Maintenance and Operations

Common Causes—and Remedies—for Indoor Mold Contamination

The best way to minimize mold in a building is to know the most common places it may occur, develop routine inspection schedules, and follow procedures for ongoing maintenance. Most guidance information recognizes there are differences between “heating climates” such as during very cold winter months and “cooling climates” during the summer, but there are also many similarities when it comes to the biggest issue—managing indoor moisture levels.
moldy wall
Many buildings share common structural areas that are historically considered great mold incubators. Exterior corners are one such example, where mold occurs in both heating climates and in poorly insulated buildings in cooling climates.

The reason for this is that exterior corners are often closer in temperature to the outdoor climate due to poor indoor air circulation, exterior wind-washing, low insulation, and larger areas of heat loss.

The result can be condensation, moisture build-up and mold growth. Remedies include improving overall air circulation by removing obstructions, and installing forced air heating and ceiling fans.

Another problem area is in cooling climates when air conditioning blows cold air against the interior surface of an exterior wall creating a cold spot on the interior wall. When warmer outdoor air comes in contact with the inner wall areas of the cold spot, moisture is created and mold is able to develop.

Among the causes are poor duct design, diffuser location or diffuser performance which may be altered or relocated. In addition, other helpful activities include preventing hot air from contacting cold surfaces by controlling surface vapor pressure, ensuring proper choice and installation of vapor barriers, facing sealants and insulation, and moderating indoor temperature to avoid overcooling.

In rooms where heavy impermeable wall coverings are used, the problems caused by poorly diffused cool air can be exacerbated allowing mold growth to become extensive. Removal of impermeable wall coverings such as vinyl wallpaper can help to minimize moisture retention and mold growth potential.

Similarly, certain structural elements that conduct heat well, such as exterior frame steel studs, uninsulated window lintels and concrete slab edges, are known as “thermal bridges” and can cause localized cooling that results in the moisture condensation that is essential to mold growth. Minimizing the impact of thermal bridges can be accomplished by using insulated sheathings to reduce conduction of heat and cold.

Windows are another notorious condensation spot because they are typically the coldest surface in any room, and the first condensing surface, during the winter. Ironically, newer high-performance glazing systems that do not show condensation may be responsible for inattention to the presence of condensation in heating climates, while older windows that show condensation are a clear indicator of the need for better interior ventilation. Regular inspection of windows and window frame areas can help to identify areas of condensation and potential mold growth.

Another ironic problem is related to the use of thermal wall insulation, which keeps heat inside, but may allow inner wall surfaces or “first condensing surfaces” to remain cold, increasing the possibility of condensation within the sealed wall. There are, however, several ways to control this condensation, including one or both of the following:

  • Controlling the infiltration and exfiltration of moist air within the wall cavities, and
  • Elevating the temperature of the first condensing surface, such as by adding exterior insulation in heating climates and installing insulating sheathing to the interior of the wall framing and between the wall framing and gypsum board in cooling climate locations.


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