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What causes condensation on windows?

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

Do windows cause condensation?

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

Impact of Temperature, Humidity and Glass Choice on Center-of-Glass Condensation

The graph below shows condensation potential on the center of glass area (the area at least 2.5" from the frame/glass edge) at various outdoor temperature and indoor relative humidity conditions. Condensation can occur at any points that fall on or above the curves. As the U-factor of windows improve, there is a much smaller range of conditions where condensation will occur. These values are based on center-of-glass temperatures. Condensation may occur at lower humidity levels on the glass edge.

Benefits: Less Condensation, Efficient Windows Collaborative, University of Minnesota

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 air's temperature 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 rails 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. As the exterior temperature drops, the humidity level needs to decrease if condensation is to be controlled .

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 louvers in your attic or basement 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.

To reduce humidity levels in your home:

  1. Use ventilation fans ventilation fans in kitchens and baths to control moisture.
  2. Your clothes dryer should be vented directly to the outside. Inspect the vent duct. Make sure it is attached securely to the dryer. Check that it is clear of obstructions (e.g. lint). Check for holes that leak air. If vent duct is damaged replace it with a metal duct. The vent duct should be cleaned at least once a year. The Consumer Products Safety Commission additional safety tips for dryer vents .
  3. If you have single pane windows, especially with metal frames, install storm windows or consider replacing your existing windows with ENERGY STAR labeled windows.
  4. If you can't afford to add storm windows or replace your windows now purchase and install a shrink film or polyethylene sheet, window insulation kit from a home center or hardware store.
  5. If you have a humidifier, check it regularly for proper operation. It could be adding too much moisture to your indoor air.

    Moisture on Window, Energy Star

Mold Control

  1. Moisture control is the key to mold control, so when water leaks or spills occur indoors — act quickly.  If wet or damp materials or areas are dried 24-48 hours after a leak or spill happens, in most cases mold will not grow.
  2. Clean and repair roof gutters regularly.
  3. Make sure the ground slopes away from the building foundation, so that water does not enter or collect around the foundation.
  4. Keep air conditioning drip pans clean and the drain lines unobstructed and flowing properly
  5. Keep indoor humidity low.  If possible, keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity.  Relative humidity can be measured with a moisture or humidity meter, a small, inexpensive ($10-$50) instrument available at many hardware stores.
  6. If you see condensation or moisture collecting on windows, walls or pipes act quickly to dry the wet surface and reduce the moisture/water source.  Condensation can be a sign of high humidity.

Moisture and Mold Prevention and Control Tips, A Brief Guide to Mold, Moisture, and Your Home, U.S. EPA, Office of Air and Radiation

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