“ In fact, sweating
windows can be a godsend.
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
Washington Energy Extension Service Technote
Problems and Multiple Glazing –
University of Illinois at Urbanna-Champaign
– SmallHomes Council-Building Research
Council. –Council Notes Moisture
F6.2 Volume 1 Number 1.
Better Business Bureau of Philadelphia
Tel-Tips (412) 456-2720 – Tape # 421 Condensation
WINDOW CONDENSATION AND WHAT TO DO ABOUT IT.
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 WINDOWS? (return
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
It may strike
you as odd, but the growing condensation
problems of the nation are caused by
Yes, if 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
the extensive use of vapor barriers, energy
efficient entrance doors, and windows tend
to keep the moisture that is produced, in
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" CONDENSATION? (return
A little 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.
But what we're
talking about is excessive
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.
Don't 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
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.
equipped your windows with good storm
windows, there isn't very much more you can
do to the windows to lick condensation.
windows later…but now, let’s go back to the
beginning with the question:
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 itself.
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.
MORE 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
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
washing, more bathing, more showers, more
appliances, all add more water vapor into
homes than in former years.
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.,
So you 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.
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.
All of this
moisture must eventually escape from your
home. No wonder we've created a condensation
problem for ourselves.
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.
Let’s turn the
light of reason on this humidity myth.
See what 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
“…in the 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 or irritating.”
however, can greatly contribute to the
deterioration of a house and to the
discomfort of the occupants.”
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 weather
In other 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 troublesome
levels remain high…in the range of 35
percent, 40 percent, 50 percent or more…it
is highly unlikely this problem will be
A column by the
Associated Press Building editor sums up the
problem of reducing humidity this way. He
says there are only three ways to reduce
CONTROLLING SOURCES OF HUMIDITY: For
instance, venting all gas burners,
clothes dryers, etc., to the outdoors.
Use of kitchen or bathroom exhaust fans.
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 modern living.
Now, 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.
The 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
If you 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 Air Temperature
-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
PRACTICAL STEPS TO CONTROL CONDENSATION (return
from easy to more difficult, are the steps
you should take to reduce condensation on
Put on storm windows or insulated
If the condensation is on the storm
window, open periodically to vent excess
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
Open fireplace damper to allow easier
escape for moisture.
Open drapes and blinds to allow warm
house air to circulate against the
Air out your house a few minutes each
day. Air out kitchen, laundry and
bathrooms during use or just following
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.
If the 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
You see, 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
The number and types of windows in the
The type of double glazing system on the
The heating system; hot air or water;
perimeter or interior wall heating.
Location of heating ducts and air flow
The type of insulation and vapor
Even the type of soil and quality of
Because 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
Before we leave
the subject of reducing humidity, we would
like to add the following:
There are two
causes of condensation that are temporary.
They will disappear after a few weeks or, at
most, a season of heating.
First, 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.
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
While we have
been discussing the control of condensation
we've mentioned just about everything EXCEPT
windows. There's a good reason.
There just is
nothing much that can be done with windows
to cut down condensation.
As the 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