Requirements for a High-Performance Building Envelope

What is Leaky Building Syndrome?

The first priority of a building envelope is to stop water from leaking into your facility.

The second is to stop air from leaking into and out of your building. In a leaky building, large volumes of air can pass into and out of a building due to wind pressure, stack effect and exhaust fans.

The effects on a building suffering from Leaky Building Syndrome can be troublesome to owner and occupants – expensive for owners, and uncomfortable, unsightly, and in some extreme cases even dangerous for occupants – because of mold, deterioration, rot and undermining the building’s integrity. For these reasons, it is paramount that good practices are followed in the design and construction of a building’s envelope.

What are the Mechanics of a Leaky Building?

Wind creates a positive pressure on the windward face and negative pressure on the non-windward (leeward) facing walls, which pulls the air out of the building. Wind causes infiltration on one side of a building and exfiltration on the other. Wind effects can vary by surrounding buildings, terrain, shrubs, and trees.

Stack pressure (or the chimney effect) can also push large quantities of air through an envelope. In the summer, hot moist air outside an air-conditioned building can push the cooler indoor air down from upper floors and out through defects in the envelope at the base of the building. In the winter, the reverse occurs. Warm air in a heated building is lighter (less dense) than the cold air outside. The warm air wants to rise up and out the building through defects in the exterior envelope at the top of the building. As in a chimney, the warm air leaving the top of the building draws cold air in from the bottom of the building.

HVAC fan pressure can also contribute to air leakage. Mechanical equipment such as fans and blowers cause the movement of air within buildings and through enclosures which can generate pressure differences. If more air is exhausted from a building than is supplied, a net negative pressure is generated, which can induce unwanted airflow through the envelope into the building. It is a general practice to keep your buildings under positive pressure to reduce the potential for leakage into your facility. Air leaking into the building typically contains water vapor which can cause condensation, mold and corrosion of building elements.

How does fresh air get into a well-sealed building?

Ideally, fresh air should not come from leaks in the building envelope but should enter the building through the HVAC system so it can be conditioned prior to being circulated throughout the building. For this to occur, the envelope needs to be tight which will require well-designed and constructed glazing systems and an effective air and moisture barrier in the remaining areas where there is no glazing.

How can Building Owners Prevent Such Costly Mistakes?

As a standard practice, ZCA requires exterior envelope assemblies to be field-tested for water and air infiltration by a third-party consultant. This quality control measure is absolutely necessary to ensure the envelope is constructed per manufacturer requirements and Industry Standards for the high-performance buildings that are necessary for today’s healthy, efficient and equitable green buildings and communities.