm rating will only improve drying times by 1 percent.

In other words, a membrane that is five times more permeable will not allow the wall to dry five times faster. The membrane allows the wall to store moisture more quickly when vapor pressure is applied from the outside to the inside, but as a rule, increasing permeance has diminishing returns on drying rates, and in some cases it simply is not worth the investment to seek membranes with higher permeability. 

In addition, the drying dynamics of any wall assembly will be dictated by the material that has the lowest permeance to water vapor. The potential benefits of a highly permeable air barrier will, thus, often be mitigated by another material that determines the assembly’s drying ability. It is essential to consider the wall type in which a membrane will be installed, as factors such as insulation presence inside the wall cavity, insulation presence outside the building envelope and water store capacity of the sheathing can complicate the drying of a damp wall.

An air and vapor barrier, colloquially called a “vapor barrier,” is a material that is placed on the exterior back-up wall of a structure and is designed to control the movement of air and vapor within the structure. The vapor barrier is illustrated by the solid orange line in the wall visual in this image. Since vapor barriers manage both air and moisture transmission, it is deemed a non-permeable membrane, “impermeable membrane” or “vapor retarder.” The industry recognizes materials that have a vapor permeance of less than 1 U.S. perm as a non-permeable membrane. 

When you look at permeable and non-permeable sheet materials, you will note that the non-permeable facer is smooth. Most non-permeable facers are made of a version of polyethylene (plastic) film. Alternatively, facers on permeable sheets may be comprised of a textured synthetic fabric or specialized thermoplastic polymer films, which allow vapor to migrate through the membrane.

The Science of Wall Systems

Now that you have a general understanding of what air barriers and vapor barriers do, let’s talk a bit more about why we need them. When constructing wall systems, one can typically spend $1.50 to $2.00 per-square-foot on the barrier material that is placed on the outside of a structural wall. You may wonder if the investment is actually worth it. It is not always inexpensive to install these materials. It is, however, important. When it comes to seeking cheaper material options as part of a “value engineering” exercise, there is good reason to look elsewhere to reduce costs. 

Air barriers may be required by code, they may be installed for energy savings, or they may be intended to keep buildings dry and to make structures last longer. Air barriers keep occupants comfortable and keep building owners’ bills lower.

The three core focus areas of this section will be energy savings, the effects of air movement and structural durability. Correctly designing and installing air barriers can affect all three of these aspects in meaningful ways. 

Energy Savings

As mentioned earlier, air barriers cut down on heating and cooling costs for buildings by controlling air leakage. Air leakage refers to uncontrolled air movement leading to heating and cooling loss, subsequent high energy costs, and sometimes expedited water infiltration. In order to avoid air leakage, air barriers must be sealed, continuous and air-impermeable.

Vapor drive occurs when water vapor moves from a region of high humidity and temperature to regions of lower humidity and temperature in an attempt to reach a natural equilibrium. This graphic shows vapor drive as warm, humid exterior conditions cause moist air to pass easily to the drier interior conditions through a wall assembly that contains no air barrier membrane. To picture this phenomenon another way, visualize a hot cup of coffee that cools to room temperature, hence obtaining thermal equilibrium inside and outside the cup.

Calculating the Savings

The National Institute of Standards and Technology (NIST) is a part of the U.S. Department of Commerce. NIST develops the testing, measurements and reference materials needed to ensure the quality of energy-related products and services. In NIST’s “Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use” prepared for the U.S. Department of Energy, Office of Building Technologies, recognized leaders in building science investigated the impact of envelope air tightness on energy consumption in a typical commercial building. Cities across the U.S. were studied, to ensure a breadth of climate conditions were considered.

The study’s findings? Up to 36 percent energy cost savings can be achieved by using an air barrier. Colder climates create conditions for the most significant savings. While hot climates offer less opportunity, the opportunity for savings is not insignificant. Clearly, a properly designed and installed air barrier can play a substantial role in reducing energy costs.

Vapor drive can also occur when we attempt to warm structures. In this scenario, water vapor moves from a region of high humidity and temperature to regions of lower humidity and temperature in an attempt to reach natural equilibrium. As you can see, air and moisture easily move out through a wall assembly that contains no air barrier membrane.

When HVAC systems run continuously in an attempt to heat or cool a building, higher energy consumption costs and diminished HVAC equipment lifespans are the result. Hot, humid air creating a vapor drive can also lead to problem areas developing within the structure, whether that air is traveling into a building or out of it.

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.

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. 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.

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.

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

Wall System Constructions

Placement of certain components within a wall assembly, combined with the geography of where the project is located, will affect your determination of what the ideal wall assembly for the job should be.

