Air pressure tends to be lower inside a building than outside. If there are any gaps linking outside and inside, air can flow through them, bringing water with it (see air movement and pressure moderation). Air barriers or air seals block that air flow.

Air barriers

Where an air permeable cladding material is used or where there is a drained and vented cavity, the wall construction must incorporate an air barrier to prevent air flow from the exterior or the cavity into the building.

In most New Zealand domestic buildings, the internal plasterboard linings provide this general air barrier. It can also be provided by a rigid underlay – such as plywood or fibre-cement overlaid with a flexible underlay, a proprietary rigid air barrier or a flexible wall underlay meeting the specific requirements for an air barrier given in Table 23 of E2/AS1 – fixed to the outer face of the framing.

Under E2/AS1, unlined gable end walls must be fitted with an air barrier.

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Air seals

There is also a risk of this air flow occurring around holes or penetrations through the external envelope, such as windows, doors, pipe penetrations and meter boxes. Hence, an air seal should be fitted between the framed opening and the reveal of the window, door, pipe or meter box.

These air seals are a fundamental component for the weathertight performance of the cladding system.

Seals around windows and doors

Around doors and windows, the air seal must be installed at the inner face of the framed opening to create a complete seal between the framed opening and the window or door reveal. This seal works in conjunction with the plasterboard interior lining to form an air barrier, closing off air leaks to the interior and moderating the air pressure difference between the trim and frame cavity and the exterior.

The air seal is achieved by using an expanding foam sealant against a solid foam backing rod to the trim cavity. The air seal must not overfill the gap around the window or door joinery - the backing rod is necessary to ensure that the gap is not overfilled. (A number of different diameters will need to be available on site.) It needs to provide a complete seal around the opening. Packers for the window need to be installed so that they will allow a continuous bead of air seal (the backing rod can be fitted between packers). 

Weathertightness tests have shown that, when a wall is under pressure from wind, water will flow in around a window when a small portion of air seal is removed.

While a number of builders have said that they have sufficient control over the depth of the foam (when not using a backing rod), BRANZ does not believe that this is always the case. There have been instances where the foam is billowing out behind the window flange on the outside of the building. In these cases, not only is the gap around the window completely filled, negating the pressure moderation, but the foam could wick (move) water from the outside to the inside of the building. 

Seals around other cladding penetrations

These types of seals must also be incorporated around all cladding penetrations such as pipe penetrations and utility boxes.

The potential air flow through a window with no air seal from higher outside pressure to lower inside pressure.

Window with air seal.