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Writer's pictureCorey Provencal
Apr 2, 20136 min read

WINDOW CONDENSATION, Causes and Cures


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Window Condensation Florida Indoor Air Quality Solutions IAQS


Each winter more and more homeowners become 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.

Many home owners seem to believe that it’s a windows problem, a cheap or single pane window issue. They call their builder or the window supplier and say, “My windows are all wet, and it must be the fault of the builder or the windows.” Well, not quite… Water on windows is condensation, and it can be a problem. However, it’s not a window problem, and the solution does not come from the windows.

It may strike you as odd, but the growing condensation problems of the nation are caused by progress and improvements in the construction of your home.  Yes, if you have trouble with window condensation it’s probably because you live in a “tight” modern home that you can heat and cool for a fraction of the cost it took to heat and cool the house your parents lived in. 

This article explains the moisture problems of the “tight” home that lead to window condensation. It offers suggestions for curing condensation problems in existing homes and provides suggestions for those who are planning on building a home.  Your new home will be a “tight” home and there are more changes or upgrades that you can add to the home during construction that will help to prevent excessive moisture.  This can be a bit more difficult in an existing home where the problem already exists.

What Causes “Excessive” Condensation?

Condensation is the result of high humidity that produces a “fog” once it hits a colder surface. The humidity is caused by excess water vapor in the home. This is commonly seen in a foggy mirror after a hot shower. Condensation usually occurs first on windows because glass surfaces have the lowest temperature of any of the interior surfaces in the home.

A little condensation on the lower corners of your windows now and then probably doesn’t bother you and shouldn’t. By the time you’ve thought about it a second time it has usually gone.

What we’re talking about is excessive condensation, condensation that runs down the windows and pools on the sills, condensation that runs off windows and stains woodwork… or in serious cases even damages the walls and possibly the floors.

If you have this kind of condensation on your windows, you have a good reason to worry and a good reason to act.

It’s natural and easy in such cases to blame the builder or the windows. But it’s wrong to blame them.

The real villain is invisible. Its 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.

If you have too much water vapor (humidity) in your home there isn’t anything you can do to the windows to stop condensation.

What is Humidity?

Humidity, water vapor, moisture, steam, they’re all the same. They’re all a form of water.  Humidity is an invisible gas. It’s present in varying quantities in nearly all air. This moisture in wet air tries to flow toward drier air and mix with it to reach equilibrium.

Scientists describe the movement of water vapor as “vapor pressure’.  It’s 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, drywall, cement and brick. Right through most of the materials we use to build our homes. That’s exactly what happens when moisture seeks to escape from the humid air.  The moisture is moving from an area of greater concentration to an area of lesser concentration. 

Here in Florida, the hot humid south, the area of greater concentration is generally the exterior of the home.  However during the winter the opposite is true.  During the winter the exterior is dryer than the interior of your home.  In addition you aren’t running the air conditioner which during the summer removes some of the indoor water vapor from the air.

So we have the same daily activities in the winter as the summer.  We’re creating the same amount of water vapor but we’re not removing any of water vapor.  So in the winter the humidity in our homes rises and the outdoor temperature and humidity lowers.  And the result is condensation on your windows.  The amount of condensation will be directly related to the amount of available water vapor or amount of indoor relative humidity.

Daily occupant activity such as cooking, washing, bathing, showers, as well as the use of household appliances such as dishwashers and clothes dryers and washing machines, all introduce water vapor into homes.  Typical amounts of water vapor include:

  1. One person’s breathing produces 1/4 cup of water per hour.

  2. Cooking for a family of four produces approximately 5 pints of water in 24 hours.

  3. Showering puts 1/2 pint of water into the air.

  4. Bathing puts 1/8 pint of water into the air.

Adding only four to six pints of water to the air raises the relative humidity in a 1,000 square foot home from 15 to 60 percent, assuming the temperature is constant.

So you see that the modern family of four can easily release 150 pounds or more than 18 GALLONS of water per week into the air in their home! 

Old drafty windows allow moisture to escape through inefficient seals and cracks. Today’s technology produces more energy efficient, “tighter” homes. This is great for keeping your home more comfortable, quieter, and cleaner, BUT by sealing your home you are also keeping moisture in. In today’s homes it is very easy to build up extremely high levels of humidity.

Since the 1970’s energy crisis we’ve been working very hard to create an energy efficient home.  Part of that energy efficiency is to prevent air infiltration by sealing homes which has created the modern “tight” home. Moisture created by bathrooms, kitchens, laundry rooms and daily occupant activity no longer flows easily to the outside. Modern insulation and construction that keep the hot and cold air outside also keep moisture in; so, it is very easy to build up excessive and even harmful moisture levels in our homes. The condensation is a big red flag that you have way too much humidity trapped in your home.

How to Reduce Humidity

Be the Master of your Domain and take control of your indoor environment: For instance, venting all bath vents, clothes dryers, kitchen exhaust hoods to the exterior of your home.  And of course you have to use your kitchen or bathroom exhaust fans.  We strongly encourage you to consider purchasing or upgrading to bathroom exhaust fans that are humidity controlled.  That’s right started and stopped automatically based on the amount of humidity in the bathroom.

Now, before we summarize specific steps for reducing humidity in your home, let’s include some basic data about recommended indoor relative humidity. You can refer to it if you are inclined to test the moisture levels in your own home.

Here are a few easy steps you can take to reduce condensation on your windows.

  1. Install a humidistat

  2. Do not install wallpaper on exterior or bathroom walls

  3. Do not place furniture in front of windows. 

  4. Do not keep window coverings such as curtains and blinds fully closed

  5. Ensure that your bathroom exhaust fan is ducted to the exterior of your home

  6. Run your bathroom exhaust fan

  7. Run your bath exhaust fan some more

  8. If possible duct your kitchen exhaust fan to the exterior

  9. Run your kitchen exhaust fan while cooking

10. Ensure that your dryer vent is clean

You see, the basic principle of reducing window condensation is extremely simple. When there is too much condensation on your windows, it means that humidity is too high in your home. You should take necessary steps to reduce humidity until condensation disappears.

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. In the moisture content of the inside air, lies both the cause and the cure.

John Lapotaire, CIEC

Florida Indoor Air Quality Solutions IAQS

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