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Why do windows sweat?
Air is composed
of several gases, one of which is moisture vapor. When air comes in contact
with the DEW-POINT TEMPERATURE, the air is chilled and CONDENSATION takes
place with a residue of water on the surface of the glass.
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What is Relative Humidity?
When air will
hold no moisture, it is said to be saturated. Relative Humidity is the
percentage of moisture in the air in relation to complete saturation. Example:
100% Relative Humidity would be raining. (0% Relative Humidity would be
excessively high inside humidity. 20% Relative Humidity would be
comparatively dry air
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What is recommended inside Relative
Humidity?
The National
Paint, Varnish and Lacquer Association, in their Bulletin #572, recommends
the following humidity be maintained to avoid excessive condensation:
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When outside temperature is:
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Recommended Humidity
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0 or
below
0 to 20 above
20 and over
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not more than 20%
not more than 30%
not more than 40%
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What can be done to relieve or correct steaming, sweating, or
frosting on primary house windows?
The installation
of storm windows usually raises the temperature of the inside window above
dew-point, thereby relieving the streaming, sweating, or frosting of the
inside window.
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What
causes sweating on the inner surface of storm windows?
In winter,
outside Relative Humidity is comparatively low. When an excessively high
inside humidity is created, it causes a vapor pressure. Moisture vapor will
tend to equalize. Therefore, the high inside humidity will leak through
cracks around the inside sash, into the air space between the primary and
storm sash and condense on the inner surface of the cold storm sash.
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How
can sweating on the storm sash be reduced?
1.
Measure or determine the amount of inside Relative Humidity. If
excessively high humidity is observed, the reduction of humidity percentage
will usually solve the problem of sweating.
2.
Determine if inside primary window is reasonably tight so that a
minimum of inside humidity is allowed to leak into the air space between
the windows.
3.
Provide adequate ventilation or "breathing" space in edges
of storm sash. The amount of "breathing" space required depends
upon the amount of inside relative humidity.
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What
is the function of storm window "breathing" holes?
"Breathing" holes on the edges if storm windows permit the
moisture in the air space to be exhausted and absorbed by the comparatively
dry, cold, outside air. A good example of this process is the automobile.
Upon entering a cold car, we see the windows begin to stream from warm,
moist breath. When the car windows are open slightly, the streaming
disappears.
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How can
humidity be checked?
There are inexpensive humidistats available,
which register inside humidity much like a dial type thermometer.
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What
causes humidity to be too high?
Anything which tends to evaporate moisture will contribute to excessive
humidity. Below are listed a few of the chief sources of moisture
evaporation:
1.
Uncontrolled furnace humidifiers
2.
Open faced, unvented gas heaters
3.
Damp basement wall and floors
4.
Excessive boiling when cooking
5.
Laundry hung up to dry
6.
Radiator water pans
7.
Large numbers of plants, watered daily
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What can
be done to reduce high humidity?
Control or elimination of some of the following offenders will tend to
reduce excessive humidity.
1.
If furnace is equipped with humidifier, turn off until sweating
subsides.
2.
Open faced gas heaters should be used as little as possible and, by
all means, connected to a chimney.
3.
Basement floors and walls should be treated with efficient
waterproofing.
4.
Kitchen exhaust fan, used while cooking, is helpful. At least close
kitchen door to rest of house and open window slightly for ventilation.
5.
Windows in laundry room should be opened for ventilation when
laundering.
6.
Bathroom door should be closed and window ventilation provided after
hot showers.
7.
Radiator pans should be removed until sweating has been eliminated.
8.
Large numbers of plants should be concentrated in sun room or other
seldom used rooms during critical cold weather.
9.
The entire house may be ventilated briefly once a day to empty humid
air to outside and replace with drier air.
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