To maximize indoor air quality, minimize air leakage

It's a four-letter word no building owner wants to hear: mold.

Yet, it's an all-too-common occurrence in buildings where moist air accumulates on vulnerable wall components, including insulation, exterior sheathing or interior wall boards. This building envelope penetration can dramatically impact indoor air quality and occupant health.

According to the Centers for Disease Control (CDC), exposure to damp and moldy environments can cause a variety of health issues, from irritation of the throat, eyes and skin to upper respiratory tract symptoms; people with mold allergies may experience more severe reactions.

Air leakage is a major cause of moisture intrusion, setting up the conditions for mold growth. Even a relatively small void in a wall surface can allow a lot of moisture into a building. Consider this: Air leaking through a one-inch hole in a 4-by-8-foot sheet of gypsum board can carry with it 30 quarts of water during a single heating season; the same gypsum board with no hole will allow just 1/3 quart of water to pass through via vapor diffusion.2 This moisture can collect in wall structures. When the relative humidity (RH) of these areas reaches 80% to 95% in warm temperatures it can create the ideal conditions for mold to grow.

A properly installed, continuous air barrier can minimize this leakage and dramatically reduce moisture intrusion through wall structures. "Continuous" means that the air barrier must be solidly adhered to the substrate, with any seams around windows, doors and other penetrations properly detailed and flashed. The goal is to deny the outside air—and the moisture it contains—any point of entry.

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What about moisture generated inside the building? Under some environmental conditions, it may be desirable to have a wall that "breathes"—allowing moisture vapor inside the building to escape, while resisting outside air leakage. This calls for a vapor permeable air barrier.

In cold climates with little exterior insulation, it may be preferable to use a vapor impermeable air barrier. This acts as a vapor barrier as well as an air barrier, minimizing the risk of water vapor condensing within the external wall.

Which type of air barrier is right for your location and wall structure? Answering that question definitively may require a hydrothermal analysis. This analysis looks at a number of factors, including climactic conditions, the location of insulation relative to the stud cavity (inside, outside or both), and how absorptive the cladding material is.

With the right air barrier, properly installed, you can minimize your chances of hearing that dreaded four-letter word: mold.