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By Nancy B. Solomon, AIA

Meanwhile, growing concerns over indoor air quality encouraged significant air-exchange requirements that are frequently satisfied by purposely drawing outside air into the building. Unless this air is dehumidified before mixing with the already conditioned air, the extra moisture from the outside is often too much for the air-conditioning to handle, especially in hot, humid climates. If the warm moist air hits a cooler surface, such as the interior gypsum board of an air-conditioned room or the cold-water supply pipe in a ceiling plenum, the vapor from the moist air will condense and mold will form on the wallboard or on the ceiling tiles below the pipe. In its effort to save energy, the industry forgot about the properties of moisture.

 

Georgia Pacific’s DensGlass Gold Exterior Guard, a paperless glass-matted panel, has become a popular choice for exterior sheathing because it eliminates paper, a source for mold if the material gets wet during construction. Photography: Courtesy Georgia Pacific

This complex combination of multidisciplinary and sometimes competing interests and seemingly invisible forces unintentionally created a situation in which greater amounts of moisture were being trapped in inappropriate places for longer periods of time and coming into contact with more vulnerable materials, leading to mold and other health and construction concerns. And because HVAC systems provide such efficient distribution pathways, conditioned air that has become contaminated with mold has sufficient opportunity to spread the spores, odors, and potential toxins around for people to inhale, causing problems to multiply.

To make matters worse, certain standards and codes have been written and adopted that, in some regions, unintentionally exacerbate the moisture problem. According to Lstiburek, the state of Minnesota requires insulation and a vapor barrier on the interior of a basement wall, blocking the only direction in which the subterranean wall could possibly dry out; and in Miami, where the air in the attic is typically drier than the outdoors, ventilation systems originally designed to take moisture out of a humid attic in the north are inappropriately required. Lstiburek also believes that the amount of fresh air that is often brought into commercial buildings (currently stipulated at 20 cfm/person by ASHRAE Standard 62.1, “Ventilation and Acceptable Indoor Air Quality”) is too high: In hot, humid climates it actually brings too much moisture inside, thus causing the very problems that the standard intended to prevent. By stating that “some existing codes may inadvertently promote dampness,” the Institute of Medicine’s report supports Lstiburek’s contention that, at least in some regions, the moisture problem has become institutionalized.