Building envelope essentials: The role of a water-resistive air barrier

By Paul Johannesson

As climate change is anticipated, bringing with it an uncertain future, the construction industry needs to seriously consider the preparedness and the proven performance levels of buildings. It also needs to ensure the “large membrane” that separates the exterior environment from the interior environment, also known as the building envelope, is up to the task.

The idea of building resilience has become more topical as everyone comes to terms with the fact that changing climate conditions seem inevitable. The Intergovernmental Panel on Climate Change (IPCC) states, “Taken as a whole, the range of published evidence indicates that the net damage costs of climate change are likely to be significant and to increase over time.”1

Will there be an increase in both the frequency and intensification of severe weather events?  Will there be a need to resist increased water, wind, and temperature extremes in the exterior environment? Will shifting the focus from the approach prescribed within the current codes and standards to a higher performance-based design and commissioning approach be the answer?

The building envelope

The building envelope can be imagined as a “thick membrane” that separates the exterior environment from the interior environment on all sides of a building—similar to a balloon. It has been mistakenly conceived as one or two layers of perfectly sealed materials; and there exists a history of building failures to prove it. It is best described as “part of any building that physically separates the exterior environment from the interior environment(s) is called the building enclosure or building envelope.”2

It is important to consider that this envelope or enclosure is a very complex system of base materials, accessories, pre-manufactured components, riddled with seams and penetrations, and is highly reliant on a systems approach of connecting them to create a weather tight separator of dissimilar environments. These environments are often dissimilar in temperature, relative humidity, wind pressure, and vapour pressure. Each building envelope deals with differences as unique as the climate region or zone it is constructed in. Thankfully, a commonly accepted approach to building envelope and enclosure design has emerged over the years.

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