One factor to keep in mind is dew point—the temperature at which air is saturated with water vapor, causing vapor to change from a gas to a liquid. When air has reached the dew-point temperature at a particular pressure, the water vapor in the air is in equilibrium with liquid water, meaning water vapor is condensing at the same rate at which liquid water is evaporating. One of the main elements that affects where the dew point develops is the insulation. As a result, the position of the insulation influences where the dew point develops within a wall assembly.

You may note that a non-permeable membrane has been placed on the exterior sheathing in conjunction with the use of batt insulation in the interior stud cavity. As a result, this wall will not perform well. The warm, conditioned air on the interior will drive outward to try to balance with the cold exterior air, but because a non-permeable membrane (the vapor barrier designated by the orange line) is present, the vapor will get trapped in the insulation and collect—not a good thing.

All conditions for this example are the same as in the previous, except we moved the non-permeable membrane to the interior of the wall and placed a permeable membrane against the exterior sheathing. The warm, conditioned air is stopped before it can reach the insulation and collect. Exterior conditions will change as the climate changes from cold to warm, and moisture vapor will be permitted to enter the assembly because we have a permeable membrane on exterior sheathing. Moisture that enters the wall assembly will be allowed to exit because of the permeable membrane. This is considered a “good wall” or “permeable wall.”

This same wall construction works well in hot climates. It is worth noting, though, that this wall performs on paper and, if a modeling program was run with this wall, it would perform well. There are a few things to consider regarding this wall type, however that may not be apparent from the diagram. The vapor barrier that is present on the interior stud cavity is a loose mechanically fastened piece of polyethylene. The material is attached with numerous fasteners resulting in many penetrations in addition to penetrations coming from electrical outlets, pipe penetrations and the like. This will compromise the performance and functionality of the vapor barrier in this example. Additionally, on multiple level constructions, the polyethylene starts and stops at every floor, making it very difficult to detail and tie in correctly. These are the sort of challenges that you will encounter in real-world applications, but may not always foresee without careful analysis of the potential variables.

Let’s look at a different wall system construction.

In this wall assembly example, the insulation is in the form of rigid insulation. There is no batt insulation in the interior stud cavity. The warm conditioned interior air tries to drive outward to the cold exterior air, but it is restricted by a fully adhered air and vapor barrier. With the absence of batt insulation in the stud cavity, there is nothing on which the moisture can collect and compromise the integrity of the wall. Steel studs and exterior sheathing also have a much better tolerance to moisture until conditions change and drying can occur.

This wall has been labeled “the perfect wall.” The placement of the rigid insulation in the exterior cavity conditions the exterior space while pushing the dew point to the exterior cavity as well. This ensures that any moisture that accumulates as the result of the dew point will occur in the exterior cavity. It will then be able to exit out a weep system in the brick veneer. The installation of the air and vapor barrier on the exterior sheathing helps ensure a quality installation because it can be easily inspected from outside the building. With rigid  insulation located in the exterior cavity, this wall also satisfies the International Energy Conservation Code (IECC) requirements for continuous insulation.

If we were to deploy this “perfect wall” in a hot climate, we would see the warm, humid air driving toward the interior cool, conditioned air. With the presence of rigid insulation against a fully adhered air and vapor barrier membrane, the warm humid air is prevented from entering and meeting the cool conditioned air. The “perfect wall,” when designed and installed correctly, works in any climate and any geographical location.

Now let’s factor in a more specific climate consideration. Will this wall work in a hot, humid climate, like one might find in Florida?

This is a common wall assembly in the extreme southern geography of the U.S., e.g., Miami. A permeable membrane is applied to the exterior face of the block, but when the climate is continuously hot and humid, this construction will not perform well. The hot, humid air will drive to reach the cool, dry interior air and will bring a tremendous amount of vapor with it, causing moisture collection in the interior space. In climates where there is little variation in temperature, there is little to no opportunity for the wall to dry out. This is an example of where a permeable membrane is not the best option.

A better option would be this same construction, but with an impermeable vapor barrier instead of the air barrier. Hot humid air will be prevented from driving inward from a fully adhered air and vapor barrier. This keeps the outside air and interior air separated, and eliminates the possibility of condensation within the wall.

Now let’s consider a wall assembly example in a climate where there are no extreme changes between the outside and inside air characteristics. In this scenario, either a permeable or non-permeable membrane will perform well. The struggle of hot, humid air driving to cool, conditioned air is not present. There will only be minor swings in temperatures, so any moisture that develops will have an opportunity to dry once the temperatures return to a consistent level.

Wall System Options

One way in which barrier membranes vary is the manner in which they are applied within the wall assembly. The following are a few common barrier types you might encounter.


Self-adhesive membranes are robust, strong choices for a number of reasons. They prevent lateral migration of air, vapor and water, and when there is a breach, the breach stays localized. Air and water cannot travel behind the membrane because the full back surface of the membrane is adhered, and it therefore cannot enter the structure.

