Air Barriers

Temperature differential is the driving force behind moisture condensation problems within the building. It can be controlled relatively well by the use of proper equipment. The temperature outside can’t be controlled but the inside one can be in order to avoid pressure differences.

The larger the temperature difference, the more drive across a wall. Hence, it becomes worse in the cold winters. Temperature drives air and vapor leakage as well as basic heat loss through the building envelope.

The direction of the wind plays an important role. Depending on the wind direction, a high or low pressure area is created, pushing or pulling moisture through the building envelope. On the windward side it is positive and on the other side there is the negative impact. There are several effects that can be seen on the walls based on the wind direction that they face.

What can be the sources of moisture inside a home?

There are several sources of moisture within a home. These include things like shower, dishwashers, and the occupants. As a matter of fact approximately 60 % of all moisture produced with a home come from the occupants themselves.

Moisture moves through the envelope in 3 ways and makes its way through the building whether it is in or out.

Diffuse flow migrates through an assembly and once it is cold it turns to water and then to ice. This settling of the water can be problematic. To reduce or eliminate this a suitable material as to be introduced into the wall assembly. Every material has different permeance and this determines the amount of moisture that the material allows to pass through.

The other is through channels and channel flow. This is when moisture travels through and within a wall assembly via air spaces and gaps between materials or components of a wall assembly.

The third one is orifice flow. These includes things like services penetrations, windows, doors and structural elements.

The principal function of the air barrier is to prevent both the infiltration of outdoor air into a building and the exfiltration of indoor air to the outside. This applies whether the air is humid or dry. Air leakage can cause problems with the deposition of moisture in the walls, the loss of energy and infiltration of rain.

An air barrier is different than a vapor barrier in several ways. Firstly, air barrier goes on to stop the air movement through the cracks in the wall compared to vapor barrier which stops the diffusing through the wall.

Air Barrier is a system unlike vapor barrier which is usually a product like a poly film. Also, air barrier is continuous and all joints need to be completely sealed.

Air barriers must also be more durable.

The three important criteria used for the air barrier design are

  • Continuity
  • Impermeability
  • Durability


Continuity means more than being without holes. Because the component that performs the role of the air barrier changes from the wall to the window to the roof, continuity means that all these assemblies must be connected together so as to ensure that there is no break in the air tightness of the envelope.

Air Impermeability

A major requirement of an air barrier is that it offers a high resistance to air flow but not to water vapour. This a very important. The reason being is that a material that a high resistance to water vapour would not let out any moisture trapped within the wall cavity if it was installed on the outside of the wall.

A perfect example of this is plywood or OSD. These products although not used as air barriers doe act as air barriers on the outside of a building. But the problem becomes that they also act as vapour barriers. This means they trap moisture within the wall assembly. As such most building codes now require that plywood and OSD be installed with gaps at their joints to allow the moisture to escape to the exterior.

While absolute air impermeability may not be achieved, materials such as glass, sheet metal, gypsum board, cast-in-place concrete and a properly supported polyethylene sheet offer a much higher resistance to air flow than do more porous materials such as concrete blocks, fiber board sheathing, and expanded polystyrene insulation. A second major consideration is that individual panels be joined into an airtight assembly. The joints between gypsum boards can be taped quickly and effectively, sheet metal panels can be lapped with tape, precast panels can be sealed with rope and sealants.


The air tightness system must outlast the building itself. For this to happen, the materials used must have a proven track record or the material should be positioned in such a way that it is accessible for inspection and maintenance.

Durability is not an intrinsic property of a material but depends largely on how a material reacts to a specific environment such as moisture, temperature, ultra-violet radiation, and to the presence of other materials (incompatibility).