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What Causes Window Condensation?

What Causes Window Condensation?

What is Condensation?
Condensation is the fog that suddenly appears in cold weather on the glass of windows and sliding glass doors. It can block out the view, drip on the floor, freeze on's annoying! 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 only provides a cold surface on which humidity can visibly condense. The fog on your windows is a form of condensation; so is the water that forms on the outside of a glass of iced tea in the summer and on the bathroom mirrors and walls after someone takes a hot shower. Condensation usually occurs first on windows because glass surfaces have the lowest temperature of any of the interior surfaces in the house. When the warm, moist air comes into contact with the cooler glasssurfaces, the moisture condenses.
The important thing is, your foggy windows and sliding doors are trying to tell you to reduce indoor humidity before it causes hidden, costly problems elsewhere in your home - problems like peeling paint, rotting wood, buckling floors, insulation deterioration, mildew, even moisture spots in ceilings and walls.
Foggy windows and sliding doors are the indicators, the warning signs, that humidity could be damaging your home.
How can my home have indoor humidity?
Humidity is water vapor, or moisture in the air. All air contains a certain amount of moisture, even indoors.
Where does the moisture come from?
There are many things that generate indoor moisture; humidifiers will, heating systems, even plants. Cooking three meals a day adds four to five pints of water to the air. Each shower contributes half a pint. In fact, every activity that uses water (like dishwashing, mopping floors, doing laundry) adds moisture to the air.
More water vapor in the air means a higher indoor humidity. High indoor humidity means condensation.
How much indoor humidity is too much?
The householder can use the windows as a guide to the proper humidity level within the house. If objectionable condensation occurs on the inside surface of the windows, the humidity level is too high.
To avoid excess condensation, the following winter humidities are recommended in the house:
Outside-Inside Relative Temperature Humidity
  • -20 degrees F: 15 to 20%
  • -10 degrees F: 15 to 20%
  • 0 degrees F: 20 to 25%
  • 10 degrees F: 25 to 30%
  • 20 degrees F: 30 to 35%
(The indoor humidity can be checked with a sling psychrometer or humidistat)
Will reducing the humidity in my home during winter help control condensation?
Eliminate the excessive humidity and you will eliminate most (possibly all) the condensation.
How can I reduce the humidity in my home?
Control the source of moisture and increase ventilation.
As a temporary solution to an acute problem, open a window in each room for just a few minutes. Opening windows allows the stale, humid air to escape, and fresh dry air to enter. After a shower, for example, open the bathroom window or turn on the exhaust fan, so steam can go outside instead of remaining in the house.
Vent all gas burners, clothes dryers, etc. to the outdoors. Install kitchen and bathroom exhaust fans. If there are a large number of plants in the house during winter, concentrate them in one sunny room and avoid over watering.
Keep basements as dry as possible by waterproofing floors and walls. Run a dehumidifier if necessary. Make sure attic vents are open and unobstructed.
Opening the windows slightly throughout the house for a brief time each day will go far toward allowing humid air to escape and drier air to enter. The heat loss will be minimal.
Consult a local heating and ventilating contractor to help determine whether ventilation is adequate and whether it can be improved.
Ensure humidifiers are correctly set according to the outside temperature.
Installation of storm windows will often relieve condensation on the prime house windows by keeping the interior glass warmer.
Does condensation occur more often in particular climates or types of homes?
Absolutely! Condensation is more apt to occur in climates where the average January temperature is 35o or colder because there will be greater extremes between indoor and outdoor temperatures affecting the glass surfaces in the home.
During the summer and fall, all parts of a house pick up moisture from damp air. In the fall, when the windows are closed and heat is turned on, this moisture will pass into the air of the house. For a week or two there is likely to be 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 in the building materials used. Building materials need about one year to dry out, so excessive condensation can be expected in the first heating season. Even after the first year, if the humidity level is too high, condensation may still be a problem because today's homes are much "tighter" (in the interests of energy efficiency) than older homes. New materials and techniques in weatherstripping, insulation, vapor barriers, etc., which are intended to keep out cold air, also lock moisture inside. As a result, moisture created by bathrooms, kitchens, laundries, and occupants no longer flows to the outside unless mechanically ventilated.
Are there any cases where window condensation is only temporary?
Yes, there are primarily three.
  • New construction or remodeling
  • The beginning of each heating season
  • Quick changes in temperature
Wood, plaster, cement, and other building materials used in new construction and remodeling produce a great deal of moisture. During the heating season, there may be a significant amount of temporary condensation. After the first few weeks of heating, it will dry out, and you'll have less condensation troubles.
Sharp, quick drops in temperature can also cause temporary condensation problems during the heating season.
Why, if my old windows did not have condensation, do my new windows have it now?
One of the reasons you probably replaced your old windows was because they were drafty, and when the wind wasn't blowing in, those slight cracks allowed the excess humidity that is in your house to escape. With new, sealed-up windows, that humidity is unable to escape, therefore, it is showing itself on the glass.This is your first indication that you have too much humidity in your home.
The condensation resistance factor (CRF) of your new windows will also play a part in the amount of condensation that appears. The higher the CRF, the less condensation will appear relative to the humidity of your home.
You say I should have less humidity, but I have been told that more humidity is healthier.
At one time, it was believed that humidifiers helped people stay healthy during the winter months. Recent tests have shown that for usually healthy people, this is not the case. Infact, humidifiers may actually cause health problems. Additionally the Association of Home Appliance Manufacturers TM makes no medical claims for humidifiers. An Association spokesperson said, "We do not have the evidence of medical benefits. However," the Association said, "Manufacturers do maintain the humidifiers help plants and furniture only".
Does the amount of condensation depend on the type of window?
Sometimes. Recessed windows, like bay or bow windows, usually experience more condensation than other window styles. This is because air circulated around those window types is usually more restricted, and since they hang away from the insulated house wall, bays and bows could be a few degrees cooler in temperature. To diminish excessive condensation, the smart installer will insulate under the seat and head of the unit. As a secondary measure, placing a common electric fan near the window to produce air circulation may also be helpful.
Do drapes and window shades cause window condensation?
Drapes and other window coverings do not cause the window condensation, but they can contribute to the problem by restricting the flow of warm room air over the glass surface. Therefore, condensation is more apt to occur when drapes are closed and shades are pulled down. Today's heavily insulated drapes and tighter shades can contribute to the problem even more.
Remember......Vinyl windows resist condensation!
Windows with vinyl frames help guard against the damaging efforts of window condensation because of the higher insulating value of the vinyl. Also, double glazed windows are far more resistant to condensation than single pane windows. Single pane windows allow for a higher percent of the indoor relative humidity.


The Condensation Problem - Here are the causes and cares: Canadian Builder. VolXIII No 7
Condensation Problems in Your Home: Prevention and Solution: U.S. Dept of Agriculture Forest Services. Agriculture Info Bulletin NO 373
Technical Bulletin 01: Ethyl Corporation
The Condensation Answer Book: Anderson
Getting at the Source of Window Condensation: Vinyl Window and Door Institute

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What Causes Condensation?

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