Relative humidity (RH) and condensation affect buildings in several ways.  In heating climates, such as the Pacific Northwest, condensation is regularly observed on windows, skylights and basement walls.  This occurs when the warm, moist interior air comes in contact with the colder exterior temperatures.  The point where this occurs is referred to as the first condensing surface.

The first condensing surface controls the moisture behavior of that particular room because it sets or limits the amount of absolute humidity that can exist.  Windows are usually the coldest surfaces in a room, therefore the place where you”re most likely to find condensation.

Relative humidity levels should be kept well under 60% at 70F within the home/building.  Structural changes and mechanical de-humidification may be necessary to maintain these levels.  It is important to note that although the relative humidity of a given area may not be excessively high, the RH near the first condensing surface could be at saturation levels.

The observation of condensation within your home or building should steer your investigation into several different areas:

  • How often condensation is observed within the home/building?
  • Is the house adequately insulated?
  • Are there unheated areas, such as a basement or closet?
  • Do bathrooms and kitchens have adequate ventilation via windows and fans?
  • Is there sufficient ventilation in the attic? (Attics are susceptible to condensation and should be carefully observed for surface moisture.)
  • Is the heater turned down at night thereby increasing the relative humidity?

The problems of mold and mildew can be as extensive in cooling climates as in heating climates.  A common example can be found in rooms where conditioned air blows against the interior surface of an exterior wall.  Poor duct design, diffuser location or diffuser performance issues can creat a cold spot at the interior finish surfaces and when outdoor air comes in contact with the cavity side of the cooled interior surface, a mold problem can occur within the wall cavity.  This condition is more likely in rooms decorated with low maintenance interior finishes, like vinyl wallpaper, that can trap moisture between interior finish and gypsum board.

Also, when interior finishes are coupled with cold spots and exterior moisture, mold growth can be rampant.

Common areas of elevated surface moisture include:

  • Exterior walls, expecially 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.
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