Technical Updates

In the summer of 1988, construction of a large luxury resort was coming to a close. Because the vinyl wall covering on the interior side of the exterior walls had an impermeable finish, it functioned as a vapor retarder (also referred to as a vapor barrier). The HVAC system consisted of a continuous toilet exhaust and packaged terminal air-conditioner (PTAC) units. The outside air exchange rate in each guest room averaged six times an hour, all from infiltration. In this case, problems developed inside the building and inside the wall.

The combined effect of excessive outside air infiltration and an improperly located vapor retarder caused $5.5 million in moisture and mold damage, even before the facility was opened. If these same design combinations had occurred in a more temperate climate, the problems would have been limited to increased energy consumption and possibly to complaints about guest comfort.

This is one example of how hot, humid climates present unique challenges that are often overlooked by the design and construction community. However, challenges also occur for buildings located in other climates. Meeting these challenges depends on understanding a building’s local climate conditions and how those contribute to IAQ problems.

Cold climates offer challenges for moisture flow through the building envelope that are similar to those in hot, humid climes. Cold climates are defined by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) as those that experience at last 4,000 heating degree days (HDD at 65 degrees Fahrenheit [18 degrees Celsius] base) per year. Most problems occur during the winter, when the warm and relatively moist interior air is forced (due to high differential vapor pressures between indoors and outdoors) to the dryer and colder outdoor conditions. Moisture flow can be trapped and condensed on an improperly located vapor retarder. In addition, if the building is air-conditioned during the summer, the wall systems designed to address the heating condition can experience moisture damage inside the walls during the air-conditioned months. Therefore, few locations in the United States are completely free of potential moisture problems.

To be continued…