“ In fact, sweating windows can be a
They serve as danger signal, showing when
indoor moisture is trying to get out…”
From a Column by David Bareuther,
Building Editor, The Associated Press
Some of the information in this booklet was derived from
the following sources:
Moisture and Home Energy Conservation,
U. S. Department of Energy – DOE/CE 15095-4 September
Washington Energy Extension Service Technote –
Moisture Problems and Multiple Glazing – TN1003
University of Illinois at Urbanna-Champaign – SmallHomes
Council-Building Research Council. – Council Notes
Moisture Condensation – F6.2 Volume 1 Number 1.
Better Business Bureau of Philadelphia Tel-Tips (412)
456-2720 – Tape # 421 Condensation on Windows.
WINDOW CONDENSATION AND WHAT TO DO
Why Is There Moisture On My Windows?
What Causes "Trouble" Condensation?
What Is Humidity?
What Can Be Done To Reduce Humidity?
What Can Be Done To Control Condensation?
WHY IS THERE MOISTURE ON MY
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winter sees more and more home owners vitally interested
in the subject of window condensation. It's not a happy
interest. It stems from bad experiences with window
condensation which range from irritating to downright
strike you as odd, but the growing condensation problems
of the nation are caused by progress.
you have trouble with window condensation it's probably
because you live in an energy efficient new or remodeled
home that you can heat for a fraction of the money it
took to heat the house your parents were raised in; a
home that's cleaner and more comfortable besides!
addition, the extensive use of vapor barriers, energy
efficient entrance doors, and windows tend to keep the
moisture that is produced, in the structure.
booklet explains the moisture problem of the modern,
energy efficient home, whether new or remodeled. It
also offers suggestions for reducing condensation
problems in your home.
WHAT CAUSES "TROUBLE"
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fog on the lower corners of your windows now and then
probably doesn't bother you. By the time you've thought
about it a second time it has usually gone away.
we're talking about is excessive
condensation...troublesome condensation… condensation
that covers whole windows with fog or frost...water that
runs off windows to stain woodwork, or in serious cases
even damage the wallpaper or plaster. If you have this
kind of condensation on your windows, you have good
reasons to worry...and good reasons to act.
worry so much about the moisture on your windows or
storms...this is just a symptom of excess humidity
throughout your home. You should worry more about what
excess moisture may be doing elsewhere in your home. It
may be freezing in the insulation, melting and damaging
your ceiling and walls exactly like a roof leak when
warm weather comes. Or it may be forcing its way out
through siding to form blisters under your exterior
paint. That means the most expensive kind of paint job.
natural and easy in such cases to blame the paint, or
the insulation, or the windows. But you’re blaming a
symptom not the cause.
villain is invisible. It's water vapor... too much water
vapor. The best and usually the only way to prevent this
trouble is to get rid of excess water vapor.
you've equipped your windows with good storm windows,
there isn't very much more you can do to the windows to
about windows later…but now, let’s go back to the
beginning with the question:
Humidity, water vapor, moisture, steam. They're all the
same. They are all a form of water. Humidity is present
in varying quantities in all air. Moisture in wet air
tries to flow toward drier air, mix with it, and balance
Scientists describe this force as "vapor pressure." It
is often a very powerful force indeed. It can act
independently of the flow of the air which holds the
moisture. Vapor pressure can force moisture easily
through wood, plaster, brick, cement, or around the
window... right through most of the materials we use to
build our homes. That is exactly what happens when
moisture seeks to escape from the humid air usually
found inside your home to the drier winter air outside.
MOISTURE TRAPPED IN LESS SPACE
building materials stop water vapor. Glass is one of
these. Also on this list are some varnishes, paints,
tiles, plastic wall coverings. Vapor-seal insulation is
designed specifically to stop the escape of water vapor
and protect the insulation, exterior paint and your
walls form the ravages of water.
