It’s more than 30 years since E2/AS1, the Acceptable Solution for proving compliance with Building Code clause E2 External moisture, was introduced and widely adopted by the industry.
Increased understanding of weathertightness
E2/AS1 can be credited with bringing a new understanding of weathertight design and construction principles and practices to the wider industry – resulting in more resilient and higher-performing residential dwellings.
Frequently, however, design and building practitioners don’t fully understand the rationale behind some of the requirements in critical areas of performance. One area of concern is incorporating air seals in openings and penetrations in a building’s exterior envelope.
Here, we look at why air seals are necessary and the key aspects of installation.
Air pressure
Even in situations with relatively low wind speed, wind acting on a building creates higher air pressure on the building’s external envelope compared to the pressure on the building’s interior.
If there are any gaps in the exterior envelope, no matter how small, this pressure differential can create an air flow or leakage path from the exterior of the building to the interior – from high to low pressure.
When rainwater is present on the building’s exterior, it can be driven by the pressure differential along any air leakage path, which then becomes a water leakage path.
Some water leakage paths may carry water into the exterior wall assembly, but gaps around windows and doors and other openings such as meter boxes run continuously through the wall assembly from the exterior to the interior. They create the potential for water to be driven into the interior. This potential also exists with pipe/service penetrations through the exterior envelope.
Pressure moderation
To negate this pressure-driving effect, we need to moderate the air pressure within the trim cavity around exterior window and door joinery. This requires air seals to be installed in the trim cavity.
Installing an air seal at the interior face of the exterior wall framing allows higher-pressure air from the exterior to enter the trim cavity, but at this point, it is blocked from entering the building’s interior by the air seal.
The air within the trim cavity moderates to that of the external air pressure, negating the driving effect of the pressure differential and eliminating the potential for water to enter the building’s interior through the trim cavity.
E2/AS1 air seals to exterior joinery/meter boxes
Air seals need to be installed around the entire trim cavity, sealing off the gap between the rough opening frame and the reveals of the exterior joinery. With meter boxes, the seal needs to seal off the gap between the rough opening frame and the actual body of the meter box within the framing. The seal needs to be located on the internal line of the exterior framing.
Seals must be formed with either a self-expanding polyurethane foam or a compliant sealant, which is installed over a polyethylene foam (PEF) backing rod.
The backing rod ensures that, when the foam/sealant is installed, it does not migrate further into the trim cavity where it can come into contact with water on the exterior face of the wall framing behind the cladding/window flange.
This could cause the seal to deteriorate or water to wick into the building’s interior.
Both the seal and backing rod must be accurately installed so they are continuous with no gaps. Even small gaps can create an air/water leakage path. Joinery packers need to be installed to allow a continuous run of air seal around the perimeter. The backing rod can run between any packers.
E2/AS1 air seals to pipe penetrations
Exterior pipe/service penetrations must be installed prior to cladding installation. The penetration must be taped with flexible flashing tape around its circumference to the flexible wall underlay or rigid air barrier on the outside face of the framing.
Air seals need to be installed around all pipe/service penetrations, effectively sealing off the gap between the penetration and the external cladding. The sealant used needs to be compliant and accurately installed. This is then followed by the installation of a flange plate over the penetration, which is then sealed to the outer face of the cladding.
Building envelope airtightness
Another benefit of air seals around openings in the exterior envelope is that they contribute to the overall airtightness of the building. Although this may not have been a consideration at the time E2/AS1 was introduced, sealing off these air leakage paths has a large impact on our ability to manage the temperature of the building’s internal environment.
Eliminating air leakage through the exterior envelope also improves a building’s energy efficiency as it removes the potential for heat loss in the cooler months and heat gain in the warmer months. This ensures that heating and cooling of the building are more efficient and that the benefits of increased insulation are realised.
The key to effective air seals in exterior penetrations is directly related to the accuracy of installation of the seals. There is evidence that seals are, at times, being poorly installed. The challenge is to understand the importance of these seals and give due regard to their accurate installation.