Use this tag for questions that involve the unwanted appearance of condensate or liquid water from an air stream that contains water vapor. Most often water condenses out of a warm air stream containing water vapor due to a change to a lower air temperature or the air stream passing over a cool or cold object such as an HVAC evaporator coil.

Condensation is when the water vapor suspended in an air stream becomes droplets of liquid water.

Common examples are:

  • water droplets condensing on a container of a cold beverage on a warm, humid day
  • steam condensing on a cold ceramic floor or mirror in a bathroom
  • warm, moist air flowing into a refrigerated space causing mist to form in the air of the refrigerated space
  • ice that forms in a freezer due to humid air which flows into the freezer when the door is open or ajar or a seal is damaged

Condensation in building construction is an unwanted phenomenon as it may cause dampness, mold health issues, wood rot, corrosion, weakening of mortar and masonry walls, and energy penalties due to increased heat transfer. To alleviate these issues, the indoor air humidity needs to be lowered, or air ventilation in the building needs to be improved. This can be done in a number of ways, for example opening windows, turning on extractor fans, using dehumidifiers, drying clothes outside and covering pots and pans whilst cooking. Air conditioning or ventilation systems can be installed that help remove moisture from the air, and move air throughout a building.[8] The amount of water vapor that can be stored in the air can be increased simply by increasing the temperature.[8] However, this can be a double edged sword as most condensation in the home occurs when warm, moisture heavy air comes into contact with a cool surface. As the air is cooled, it can no longer hold as much water vapor. This leads to deposition of water on the cool surface. This is very apparent when central heating is used in combination with single glazed windows in winter.