The importance of air seals

Greater understanding of how to incorporate air seals in openings and penetrations in the exterior envelope of a building is necessary to prevent wind-driven water from entering

It has been 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 to the wider industry, resulting in more resilient and higher-performing residential dwellings. However, design and building practitioners frequently do not fully understand the rationale behind some of the requirements in critical performance areas.

One area of concern is incorporating air seals in openings and penetrations in a building’s exterior envelope. This article examines why air seals are necessary and the key aspects of their 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 airflow or leakage path from the exterior of the building to the interior—moving 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, doors, and other openings—such as meter boxes—run continuously through the wall assembly from the exterior to the interior. This creates the potential for water to be driven into the interior. This risk also exists with pipe and service penetrations through the exterior envelope.

Pressure moderation

To negate this pressure-driving effect, it is necessary 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. However, at this point, it is blocked from entering the building’s interior by the air seal.

The air within the trim cavity then moderates to match the external air pressure, negating the pressure differential and eliminating the potential for water to enter the building’s interior through the trim cavity.

E2/AS1 air seals for exterior joinery and 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.

For meter boxes, the seal must close the gap between the rough opening frame and the actual body of the meter box within the framing. The seal should 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 or sealant is installed, it does not migrate further into the trim cavity, where it could come into contact with water on the exterior face of the wall framing behind the cladding or window flange. This could cause the seal to deteriorate or allow water to wick into the building’s interior.

Both the seal and backing rod must be accurately installed to ensure continuity with no gaps. Even small gaps can create an air or water leakage path. Joinery packers need to be installed to allow a continuous run of air seal around the perimeter, with the backing rod running between any packers.

E2/AS1 air seals for pipe penetrations

Exterior pipe and service penetrations must be installed before the cladding. The penetration must be taped with flexible flashing tape around its circumference, sealing it to the flexible wall underlay or rigid air barrier on the outside face of the framing.

Air seals must be installed around all pipe and service penetrations, effectively sealing the gap between the penetration and the external cladding. The sealant used must be compliant and precisely applied. Following this, a flange plate is installed over the penetration and sealed to the outer face of the cladding.

Building envelope airtightness

Another benefit of air seals around openings in the exterior envelope is their contribution to the overall airtightness of the building. Although this may not have been a key consideration when E2/AS1 was introduced, sealing these air leakage paths significantly impacts the ability to manage the building’s internal temperature.

Eliminating air leakage through the exterior envelope also improves energy efficiency by preventing heat loss during colder months and heat gain in warmer months. This ensures that heating and cooling systems function more efficiently, maximizing the benefits of increased insulation.

The key to effective air seals in exterior penetrations lies in the accuracy of installation. There is evidence that, at times, seals are being poorly installed. The challenge is to understand the importance of these seals and ensure they are installed with the necessary precision.

 

Article by Greg Burn, Building Consultant, Structure Limited, Auckland. This article was first published in Issue 204 of BRANZ Build magazine. www.buildmagazine.org.nz