7 Air Barrier Facts Architects and Specifiers Need to Know

Russ Snow is a proven industry thought leader on air barriers, most recently receiving the distinction of a fellowship induction from the Air Barrier Association of America (more on that here) for his commitment to influencing industry best practices.

Russ briefed me today regarding air barrier trends in Q2 of 2021. Here’s an excerpt from our conversation:

Q: Are there any new trends in air barriers?

Russ: This isn’t necessarily a trend, but I want to point out that within the International Building Code (IBC), there are requirements for air barrier materials, assemblies, and whole building testing.  Why is this important? Well, U.S. states and certain regions have adopted versions of the code, making it mandatory for an air barrier to be used.

Q: What about new materials?

Russ: There’s ongoing research regarding different sorts of materials. I have seen that fluid-applied materials seem to be more prominent than sheet materials. There are a number of different types of materials and chemistries on the market now, all with their own pros and cons; fluid-applied, sheet-applied, and also some that I call an “all-in-one system,” where the air and moisture barrier is integral with the sheathing.  All of these materials have their place, and as mentioned, have their pros and cons. Ultimately, regardless of the type of material, it is essential that they are installed correctly to provide a continuous air barrier. Having a trained installer familiar with the air barrier material is critical, because detailing at transitions and fasteners – when not done correctly – will be where the issues arise. Regardless of the type of material, air does not get through the field of the material.

Q: I noticed one of the top search terms for air barriers was “Do you use air barriers for metal buildings?” because of the increase in warehouse building. Is there a definitive answer?

Russ: The most common phrase you’ll hear in our industry – and I know it’s frustrating – is: “It depends.” That’s because there are so many different factors when it comes to the selection of an air barrier material. There’s the wall assembly, the design structure, your interior climate, the exterior climate, and the local code requirements. It would be nice if there was one thing that worked for everything, but there isn’t. With metal buildings, yes, there is an air barrier, but depending on the type, the exterior metal could be the air barrier, the interior vapor barrier could also be the air barrier, or it could be buried within the overall assembly. The critical areas are the joints and transitions, to ensure continuity of the air barrier.

Q: That raises another point. Another popular search term is “What’s the difference between an air barrier and a vapor barrier?”

Russ: A full explanation on this could take a lot of time, but briefly, an air barrier will control air flow, and a vapor barrier will control vapor movement.  There are many things that can perform as an air barrier, but not everything can be a vapor barrier.  When incorporating these within an assembly, an air barrier will work anywhere within the assembly; a vapor barrier needs to be incorporated to avoid condensation and not prevent drying of the wall.

A common rule of thumb is that you place a vapor barrier on the warm-in-winter side of the wall to avoid the potential for condensation, if it is on the cold side.  If you have an air/vapor barrier, such as W. R. MEADOWS’ AIR-SHIELD, that is both an air barrier and vapor barrier, then where do you place that within a wall? You want it on the warm side of the wall with enough insulation outboard of that to keep it warm.

In the South, where things are reversed, a similar wall assembly may work, or it may not. To avoid the potential of moisture build-up and to allow drying, a lot of wall assemblies are now using vapor-permeable air barriers, such as W. R. MEADOWS’ AIR-SHIELD LMP.  They can all work, but it comes down to a number of considerations in the design. It’s one of those questions where there’s not a definitive answer, and I will defer to the “it-depends-scenario.”  Personally, I love discussing the above-grade wall assemblies, because there are so many factors involved, and it does get very technical in some cases. Maybe because of my science background, I like to work towards a solution versus having a cookie-cutter answer.

Q: How do you know whether to use permeable or impermeable air barriers?

Russ: This is the harder one to answer and again, there are a lot of factors involved in the solution. It’s dependent upon the components in your wall assembly, the exterior climate, interior climate and the use of the building. It could also be dependent upon what the code requirements are. When you get into vapor-permeable products, different products have different permeabilities. The decision on what to use should take into consideration all of the above factors, and not just the published value of a material. The whole idea is to control moisture and allow the assembly to be able to dry.

Those would be the main factors. W. R. MEADOWS has published a white paper available on the website, which gets into a fair amount of detail on this topic.

Q: What about air barriers and fire ratings?

Russ: This is where you get into the combustibility of materials, such as asphaltic versus non-asphaltic, for example. Again, due to the wide range of materials, this does vary and this is a very in-depth topic. One of the industry standards prominent now is NFPA 285 (National Fire Protection Agency). This is an overall assembly fire test. Back in 2012, the International Building Code had included the water-resistive barrier/air barrier to be required as part of the tested assembly, resulting in a fair amount of additional testing, whether it be a full assembly test, or a specific material test for flame spread, smoke development, or heat of combustion. To many, NFPA 285 can be very confusing, but an approved assembly with certain components is only good for that certain assembly. In some cases, there is also the potential to obtain an engineering judgement from a fire consultant, where a material can be replaced within an assembly by another, assuming that the material testing for the proposed product is no worse than the material within the assembly, and is not going to affect the overall performance of the assembly. For example, in our case, if we wanted to have AIR-SHIELD LMP included within an assembly where it is not, we would provide material testing to the fire consultant for their review and potential approval.  If deemed acceptable, then we would receive this engineering judgement showing AIR-SHIELD LMP within the assembly.

DD: Thanks, Russ, for your time and attention today. For those of you reading along, please feel free to connect with Russ Snow on LinkedIn here: https://www.linkedin.com/in/russell-snow-248189b/.

About Russ Snow:

Russ Snow served as chair for the Air Barrier Association of America from 2016 to 2019, and has assumed varied executive committee roles since 2015. With more than 25 years’ construction industry experience, Russ is knowledgeable about building envelope, concrete restoration and concrete treatment products. Russ develops CEU programs through AEC Daily, the American Institute of Architects, and other organizations. He serves on the board of directors for the National Air Barrier Association. Russ has also served as the Regional Sales Manager/Product Group Manager – Building Envelope since 2019, and as the Building Science Specialist for W. R. MEADOWS for more than a decade.