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Questions & Answers To Demystify Condensation

What is condensation?

Condensation is the fog that suddenly appears in cold weather on the glass of windows and doors. It’s the formation of moisture on the inside surface of the window. It can obstruct the view, drip on the floor, freeze on glass. It’s natural to blame the windows, but you shouldn’t.

What causes window condensation?

Window condensation is the result of excess humidity in your home. The glass provides a cold surface on which humidity can visibly condense. The fog on your windows is a form of condensation similar to the water you find on the outside of a glass of fresh water in summer. Condensation usually occurs first on windows because glass surfaces have the lowest temperature of any of the interior surfaces in the house. When condensation occurs on your windows and doors, it’s a sign that you should reduce indoor humidity before it causes problems in your home like mildew, damaged paint surfaces, rotting wood, or moisture spots.

Why does my home have indoor humidity?

Moisture is naturally present in the air in the form of vapor and it condenses when in contact with cold surfaces.

Where does the moisture come from?

Many factors can generate moisture; heating system, humidifiers and plants. Everything from cooking to bathing to watering the plants and doing laundry adds moisture to the air.

My old glass or windows didn’t have condensation, why do my new glass or windows have some?

It is common for homeowners who replace windows or glass to suddenly start having condensation. This is because they now have windows that are airtight and that make the excess humidity impossible to escape. Your old windows or glass were probably draughty and when wind was blowing in, the slight cracks were allowing the excess humidity in your house to escape. The new windows are now indicating that you have excessive moisture in your home. Moreover, wood, plaster, cement, and other building material used in new construction and remodeling produce a big amount of moisture. When the heating season begins, there may be a certain amount of temporary condensation that appears. Sharp quick drops in temperature can also create temporary condensation problems during the heating season.

How can I reduce the humidity in my house?

  • Opening a window for a short time will ventilate the house and dilute the humidity level. The heat loss will not be important.
  • Install kitchen and bathroom exhaust fans.
  • Control the source of moisture and increase ventilation
  • Limit the use of humidifiers
  • Dehumidifiers can be used if necessary

Can condensation be temporary?

Yes in the following cases:
  • New construction or remodeling
  • At the beginning of each heating season
  • When quick changes in temperature occur

Recommended Winter Humidity

Outside Temperature

-20°F (-29°C)
-10°F (-23°C)
0°F (-18°C)
10°F (-12°C)
20°F (-7°C)  

Inside Relative Humidity

15 to 20%
15 to 20%
20 to 25%
25 to 30%
30 to 35%


  • It’s simple. If condensation occurs on the inside surface of the windows, the humidity level is too high.
  • Foggy windows and doors are an indicator that humidity could be damaging your home.
  • More water vapor in the air means higher indoor humidity. High indoor humidity means condensation.
  • During the first year after construction or remodeling, it is likely a house will have more condensation present because of the massive amount of moisture present in the building materials used

Talk to a Glass Specialist