Since moisture is a pivotal factor in determining mold growth, the humidity, condensation, and climate of an environment are all important factors to consider when evaluating mold safety in a building. The relative humidity of a room in a building is determined by the point of contact between the warm, moist internal air of a building and the cold, frigid air in the outside and is called the first condensing surface. Typically the first condensing surface is an outside-facing window in the room.

Ideal relative humidity levels depend on various factors, but keeping your building under 60% humidity at 70F is a safe bet to prevent mold. Relative humidity can be modified through structural changes in a building and mechanical de-humidification, though it’s important to note that the relative humidity near the first condensing surface could be higher than other places in a room.

When thinking about condensation and humidity, you should be asking yourself the following questions:

  • How often is condensation observed in the building?
  • Is the building properly insulated?
  • Are unheated areas, such as basements, susceptible to dampness caused by condensation?
  • Do the bathrooms and kitchens have adequate ventilation via windows and fans?
  • Is there sufficient ventilation in the attic to offset it’s susceptibility for condensation?
  • Is the heater turned down at night thereby increasing the relative humidity?

Mold and mildew can be an extensive problem when using a cooling system, just as it is with a heating system. For instance, poor duct design or diffuser issues can cause conditioned air to blow against the interior surface of an exterior wall. This can create a cold spot in the wall cavity that can be an optimal place for mold growth. This is more likely with materials like vinyl wallpaper that can trap moisture.

Common areas of elevated surface moisture include:

  • Exterior walls, especially corners
  • Areas where furniture is in direct contact with exterior walls, limiting airflow
  • Closets adjacent to exterior walls
  • Single pane or older style double pane windows. (Metal frame windows typically have increased likelihood of condensation)
  • Roof sheathing; look for sufficient ventilation in the attic area
  • Bathroom or kitchen ceilings where there is improper or unused ventilation
  • Wall cavities near the A/C unit.
  • Interior finishes with cold spots and exterior moisture
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