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Excessive condensation of moisture on windows is a source of increasing
concern in homes in winter.  It not only creates a nuisance by limiting
visibility and by wetting adjacent walls and floor surfaces, but in more
severe cases it can cause rotting of wood and deterioration of paint and

To the builder the problem takes on a special significance because
the homeowner frequently assumes that window condensation is a fault of
construction.  He does not readily appreciate that his own living habits are
of prime importance, nor that a well built house is often more vulnerable
to excess moisture problems than one that is loosely constructed.

Fortunately most cases of window condensation can be controlled by the
occupant once he understands the basic causes of the problem.  It is our
purpose to discuss these causes and to provide suggestions for controlling
excessive moisture in houses.

Some things are pretty basic.  If you have too much moisture in the home, then
look for some of these occurrences:


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.  If, at the same temperature, it contains
only one-half the water vapor it is capable of holding, then the relative
humidity is 50 percent.

If the temperature changes but no water vapor is
added or taken away, then the relative humidity will also change and will
increase as the temperature falls.  The relative humidity will continue to
rise with falling temperature 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 (when the temperature is above freezing) or as frost (when the
temperature is below freezing).

Air cooled by contact with the cold surfaces
of windows will therefore deposit some of its water vapor on the glass or
the metal sash whenever it has more water vapor than it can hold at its new
temperature that it achieved as it brushed against the cooler glass or sash.

Condensation may occur on either the inside surface of the inner window
or the inner surface of the outer window.  The first case indicated that
there is too much water vapor in the air for the weather conditions prevailing
at the time.  The second cause indicates air leakage outward around the inner
window, and will occur even when the amount of water vapor in the air is
quite low.  This latter form of condensation is more prevalent on upstairs
storm windows and the down-wind side of the house.

Condensation usually occurs first on windows because they have the lowest
temperature of any of the interior surfaces in the house.  It seldom appears
on walls because they are normally warmer, although occasionally condensation
may occur on cold spots such as nail heads and in the corners of outside walls
and closets where the insulation value is reduced and circulation of warm room
air is restricted.  In extreme cases this has led to mildew and the growth
of mold.


Humidity levels should be controlled so that little or no condensation
appears on the inside surface of the glass.  With double glazing this
still permits quite high relative humidity except during the most
severe weather as indicated in the following which shows the maximum
humidity levels that can be tolerated if condensation is to be avoided
in the cold weather.

   Outside Air             Desirable Maximum
Temperature, °F           Humidity, %
	-20                      	20
	-10                      	25
	 0                       	30
	 10                      	35
	 20                      	40

If only single glazing is used, much lower humidity will produce
condensation (less than 12% at 0° F); storm windows can be installed,
however, to provide the thermal equivalent of double glazing and thus permit
these higher humidity levels to be maintained.

In practice, condensation will occur first over the lower part of the window
because the glass surface temperatures are not uniform, being lower at the
bottom than at the top.  

Condensation at the base of the window and also at the sides tends to be more 
severe with metal sash and with some special units such as factory sealed double 
glazing where the method of assembly results in increased heat transfer at the 

Drapes or other window coverings can contribute to the problem by restricting 
the flow of warm room air over the glass surface.

The homeowner need not measure the humidity directly, he can simply use the
windows as a guide to the proper humidity level within the house.  As soon as
the objectionable condensation occurs on the inside surface of the window,
steps should be taken to reduce the relative humidity by controlling the
moisture sources or by increasing ventilation.

It is common belief that for health reasons there should be a lot of moisture
in the air during the winter months.  There is, however, no conclusive
evidence that either the health or the comfort of most people will be
adversely affected if humidity is kept at a level that will prevent excessive
condensation on the interior surfaces of double windows. Comfort and nasal
dryness will, however, be noticeably affected if the percent of relative
humidity drops excessively low.

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