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The cause of condensation on windows


Photo of toy house wrapped in striped scarf.

Weatherizing a home can sometimes cause problems with condensation.

A customer recently e-mailed us about condensation on new windows placed in the east side of his home. The window contractor, Tom Heslin of Integrity Glass in Chelan, explained that the condensation is due to humidity and can occur when new windows and/or insulation have been installed.

“The home has been made weather-tight and no longer is drafty or breathes like it used to before it was weatherized properly,” Heslin said.

Heslin referred to a Q and A about condensation on the website of Soft-Lite Gorell, an Energy Star award-winning manufacturer of windows and doors. The information is reprinted below with thanks to Heslin and Soft-Lite Gorell.

What causes exterior condensation?

Exterior condensation occurs when moist air comes into contact with cool surfaces, such as glass. This type of condensation appears when the dew point in the air is higher than the temperature of the glass. This occurs when a cool night follows a warmer day, most typically during the spring and fall seasons.

How does low-emissivity (Low-E) glass affect exterior condensation?

Low-E glass reduces heat conducted through the glass from the warm interior of the home to the outside glass surface. Heat conduction can be reduced by as much as 50 percent with an efficient Low-E coated glass. This reflected heat energy reduces the outside glass temperature and can result in condensation on the glass. Exterior condensation is actually an indication that the insulating glass in the window is performing as it should.

What causes condensation on the inside glass of windows?

Whenever there is excess humidity in a home, it manifests itself in the form of condensation on the coldest area of a wall, which is normally the windows. The warmer the air, the more moisture it will retain, so when air in your home comes in contact with the colder glass surface, it is subsequently cooled and moisture is released in the form of condensation on the glass.

Do windows cause condensation?

No, condensation on windows is not the fault of the window. However, by replacing drafty windows and doors or installing a new roof or siding, you are reducing air flow in your home and making it tighter. Tighter homes actually retain more humidity.

Where on a window does condensation normally form and why?

Condensation often forms at the meeting rail and at the bottom of the lower sash on the interior of the glass. This is because when warm air cools, it falls down across the interior surface of the window at the same time the temperature of the air is falling. The air contacts the horizontal surface of the meeting rail, which acts like a dam, slowing the air’s rate of fall and creating the perfect opportunity for the trapped water vapor to escape and form on the meeting rail’s surface. The air then rolls over the edge of the meeting rail and again gains speed until it encounters the lower handle of the sash. At this point, the water vapor again makes its exit and lies at the bottom of the sash.

Can I reduce the condensation on my windows?

Yes. In order to reduce condensation, humidity must be controlled and air movement must be generated.

What steps can I take to reduce humidity in my home?

The two main things you can do are to control sources of moisture and increase ventilation. To decrease or control excess humidity and condensation:

  • Use exhaust fans in your kitchen, laundry and bathrooms.
  • Vent gas burners, clothes dryers, etc. to the outdoors.
  • Shut off furnace humidifiers and other humidifying devices in your home.
  • Be sure that the ventilating louvers in your attic, basement or crawl spaces are open and amply sized.
  • Open fireplace dampers to allow an escape route for moisture-laden air.
  • Air out your house a few minutes each day.

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A comment from Mark Wiser, senior residential energy adviser:

Good discussion on condensation, which with the change in seasons, is becoming pretty prevalent in Chelan County. There are a couple points I’d like to comment on for our customers’ consideration. First of all we don’t recommend opening fireplace dampers to help remove excess moisture in the air. Opening a fireplace damper creates uncontrolled heat loss from the home, which may increase as outside temperatures decrease. A better strategy is to install a whole house exhaust fan on a timer (many newer homes already have them). These are often placed in utilitiy rooms. With this system, a homeowner can adjust the time the fans are on in order to exhaust only enough inside air to eliminate moisture buidup. The homeowner can increase or decrease as needed, thus reducing heat loss while controlling moisture. A whole house fan should be operated a couple hours a day under normal conditions as well, simply to exhaust stale inside air and replace with fresh.

Often condensation on inside windows and walls occurs first in a bedroom on the north side of a home. This is typically the coldest room in the home. The condensation occurs behind a closed curtain or behind furniture or the bed on the north wall. Sometimes this can be eliminated simply by opening the curtain or moving the furniture away from the wall. This allows air to move before condensation appears. Please feel free to contact me at 509.661.4382 or at mark.wiser@chelanpud.org – Mark Wiser 

  1. Mark Wiser says:

    Good discussion on condensation, which with the change in seasons, is becoming pretty prevalent in Chelan County. There are a couple points I’d like to comment on for our customers’ consideration. First of all we don’t recommend opening fireplace dampers to help remove excess moisture in the air. Opening a fireplace damper creates uncontrolled heat loss from the home, which may increase as outside temperatures decrease. A better strategy is to install a whole house exhaust fan on a timer (many newer homes already have them). These are often placed in utilitiy rooms. With this system, a homeowner can adjust the time the fans are on in order to exhaust only enough inside air to eliminate moisture buidup. The homeowner can increase or decrease as needed, thus reducing heat loss while controlling moisture. A whole house fan should be operated a couple hours a day under normal conditions as well, simply to exhaust stale inside air and replace with fresh.

    Often condensation on inside windows and walls occurs first in a bedroom on the north side of a home. This is typically the coldest room in the home. The condensation occurs behind a closed curtain or behind furniture or the bed on the north wall. Sometimes this can be eliminated simply by opening the curtain or moving the furniture away from the wall. This allows air to move before condensation appears. Please feel free to contact me at 509.661.4382 or at mark.wiser@chelanpud.org – Mark Wiser, Senior Energy Advisor, Chelan County PUD

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