Top 5 Common Questions On Window Condensation

“ 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 September 1983
  • 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 About It.

Why Is There Moisture On My Windows?

Each 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 expensive.

It may strike you as odd, but the growing condensation problems of the nation are caused by progress.

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 besides!

In addition, the extensive use of vapor barriers, energy efficient entrance doors, and windows tend to keep the moisture that is produced, in the structure.

This 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” Condensation?

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…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.

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 job.

It’s 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.

The real 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.

Once you’ve equipped your windows with good storm windows, there isn’t very much more you can do to the windows to lick condensation.

More about windows later…but now, let’s go back to the beginning with the question:

What Is Humidity?

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 itself.

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.

More Moisture Trapped In Less Space

Certain 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.

First, more washing, more bathing, more showers, more appliances, all add more water vapor into homes than in former years.

“Heating 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.

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.

Now, increased production of humidity is only part of the story.

Houses 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.

The Great Humidity Myth

Some 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.

But with 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 Antonio:

“…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.”

“High humidity, however, can greatly contribute to the deterioration of a house and to the discomfort of the occupants.”

Recommended Humidity Levels Are Healthy

Most 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 stripping.

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 condensation.

If 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.

How To Reduce Humidity

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 humidity.

  1. CONTROLLING SOURCES OF HUMIDITY: For instance, venting all gas burners, clothes dryers, etc., to the outdoors. Use of kitchen or bathroom exhaust fans.
  2. 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.
  3. 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 windows.

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

These humidities are generally considered to be comfortable.

Practical Steps To Control Condensation

Here, arranged from easy to more difficult, are the steps you should take to reduce condensation on your windows.

  1. Put on storm windows or insulated replacement windows.
  2. If the condensation is on the storm window, open periodically to vent excess moisture.
  3. Shut off furnace humidifier and any other humidifying devices in your home.
  4. Be sure that louvers in attic or basement crawl spaces are open and are of adequate size.
  5. Run kitchen or other ventilating fans longer and more often than has been your custom.
  6. Open fireplace damper to allow easier escape for moisture.
  7. Open drapes and blinds to allow warm house air to circulate against the window.
  8. Air out your house a few minutes each day. Air out kitchen, laundry and bathrooms during use or just following use.
  9. 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 replacing window sash damaged by excessive water vapor.

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 disappears.

But in 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.

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 manufacturers.

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.

Second, 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.

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 the cure.