Increased use of these "moisture trapping" materials in
the last few years has created the modern "tight" home.
Moisture created by bathrooms, kitchens, laundries and
occupants no longer flows easily to the outside. The
modern insulation and construction that keep the warm
air in and the cold air out also keep the moisture in.
So it is very easy to build up excessive and even
harmful moisture levels in such homes. The result is
condensation on the window or even on the storm windows.
more washing, more bathing, more showers, more
appliances, all add more water vapor into homes than in
and Ventilating” magazine provides builders with
reference data on sources of water vapor. For instance,
cooking for a family of four adds 4.5 lbs. of moisture a
day to a house. Each shower contributes half a pound; a
weekly laundry, 30 lbs.; human occupancy, 6 to 8 lbs.
per day; dish washing 1.2 lbs., etc., etc.
see that the modern living of a family of four can
easily release 150 pounds, or more than 18 gallons of
water per week into the air in your home! And houses
with no basements can have additional moisture problems.
increased production of humidity is only part of the
generally have been growing smaller and this means an
even greater concentration of water vapor which is
trapped by modern tight construction. It means more
moisture contained in less space.
this moisture must eventually escape from your home. No
wonder we've created a condensation problem for
THE GREAT HUMIDITY MYTH
humidity is necessary for comfort and may help health,
and with older houses it was (and still is) a struggle
to keep enough moisture inside the house.
modern, energy efficient new construction or remodeled
homes the situation is completely reversed. The problem
is to get rid of moisture. Yet many home owners
continue to put additional moisture into the air. They
certainly aren’t discouraged by people who sell
humidifying devices or people who install them in
heating plants. They aren’t discouraged by the danger
sign of condensation on windows. Sometimes they aren’t
even discouraged by an exterior paint job costing
several hundred dollars.
turn the light of reason on this humidity myth.
the director of a leading research organization says.
This quote is from the book, New Frontiers or Home
Builders, by C.W. Smith, director of the Housing
Research Foundation of the Southwest Research Institute
at San Antonio:
more tightly built modern houses the moisture given off
by showers, laundry equipment, cooking and by the
occupants themselves puts more humidity into the air
than is needed and there is little likelihood that the
humidity level would ever become so low as to be harmful
humidity, however, can greatly contribute to the
deterioration of a house and to the discomfort of the
RECOMMENDED HUMIDITY LEVELS ARE HEALTHY
authorities agree with Prof. C.P. Yaglou of the Harvard
School of Public Health that any inside relative
humidity higher than 40 percent is undesirable in
winter. These same authorities agree that the
humidities recommended for homes by the University of
Minnesota Engineering Experiment Station are normally
adequate for comfort and health. In fact these
humidities are higher than could be attained in houses
built before the days of modern insulation, heating and
words, the first step in solving condensation problems
in your home is a willingness to reduce humidity. If you
will decide to keep moisture down to levels recommended
by engineering research organizations and by most paint,
window, insulation and heating manufacturers, you are on
the only possible path that leads to control of
humidity levels remain high…in the range of 35 percent,
40 percent, 50 percent or more…it is highly unlikely
this problem will be eliminated.
by the Associated Press Building editor sums up the
problem of reducing humidity this way. He says there are
only three ways to reduce humidity.
CONTROLLING SOURCES OF HUMIDITY: For instance,
venting all gas burners, clothes dryers, etc., to
the outdoors. Use of kitchen or bathroom exhaust
WINTER VENTILATION: Because outside air usually
contains less water vapor, it will "dilute" the
humidity of inside air. This takes place
automatically in older houses through constant
infiltration of outside air.
HEAT: The process of heating your home will reduce
the relative humidity — providing its dry heat. It
will counterbalance most of the moisture produced by
before we continue, let's include some basic data about
recommended moisture. You can refer to it if you are
inclined to test the moisture levels in your own home.
table below is the result of long and careful
experiments at the University of Minnesota's engineering
laboratories. It shows the maximum safe humidity levels
for your home... not just for the windows, but for your
paint, insulation and structural members, too. In
most cases, reducing moisture to these levels will
minimize troublesome condensation on windows.
test humidity in your home, be sure to use an accurate
instrument, preferably a good sling psychrometer.
