Why do Windows Fog?

• Windows "fog" when the temperature of the glass falls below the dew point of the air that is in contact with the window. All air holds moisture, and warmer air can hold more moisture than colder air. If the air falls below a certain temperature, called the dew point, the moisture will condense. Because your lemonade glass is colder than the dew point temperature on a summer afternoon, water condenses out of the air and onto the glass.
  
  

Fogging on the Inside or the Outside

• Under certain conditions, moisture will frequently fog on either the inside or outside pane of a window.
  
  If you take a hot shower, moisture can condense on the cool bathroom window and mirror. You can prevent this by using the exhaust fan while the shower is running. The inside panes of windows can fog under other conditions that cause the air inside to be warm and moist while the outdoor air is cold; for example, boiling water on the stove may cause condensation to appear on some windows. Rarely, windows on a newly constructed home will fog as the building materials are drying.
  
  If a cool evening follows a hot and humid day, frequently there will be moisture condensing on the outside panes of a window, as the glass will cool faster than the surrounding air. If there is condensation on the outside of a window while the air conditioner is on, that is a sign that the window is leaking a lot of your air conditioning dollars to the neighborhood.
  
  

Fogging Between the Panes

• If the condensation appears between the two panes of glass, it means that the window is damaged. The space between the two panes is supposed to be sealed. Over time with thermal expansion and contraction, the seals can become brittle and crack. When the seal cracks, air and the moisture it is carrying can get into the gap between the panes. This moisture will condense on the cooler pane of the window.
  
  If you are unable to physically touch a window, you need to examine the environmental conditions to figure out whether the condensation is between panes or not. Moisture will always condense on the side of the glass that is colder than the air surrounding it. During the winter, if you can't touch the moisture, it is between the panes. During the summer, condensation will appear either between the panes or on the outside of the window.
  
  

Can the Fogging be Fixed?

• In theory, a window with moisture between the panes can be fixed. You would have to remove one pane, remove the failed seal, remove moisture, install a new seal, make sure that it is airtight and re-install the window pane. This repair procedure requires a lot of specialized tools and skills, so it should be handled by a trained glazier. Some window manufacturers sell the parts to repair their windows. Other windows use custom gases, like Argon, or a partial vacuum between the panes and cannot be repaired.
  
  Depending on the age and quality of the windows, it can be less expensive in the long run to replace the windows than to repair them.