Air & Vapor Barriers

It’s widely understood that uncontrolled moisture can be extremely problematic to buildings and their occupants. In many cases, this moisture can compromise the building’s indoor air quality through mold growth, as well as possibly jeopardizing the building’s structural integrity by way of material rotting and/or rusting.  So, while everyone can agree on the importance of mitigating moisture, it’s become a bit more challenging however on arriving at a consensus on how best to go about it. It’s not surprising there exists so much disagreement in the industry. With many manufacturers’ marketing and sales reps making unsubstantiated claims, some technical literatures being quite illusive, and even many professionals often resorting to outdated practices, it’s no wonder the industry remains quite divided on what best practices entails. This concern can be said of many topics in the construction industry, but it’s especially true in reference to the subject of air barriers and vapor retarders.

The industry’s division on the topic of air and vapor barriers can also be attributed in part to continuously evolving concepts and practices in response to failed methods that at a certain point in time where considered best practice. These failures and their learned lessons, although beneficial and characteristic of the scientific method, are not always ideal in an industry where many resign to outdated methods through the disposition of resorting to one’s own experiences as an all-encompassing example of the industry’s proven approach. One thing we can be confident of, is that in time the pendulum will swing once again and come to challenge some of our current assumptions. That being said, for now we can only hope to apply those methods that have thus far proved to be successful and are supported by current research and industry standards. Hence in a quick recap of some of our industry’s prior methods, we have moved away from applying polyethylene on the warm side of the wall, to airtight drywall considerations, to later doing away completely with vapor barriers in certain climate zones. Now, once again we try to understand the science behind the failures in the hopes of mitigating the moisture problems that seem to always be one step ahead of our profession. 

In the building industry the following are usually considered in order of importance as relating to successful building envelopes: water intrusion, air permeability, vapor diffusion and thermal performance; air and vapor barriers reside within the second and third category.

In climates that are mostly hot or cold year-round it’s rather clear where the warm side of the wall resides and in what direction thermal pressures and temperatures travel, hence making it easier to determine where best along the assembly the migration of moisture and air should be controlled. This exercise however becomes a bit more challenging in places where thermal pressures and temperatures are multi-directional throughout the year. Places like the tri-state area of DC, Virginia and Maryland, where the mixed-humid climate inverts the vapor drive’s direction from winter to summer. For this reason, we’ll here be focusing our efforts mostly on Climate Zone 4, since it’s multi-directional vapor drive has been considered by many to be among the most challenging as relating to moisture control within exterior wall assemblies.

The use of air barriers and vapor retarders on exterior walls is dependent not only on the Climate Zone, but also on the material’s performance in gradients of relative humidity. Some materials store moisture (hygroscopic) at greater extent and for longer periods than others, and hence release said moisture as temperatures rise, driving it towards colder sections of the assembly. The second law of thermodynamics states that temperature gradients flow from hot to cold in an attempt to reach thermal equilibrium, as well as high pressures moving towards low pressures; in a nutshell, this translates to both the latent (vapor) and sensible (air) heats aiming to reach equilibrium. In an exterior wall assembly this translates to hot air, which carries greater amounts of moisture than cooler air, pushing its way towards the cooler temperature. As the moist air (humidity) lowers in temperature (usually passing through the insulation) there reaches a point where the vapor condenses (dew point). This condensation can translate to quite a large amount of water within the wall’s cavity, and hence stopping moisture migration becomes crucial.

As we all know, there is no such thing as a perfect building system and even less so the installation of it; hence, it becomes also critical for the assembly to be able to dry out any moisture that does make it in. Here is where things get a bit interesting, cause as we’ve stated the idea is not only to stop the moisture travelling from high to low temperatures and pressures, but also allowing for any moisture that entered the assembly to exit and dry off. This delicate balance is not always quantifiably clear and hence has become a point of slight contention within the industry. 

Figure 1: U.S. Department of Energy. IECC Climate Zone Map

VAPOR BARRIERS / RETARDERS:

Vapor barriers slow the migration of moisture through the assembly and hence should be considered more of a retarder than a literal barrier. Vapor retarders are grouped into three classes based on their permeability: Class I (<0.1 perm) referred to as a barrier; Class II (0.1< 1.0 perm) described as a retarder, and Class III (1.0 < 10 perm) considered vapor permeable. These classes exist as a way to categorize materials by the amount of vapor migration they allow to pass through, and depending on said permeability it’s said that the assembly is allowed to ‘breath’ - i.e., allowing trapped moisture a way to escape. 

The use of vapor retarders in commercial wall assemblies depends on the project’s climate zone and can be referenced in Section 1404.3 of 2018 IBC. There an account of when and where to use vapor retarders is provided, as well as which materials constitute as such from a code standpoint. Since our focus is on Climate Zone 4 (Mixed-Humid), it should be noted that although there are several who continue to design Class I & II retarders as part of the exterior wall assembly, it is not required by code nor practice. This is due to the moisture’s directional shift for roughly half of the year - i.e., vapor drive in winter from in to out, and in summer from out to in. In Climate Zone 4 providing for a vapor permeable solution allows the wall assembly to dry rather than trap the moisture within for half of the year.