Remember, too, that these relative humidities are for 70
degrees F. For higher inside temperatures, lower
humidities are required.
Outside Air Temperature
Inside Relative Humidity for 70° F. Indoor
-20 degrees F. or below
not over 15 percent
-20 degrees F. to -10
not over 20 percent
-10 degrees F. to 0
not over 25 percent
0 degrees F. to 10
not over 30 percent
10 degrees F. to 20
not over 35 percent
20 degrees F. to 40
not over 40 percent
humidities are generally considered to be comfortable.
PRACTICAL STEPS TO CONTROL
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arranged from easy to more difficult, are the steps you
should take to reduce condensation on your windows.
Put on storm windows or insulated replacement
If the condensation is on the storm window, open
periodically to vent excess moisture.
Shut off furnace humidifier and any other
humidifying devices in your home.
Be sure that louvers in attic or basement crawl
spaces are open and are of adequate size.
Run kitchen or other ventilating fans longer and
more often than has been your custom.
Open fireplace damper to allow easier escape for
Open drapes and blinds to allow warm house air to
circulate against the window.
Air out your house a few minutes each day. Air out
kitchen, laundry and bathrooms during use or just
If troublesome condensation persists see your
heating contractor about an outside air intake for
your furnace; about venting of gas-burning heaters
and appliances; or about installation of ventilating
fans. Another source is your utility company either gas or
electric. They can offer additional solutions.
simple remedies we suggest (number 1 through 7) don't
work, you have a significant condensation problem. But
the changes your heating contractor may recommend to
further reduce humidity in your home should not be very
expensive. Certainly they will be less expensive than
repainting your house or replacing window sash damaged
by excessive water vapor.
the basic principle of reducing window condensation is
extremely simple. When there's too much condensation on
your windows, it means that humidity is too high in your
home for the current conditions outside. You should take
necessary steps to reduce humidity until condensation
practice, window condensation and reducing humidity may
become very complicated, since many different conditions
may affect the way the condensation situations relate to
different structures. Let us just mention a few:
The number and types of windows in the home.
The type of double glazing system on the windows.
The heating system; hot air or water; perimeter or
interior wall heating.
Location of heating ducts and air flow patterns.
The type of insulation and vapor barrier.
Even the type of soil and quality of drainage.
of so many variables, a condensation problem can
sometimes be very tough to solve. That's why we
recommend that you put an expert to work on your problem
if the simpler steps to reduce humidity don't solve your
condensation problem. See your heating contractor first.
If he can't help, we suggest that you ask your general
contractor or lumber dealer to put you in touch with a
qualified expert. They are available both at engineering
schools and from the staffs of heating, insulation,
wallboard or window manufacturers.
we leave the subject of reducing humidity, we would like
to add the following:
are two causes of condensation that are temporary.
They will disappear after a few weeks or, at most, a
season of heating.
there is the moisture that comes from new construction
or remodeling. There's quite a lot of moisture in the
wood or the plaster or other building materials of a new
home. When the heating season starts, this moisture will
gradually flow out into the air in the home. Then it
will disappear and not cause any more trouble.
much the same sort of thing happens in a milder form at
the beginning of each heating season. During the summer,
your house has absorbed some moisture. After the first
few weeks of heating, your house will be dried out and
you'll have less trouble with condensation.
have been discussing the control of condensation we've
mentioned just about everything EXCEPT windows. There's
a good reason.
just is nothing much that can be done with windows to
cut down condensation.
building experts have often pointed out, the windows are
not to blame for condensation. They are merely an
indicator. The moisture content of the inside air,
contains both, the cause and the cure.