Self-adhesive membranes are manufactured with a controlled thickness, are externally reinforced by the surfacing film and require no mixing or special equipment to apply. When working with self-adhesive barriers, keep in mind that they tend to have seams approximately every 3 feet, and they require sealing of the overlaps to avoid fish mouthing. The material is designed to stick to itself, so sealing is easily accomplished. Sheet materials such as these may not be ideal for intricate detailing or for use with complex geometries.

Among the permeable air barrier lines available on the market, self-adhered membranes have varying adhesive properties. An effective permeable air barrier must readily adhere to most building surfaces, so look for an option with strong adhesive qualities, and ideally find one that does not require a primer for its installation to maximize installation efficiency. 

Mechanically Fastened

Mechanical fastened air barriers benefit from being relatively inexpensive and easy to install. As is the case with self-adhesive membranes, however, detailing can be complex with mechanically fastened barriers. They are loosely applied, and they are therefore vulnerable to tearing at fastener points. All joint, lap and fastener penetrations need to be completely sealed to provide a continuous barrier to avoid moisture intrusion. Mechanically fastened barriers do not protect against lateral migration of air or water, and they do not transmit negative wind load, meaning their structural integrity many not withstand high suction loads caused by wind being directed around the building at high force.

Mechanically fastened materials are used because the material itself is economical, but it is rarely installed correctly in accordance with manufacturers’ recommendations. When mechanically fastened materials are installed per the manufacturer’s recommended application procedures, the installed cost is similar to that of adhered systems. Adhered materials, however, outperform mechanically fastened materials on all fronts. For example, air and water can travel unrestricted once behind the mechanically fastened membrane. The breach is not contained or localized and the actual source of the breach is nearly impossible to locate. As previously stated, adhered materials stick well to the substrate and prevent the free flow of air or water if a breach occurs. Any breach is contained to an isolated area that is easily detected.


A third type of membrane, a liquid-applied (or fluid-applied) barrier, is typically easy to install and can save time versus sheet material applications. These liquid-applied barriers are seamless, work well on any geometries or applications with intricate details, and require minimal substrate preparation.

The biggest challenge with liquid-applied barriers is thickness control, as it is fully dependent on the installer. Mixing of the material in the container is typically required, and spraying equipment is usually involved, although many of these products may be installed with a roller. There can also be variability with the material, and subsequent application requirements, due to climatic conditions. For example, some materials cannot be installed near freezing temperature, and some may require additives depending on the temperature. There are also differences in composition and the percentage of solids content in liquid materials, and the percentage of shrinkage from wet film to dry film correlates directly to the solid’s content. Be sure to read the product data sheets and installation instructions carefully prior to starting any work.

With numerous types of wall system options available, there is more than one right answer to fit your need. The best option for you is often a matter of preference based on your circumstances and experience. For example, you many have a block structure with brick ties, in which case a liquid may be the best option for the job. Alternatively, you may have a project that requires 40 stories of barrier material where overspray could be a concern, and this application may lend itself to using a sheet material. In general, manufacturers that are dedicated to providing materials designed to protect the entire building envelope will offer sheet materials and liquid-applied barrier options that are compatible and will effectively secure all tie-ins from air-moisture intrusion. If you find a manufacturer you trust, you can likely find reliable materials with the adhesion properties you want.


The ways in which wall systems become wet and ultimately dry are complex, and there is no one simple solution to equipping wall systems with barrier material that is ideal in every situation. Sometimes an impermeable vapor barrier will be needed, sometimes a high-permeability air barrier will work well, and sometimes you will want both. There will also be applications where a self-adhesive sheet material is easiest to apply and will perform admirably, and there will be situations where fluid-applied or mechanically applied barrier material makes the most sense. One thing that is for sure, however, is that any decisions made about vapor control layers should be done in the context of holistic enclosure design.

  • Wall systems not only have a positive impact on energy savings, they also create more comfortable environments for building occupants while protecting the integrity of the structure.
  • As showcased in the examples throughout this course, it is important to understand the benefits of and differences between air barriers and vapor barriers, as one option does not fit the needs of every building design and application condition.
  • When designing the correct wall system for any given job, take into consideration climate, geography, conditioning and wall components to determine which wall system assembly will perform best.
  • There are a number of options to consider when choosing a system—from various barrier materials, to wall assembly designs, to adhesion methods that will best suit the application.
  • Whatever materials you choose, be sure they are compatible, will adhere to one another, and will physically transition smoothly from surface to surface to create safe, healthy building environments.

Finally, when you do tackle your next installation, remember to work with a manufacturer that can provide tested solutions on all areas of the structure for the highest likelihood of a successful application. When you are using materials that are designed to be used together, you need not worry about compatibility issues that could affect the longevity of your installation.

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