One key to effective air barriers is to ensure that they have no gaps or disconnections. If you’ve read our blog you know we sing the ‘continuous’ tune over an over. The best way to ensure that continuity actually happens is to choose early-on where the air barrier will sit within the wall assembly ‘sandwich’.
Though there are a myriad of different wall design options (and often several different ones on the same project), most have the same physical elements in some form – interior finish, structure, insulation, secondary drainage plane and cladding.
Because one goal of the air barrier is to ultimately support efficient temperature (thermal) control, our choice of where to plug in the air barrier is always in relation to the thermal layer. So just as every story has a beginning, middle and end, so does the air barrier – it can be in either on the outside, inside, or somewhere in-between the wall insulation.
In this article we’re going to simplify how to approach locating the air barrier. We’re also going to give you some pros & cons to help you choose what’s best for your project.
Option 1: Exterior air barrier
What do we mean by exterior? Anything outside of the thermal layer. This is the most familiar air barrier method and commonly piggy-backs the secondary drainage plane (weather barrier).
Technically we could include wall cladding as an exterior option, but even with a “continuous” cladding such as stucco, there are inevitable control joints, cracks, and transitions that just cannot effectively seal the building from air infiltration. It would be an insanely tedious installation and even more insane amount of upkeep. Realistically, the air barrier in this case has got to hide ‘inside the outside.’
This is typically found in a code-minimum building, with varying degrees of effectiveness based on the individual performance needs of the project.
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- Pretty much the same wall and products that builders already know. Minimal new training required to get the job done.
- Cost-effective – using an existing layer doesn’t require an extra pass around the structure
- Easy to QC / identify problem areas and to allow blower-door testing during construction
- Fewer penetrations as compared to interior approach
- Pretty much the same wall and products that builders already know. This is problematic when a builder keeps on going like they did 20 years ago. Carelessness installing the WRB or inattention to detail will result in a leaky building.
- Must endure the entire construction process: potential for flaws created by other trades.
- Uncontrolled penetrations from cladding attachment.
Option 2: Mid-wall air barrier
This is just as the name implies – the air barrier is sandwiched within the thermal layer of the wall. This is the second-most common location of an air barrier, thanks to our sweeping adoption of continuous insulation (CI).
CI puts a supplementary blanket of insulation around the entire exterior of the structure. This is meant to reduce thermal bridging but not usually intended to meet the full insulation requirement for the building. The mid-wall air barrier then falls in between the two layers of insulation. It may be comprised of a fully sealed weather-barrier, fully sealed foam insulation, or taped sheathing.
Another example is the potential for an air-tight layer within a double-stud wall assembly. Or you can go the old-school passive house method of the air-tight layer installed underneath TJI or larsen truss outriggers with insulation outside of the main structure (essentially beefed-up CI).
- Protected from damage during and after construction (depending on the assembly)
- Uses existing building layers (usually)
- Supports current IECC code requirements and building science trends, which encourage the use of continuous insulation
- Unknown or uncontrolled penetrations are possible and difficult to address after the fact
- Constructability: The overall assembly is usually more complex, especially with detailing floors, windows and doors. These require the air-barrier to zig and zag around structural planes or extra blocking.
- Potential for confusion between moisture /air & vapor control layers.
Option 3: Interior air barrier
In this case, the air barrier is entirely on the warm side of the insulation. There’s not a lot of material between the edge of a stud and the finished paint: a layer of gyp and maybe a vapor retarder.
In an ideal scenario, the air barrier will be created with a fully sealed vapor retarder (sheet or panel goods), plus the addition of a service cavity separating it from the gyp to reduce the number of penetrations from MEP trades.
Other examples include an ‘economy’ version in the air-tight drywall approach. This relies on caulk around the wall perimeter and spray foam at penetrations. It’s common on high-performance retrofit projects, where the shell remains intact, but the interior is exposed, and mass wall construction.
- Supports optimal building physics by controlling the thermal volume on the warm side
- Works very well where exterior access is difficult or problematic (vaulted ceilings, retrofit projects) or for separate spaces within a larger structure
- Minimal UV and weather exposure risk and usually installed after building is dried in
- Potential for damage through trades and during occupancy
- Cost – in many markets, this adds an entirely new layer to the construction sequence
- Difficult to detail between floors or other massive structural elements
- Unfamiliar sequencing or materials required
Other air barrier placement considerations:
Can I have two air barriers?
Yes! But it’s not always required, and you should definitely assess the condensation risk based on your specific climate (something SIGA’s engineering department can help with). This is primarily useful in colder climates and/or projects pushing for passive house level airtightness (you get two chances to make a great seal).
What will give me the best airtightness result?
You’ll notice that performance didn’t make the pro/con list anywhere. Whichever one you chose can give you a tight air barrier. It all depends on the constraints of skill-level of tradespeople involved, code and project performance requirements, weather conditions and construction sequencing.
Designing an air barrier into your assembly is all about constructability and coordination. Identifying the project air barrier goals early and then discussing build + cost issues with the contractor as the project progresses will help ensure a great end result.
Looking to learn more about achieving your airtightness goals? Sign up to watch our Beginner’s Guide to High-Performance Air Sealing webinar.
It’s pre-recorded, which means you can watch when it’s convenient for you.