In most other climate zones the direction of vapor drive is mostly one directional since temperatures are either cooler or warmer for most of the year. In colder climates for example, the retarder is placed within the warmer interior side of the wall assembly. However, climate isn’t static and such variations exacerbated by current climate change concerns is giving way to added instances of varying temperature periods. One way of addressing this issue on exterior wall assemblies is by considering the use of ‘Smart’ vapor retarders. These allow the wall cavity to dry out in the summer months by increasing the material’s permeance at high humidity levels, while slowing vapor drive in the cold season by decreasing its permeance; in other words, allowing membrane to dry towards the direction of higher humidity. For these to function correctly the membrane should be properly located within the wall assembly based on the project’s climate zone.

Figure 2: Water quantity transmitted by vapor diffusion vs. air leakage in one heating season under interior conditions 70 deg F and 40% RH. Image adapted from Building Science Corporation (‘Builder’s Guide for Cold Climates’ by Joe Lstiburek)

AIR BARRIERS:

Air barriers are essential for keeping conditioned air from escaping and exterior air from infiltrating, thereby lessening the building’s mechanical loads. Air barriers are considered much more critical to the building’s performance than vapor retarders. This statement isn’t meant to be interpreted as vapor retarders not being important to the overall performance of the building, but rather simply to stress that a much greater amount of moisture is transferred via air than through vapor diffusion (See Figure 2 above). This being the case, it is crucial that care be taken towards the detailing and installation of air barriers.

Air barriers should be strong enough to accommodate for temperature and pressure differentials between the building’s exterior and interior. If not properly detailed and installed air barriers can billow and rip around or detach from their mechanical attachments. This is one reason why many in the field prefer self-adhered or fluid applied to mechanically attached air barriers.

In addition, air barriers should be continuous throughout the building envelope and care should be taken around openings and between material transitions. Penetrations should also be properly sealed. There are many instances where barriers are installed and later penetrated by countless fasteners rendering them rather ineffective. Where penetrations into the membrane are inevitable of the assembly, they should be sealed with elastomeric sealants comparable to the air barrier membrane and installed per the manufacturer’s requirements. At instances where penetrations occur but aren’t able to be sealed due to construction sequencing restrictions, self-sealing air barriers meeting ASTM D1970 have been shown to be rather successful. Self-sealing is limited to the air barrier’s ability to seal itself around the attachments after being pierced, and hence should not be confused with self-healing which is a material’s ability to repair itself from damages caused by extended usage. Be wary of air barrier manufacturer's claiming the latter, since currently there aren't any ASTM test methods for self-healing.

Today, many air barriers on the market are capable of addressing both vapor retarding and reduction of air permeance. Such air barriers should be used with caution and provided only where adequate to the project’s climate zone and code requirements. Within commercial Climate Zone 4, where air barriers are placed on the exterior of the wall, providing a vapor permeable air barrier assures the wall assembly is able to dry out in the cold months when warm interior air drives vapor from in to out.

CONCLUSION:

Every material has a different vapor permeance and even tends to perform differently depending on temperature and humidity levels. It would be ideal to consider the permeability of each material in the wall’s composition and provide an aggregate account based on the assembly’s vapor diffusion resistance (Rp), much like we tend to do for the total thermal resistance (Rt); unfortunately, currently this isn’t the norm in the architectural profession. This oversight can prove problematic, especially where a designer may use certain wallcoverings with low permeability that limits the wall’s ability to ‘breath’. Design professionals should be aware of what material(s) in their wall assembly are capable of entrapping moisture and how best to mitigate this concern based on their project’s climate zone.

One rather simple and relatable example of how different materials’ permeability can affect their ability to restrict air and vapor can be observed in party balloons. There are mainly two types of balloons commonly used. One is made of rubber-latex while the other is the mylar type (polyester that looks like foil). When inflated - disregarding any air leakage through the knots and seams - the mylar balloon tends to last much longer than the latex one. Although not entirely the same, for our purpose we’ll consider the latex balloon performing similar to an air barrier, while the mylar to be comparable to a vapor barrier. The analogy is not meant to be a literal interpretation, but rather assist in visualizing some of the differences between an air vs vapor retarder. The latex balloon is able to hold air pressure in while allowing that very air (along with its moisture) to escape at a faster rate than that of the mylar, yet slower than in the absence of a membrane. Since the latex is porous, especially when stretched, it is able to slow the air transfer but not entirely stop the vapor diffusion through its surface. On the other hand, the mylar balloon (like a vapor barrier) performs much better at slowing vapor diffusion and in our case, the air as well.  

As with all code and building related discussions, designers should reference and adhere to the project’s specific code, AHJ and Climate Zone requirements.