Air barriers and vapour barriers are often confused on jobsites — and that confusion leads to moisture problems, callbacks, mold, and durability failures.
The challenge is that some materials can perform multiple functions at once. A material might act as an air barrier, a vapour barrier, a weather barrier, insulation — or some combination of all four.
Understanding the difference matters more than ever as modern homes become tighter, more energy efficient, and more dependent on correctly designed control layers.
What Is an Air Barrier?
An air barrier controls the movement of air through the building envelope. Its purpose is to stop air from moving in or out of the conditioned space. Its job is to block the air pressure differences that drive the stack and wind effects.
When air leaks through assemblies, it carries heat and moisture with it. That can create a whole series of problems depending on where the air leakage is happening:
- condensation inside walls
- comfort complaints
- ice damming
- attic moisture problems
- reduced insulation performance
- indoor air quality issues
The air barrier controls the air movement between the conditioned space and the exterior. To do so, it must be continuous across all the different components, assemblies, and planes of the building envelope. What this means is that there needs to be an air barrier system. There are two challenges in creating an air barrier system:
- Given the different components, assemblies, and planes, there can be more than one air barrier material.
- Air barrier materials can be placed almost anywhere in the different assemblies.
Common Air Barrier Materials
Lots of folks consider 6 mil polyethylene sheeting to be ‘the air barrier’. But way back before the 1970s, its initial job was to be the vapour barrier. Taping and sealing it is what makes it into an air barrier. For decades, it has been a commonly used material to create an interior air barrier, doing two jobs in one when sealed and continuous, other materials can do the job as well. Examples of interior and exterior air barrier materials include:
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Just like 6 mil poly, many of these materials are doing double duty – they have a ‘primary’ function within a building assembly AND they can function as air barriers as well if properly detailed and sealed.
What Is a Vapour Barrier?
purpose of vapour barrier — now more accurately called a vapour diffusion retarder or VDR — is to control the movement of water vapour through materials by diffusion. Unlike air leakage, which is driven by pressure differences, vapour diffusion is a slow molecular process. The job of a VDR is to prevent warm, moisture-laden air from reaching cold surfaces where condensation can occur.
Unlike air barrier materials, the VDR cannot go anywhere in the building assembly, otherwise you risk trapping moisture in places you can’t see, which can lead to problems with mold and rot. In cold climates like Canada’s, the vapour barrier should be on the inside of the insulation. In hot climates, it should be installed on the outside of the insulation. In both cases, the vapour barrier prevents warm, humid air from leaving its moisture behind as it meets a cool surface, regardless of the direction it is moving.
Air Leakage vs Vapour Diffusion
One of the most important building science concepts contractors need to understand:
Air leakage moves far more moisture than vapour diffusion.
A tiny air leak can transport significant amounts of moisture into an assembly very quickly. Vapour diffusion – the movement of moisture through materials – happens at a much slower pace. That is why building science best practices place enormous emphasis on the air barrier, continuity and proper detailing at transitions and penetrations. Airtightness testing with a blower door is the only way to ensure control layer continuity. In many cases, solving air leakage problems has the bonus of solving moisture issues in a house.
Why Moisture Control Matters
Poor moisture control can contribute to:
- wet insulation
- mold growth
- rot
- reduced thermal performance
- long-term durability failures
As assemblies become more insulated and airtight, drying potential becomes increasingly important. That means there’s more to know about the way the assembly functions than just where the air barrier and VDR go! The materials choices you make, or that your client makes, impact the drying potential in a few ways. Primarily, you need to understand whether your assemblies are permeable or impermeable.
When Materials Perform Multiple Functions
High performance assemblies get complicated because many materials perform more than one control function. A material can be an air barrier, a vapour barrier, a weather barrier, an insulator or any combination of those four functions. Other materials that can meet code requirements as both air and vapour barriers. This is where many installation mistakes happen.
It can be cost-effective to use a material that can have multiple functions. But the key to successful and durable homes is to understand the drying potential of the assembly (is it permeable or impermeable - does it dry to the interior or does it dry to the exterior. The consequence you want to avoid at all costs is the dread vapour sandwich, where you unintentionally trap moisture inside an assembly by combining materials with low drying potential on both sides.
| Material |
|
Vapour Barrier | Insulation | Weather Barrier |
| Polyethylene Sheeting | Yes | Yes |
No |
No |
| Spray Foam Insulation | Yes | Sometimes | Yes | No |
| House Wrap | Yes | No | No | Yes |
| Rigid Foam Insulation | Yes | Sometimes | Yes | Sometimes |
| OSB/Plywood Sheathing | Yes | Sometimes | No | No |
Many materials can act as an air barrier but do not, on their own, create a vapour barrier, or in the case of spray foam, for example, they have to be a certain type, and be a certain depth. Sheet goods and panel materials such as plywood and OSB, along with certain paints and coatings can act as both air and vapour barriers as described in the building code.
The Airtight Drywall Approach (ADA)
Gypsum board (drywall) is a good example of how materials can serve multiple functions. Drywall itself can act as an air barrier when:
- joints are sealed
- penetrations are controlled
- continuity is maintained
- This is known as the Airtight Drywall Approach (ADA).
Standard drywall alone is not a vapour barrier because water vapour can still diffuse through it. But, when it’s coated with a vapour-retarding latex paint, drywall can also function as a vapour retarder. This demonstrates why you need to think in terms of assembly performance rather than individual materials.
Why This Matters More Today
The latest version of the building code (NBC 2020) and the BC Energy Step Code require tiered energy performance targets. While the lower levels can be managed with a prescriptive approach (use the tables provided in the code), it does mean that insulation levels are increasing and mechanical systems are changing as well. As Code requirements are evolving, moisture risks are becoming less forgiving, and the performance path – energy modelling and blower door testing required – can actually be more cost-effective because, in part, it can reduce callbacks due to air leakage-related moisture problems.
Poorly designed or improperly installed control layers can create serious problems in modern high-performance buildings and renovations. Understanding air barriers and vapour barriers is now fundamental building science knowledge for contractors and renovators.
Learn More
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