What is condensation?

Condensation generally occurs inside the home when it is freezing outside, and the humidity level inside the house is at a higher level than recommended. When it gets frigid out, the humidity level inside needs to be reduced accordingly to avoid condensation.

Condensation typically develops on cooler surfaces inside the home, such as astragals, hinges, hardware, and glass. These surfaces are cooler because they are closer to the outside than a wall or table, and excess moisture condensates on these surfaces. Moisture in air is known as humidity.


So where is the water coming from?

An important fact to remember is the surface where condensation has formed is not obtaining water from outside the home, nor is it making its moisture; instead, it comes from the environment inside the house. The higher the humidity percentage inside the house, the quicker a cooler surface will begin to condensate. Air temperature, combined with humidity level, defines a temperature at which condensation will form. This is called the Dew Point Temperature. If condensation is developing, then the humidity percentage must be reduced, so the resulting Dew Point Temperature is below the temperature of the cooler surfaces.



Understanding Dew Point and Temperature

For instance, if the air inside is 70° F with 45% humidity, then any surfaces cooler than 48° F (calculated) will condensate. If it’s 15° F outside, then inside surfaces such as hinges or hardware, would be approximately 30 degrees warmer, or about 45° F. In this example, the 45° part temperature is below the 48° F temperature where condensation will happen, and condensation will form. If the humidity percentage is lowered to 30%, then the part temperature would need to be around 37° F (calculated dew point temperature) to condensate.


Think about a glass of ice water with condensation on the outside.

Why is there moisture on the outside of the cold glass, yet the glass is not leaking?  This is because the air next to the chilled glass has cooled to the Dew Point Temperature, and the air cannot hold any more moisture, which will cause condensation to form on the outside surface of the glass.



So what causes moisture in the home?

Moisture in the home can be caused by people taking showers, cooking food, doing laundry, or when a large group of people is in a room together. With closed curtains where air movement is restricted, the air closest to the window remains cool and can also condensate. Elevated moisture levels can also lead to mold and rot in unseen areas of the home. Reducing the humidity percentage inside the home when it’s cold outside usually solves the problem. Higher humidity may not cause an issue when outside air temperatures are 30° and higher, but when the outside temperatures drop below freezing to near zero while maintaining the same inside level of humidity, condensation will be an issue. Differences in inside and outside air temperatures, combined with different humidity levels inside, will determine when condensation occurs. If moisture is collecting on the inside glass, hinges, etc., identify and reduce the cause of the excess moisture in the air. An example of elevated humidity is a fogged mirror in a bathroom after someone takes a shower. The air is in an enclosed space and is saturated with moisture from the shower and then lays on adjacent surfaces that are cooler. Once the door is opened, the excess moisture will mix with the dryer air outside, and the condensation will go away.



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Learn what causes condensation on your windows and what you can do to avoid it



Remember putting a pot of water on ra­diators to add humidity to the air in win­ter? This was a common practice in the days when homes were built with wood board sheathing and sometimes no insu­lation.  Today, builders use panel prod­ucts for sheathing, and walls and ceilings that are insulated. Houses are "tight" and prone to moisture problems rather than difficulties with dryness.  This also includes windows.

Understanding Window Condensation

Watch our video to learn what causes condensation on your windows and what you can do to avoid it.

Condensation can form on interior glass surfaces when there is too much moisture in the air. If the interior of a structure exceeds certain limits of moisture in the air, the moisture will condense and show up on comparatively cooler surfaces, such as glass.

Windows are typically the coolest areas of interior walls; even if they have storm panels, are glazed with welded insulating glass, have Low-E4® insulating glass, or use triple pane glass. When the warm, room temperature air comes in contact with the glass surface, the air is cooled and if there is enough moisture in the air, the dew point will be reached and the water in the air will condense. A good analogy is when you have an iced drink on a warm summer day, and the glass has moisture on the outside of it. The warmer air meeting the cooler surface of the glass causes condensation to form. Recommended humidity levels in winter months should not exceed 30-35%. If these humidity levels are exceeded, you may want to take measures to reduce the interior humidity level such as:

  • Checking your ventilation
  • Using a dehumidifier
  • Turning the humidifier on your furnace down (or off)
  • Making sure blinds or curtains are open during the day
  • Leaving ceiling fans on to promote air movement
  • Use an exhaust fan in bathroom areas when showering

Additional information can be found in the Guide to Understanding Condensation.

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