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By Nancy B. Solomon, AIA

And our increased reliance on air-conditioning—even in northern states—means that the temperature gradient at an exterior wall can change dramatically over the year (in northern climates, colder air may be outside in winter but inside in summer), thus altering the dewpoint, or the position at which condensation forms, and the direction of vapor flow. Although it is still true that the vapor retarder, if needed, should go on the warm side of insulation, in some climates it becomes confusing as to which is the warmer side. Properly detailed, an air-conditioned building in Minneapolis is just as likely to have mold in the summer as one in Miami.

The scale was finally tipped after the oil crisis of the 1970s. In order to reduce fuel consumption, buildings were being better insulated. Insulation, however, reduces the ability of a wall to dry out. It can also shift the dew point within the wall to a point where, if not adequately drained or vented, mold growth and other water damage can occur.

 

A wall cavity in a building under construction in the Northeast developed mold because materials were not kept dry prior to installation and water was entering from upper stories as they were being built. Photography: Courtesy Environmental Health & Engineering

Around the same time, buildings were being fitted with more sophisticated, energy-efficient glazings, lighting systems, and appliances. These measures succeeded in reducing the heat load in buildings. However, because air-conditioning systems are typically oversized, “the cooling mode does not come on often enough or long enough to allow proper dehumidification,” explains Michael Garrison, associate professor of architecture at the University of Texas at Austin.

In addition, greater efforts were being made to seal buildings and maintain positive interior air pressure with respect to the outside air to prevent unwanted infiltration through inevitable small cracks in the envelope. This legitimate strategy, however, was often unwittingly compromised by localized pockets of negative pressure created, for example, by a bathroom exhaust fan or rooms that are not adequately served by fully ducted return air vents. Although the overall building may have positive pressure, these localized pockets could inadvertently draw in moist air from the outside via poorly sealed portions of the building envelope, often traveling long and circuitous distances via the complex network of relatively hollow wall cavities and plenums common in most buildings.