As building experts have often pointed out, windows are not to blame for condensation.  The moisture content of the inside air is the key to both the cause and the cure.
  Windows do not cause condensation, they are merely indicators.  Condensation problems arise because air can hold only a limited amount of water vapor, an amount that varies with temperature.  When air at a certain temperature contains all the water vapor it can hold it is said to have a relative humidity of 100 percent.  The moisture (water vapor) is invisible. When  air is warm, it holds more moisture than it can when it's cold.  As the temperature falls, relative humidity will continue to rise until the dew-point is reached - that is, the temperature at which the relative humidity becomes 100 percent.  Any further decrease in temperature will force some of the vapor to condense as water. 
  Air cooled by contact with cold surfaces will therefore deposit some of this water vapor on the surface whenever it has more water vapor than it can hold at its new temperature.  Condensation usually occurs first on windows because they have the lowest temperature of any of the interior surfaces in the house.
  When windows and window frames in a house show signs of sweating, fogging or frosting, there is only one reason:  TOO MUCH MOISTURE IN THE AIR INSIDE THE HOME!
  Condensation is actually a safety gauge.
Sweating windows indicate that there is excessive moisture inside the home trying to get it out.  It may be a warning signal of dangerously high humidity - humidity that can cause warp, rot, mildew, and paint failure.  It can force its way through wood, plaster, brick and cement.  It can be a source of discomfort and can cost you a great deal of money by deteriorating your home. 
  Moisture on windows also proves that you have a tight home.  Condensation is a modern problem - older "loose" homes need added humidity because the free inflitration of dry air from outdoors keeps the moisture level low.  New methods of construction create a vapor barrier that holds in the moisture released into the air buy bathing, cooking, washing and other home activities.
  Remember the first sign of excess moisture is the fogging of windows. Humidity does not have to be measured directly; simply use the window as a guide to the proper humidity level within the house. As soon as objectionable condensation occurs on the inside surface of the window, steps should be taken to reduce the relative humidity by controlling the moisture source or by increasing ventilation.
Humidity should be controlled so that little or no condensation appears on the inside surface of the glass. When outdoor temperatures are moderate, rather than very cold, a higher ventilation rate may be required to control humidity because outside air brought in at moderate temperatures contains more moisture than very cold air would.

Please Don't  Blame Your Windows for Winter's Condensation

 CUT condensation by keeping humidity to realistic levels...HOW?
Learn to recognize how moisture fills the air in your home:
 The principal sources of moisture in a typical home are the household activities which vary with the living habits of the family.  For instance, cooking for a family of four adds 4.5 pounds of moisture a day to a home.  Each shower contributes 1/2 a pound; weekly laundry adds 30 pounds; human occupancy adds 6 to 8 pounds  a day; and dishwashing adds 1.2 pounds.  Gas appliances will add moisture from the water vapor which is one of the products of combustion when gas is burnt.  Large fish tanks, watering of plants, and humidifiers are other common sources of moisture.
  Figures show that the modern living of a family of four can easily add 150 pounds, or more than 18 gallons of water per week, to the air in the home.  All of this moisture must eventually escape.
And learn to help it out.
  Natural air leakage is a major controlling factor which prevents excessive humidity within a home.  Leakage takes place through cracks around windows and doors, vents for furnaces and water heaters, and to some degree directly through walls.  Through these channels, the inside air with its accumulated moisture is replaced by outside air which, in colder weather, is much drier.  If you have trouble with condensation, part of the problem is the "tight" modern home that you live in... a home that you can heat for a fraction of the money it takes to heat an older home, and a home that's cleaner and more comfortable besides.
  If you have excessive condensation, troublesome condensation that blocks whole windows with fog or frost,with water that runs off windows to stain woodwork.. or in serious cases even to damage the paneling, you have a good reason to worry and good reason to act.
Don't expect the glass in your windows to remain free of condensation if the moisture level in your home is too high. It can't be done. The best way to reduce the relative humidity within the home is by use of fresh air.
It is more satisfactory to have a controlled source of ventilation than to depend on the uncontrolled ventilation from cracks around windows and doors. Any method of ventilation is good. Use exhaust fans to remove moisture caused by cooking, bathing, washing and drying, before the moisture is mixed with room air. Open a window or vent whenever operating exhaust fans to bring in outside air. Keep small sidewall or ceiling vents cracked slightly to allow continuous escape of humid air. Heat loss from this will be surprisingly minimal.
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