Window Condensation

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What Is Condensation?

Is your glass “sweating”– is water beading or ice forming on the inside surface of your windows and skylights? Don’t be too quick to blame the windows! There’s a good chance that what you are seeing is condensation, a sign that there is excess humidity in your home. Humidity– water vapor mixed with air– is drawn to the coolest surfaces, such as your window. Cool air cannot hold as much moisture as warm air, so windows and doors often collect this moisture and make it visible. A surface that is cooler than room temperature is more likely to show condensation.

​What Causes Condensation?

Indoor moisture is caused by a variety of factors, including cooking, showering, running dishwashers, storing firewood, pets, fish tanks, plants, clothes dryers that are not vented properly, even breathing. Your new windows or skylights are most likely showing condensation more than your old ones because they are more airtight– less air is entering your home from the outside. The air leaking from older windows/skylights evaporated the moisture before it could collect.

While some humidity is necessary for health and comfort, chronic and excessive condensation should be tipping you off to take some action before serious, costly damage, such as decay, mold, paint problems, and even major structural damage occur.

What about condensation in between the panes of glass?

This may indicate a seal failure, and you should contact your Window dealer, as this is covered by your warranty.

Why is condensation forming at the bottom of the window/skylight?

Each insulated unit is a sealed atmosphere, and the air in this atmosphere becomes layered, just as in any closed space. Warm air rises, and since humidity is attracted to cooler air, condensation will often show near the bottom of the glass.

Temporary Condensation

There are several ways to tell if the condensation on your windows/skylights and doors is temporary. When and where does condensation usually form:

  • During baths and showers, cooking, dishwashing, laundry, or other steam producing occasions?
  • During the start of each heating season? Houses absorb moisture during humid summers. This will dry out after a few weeks of heating.
  • During sharp temperature changes? Sudden drops in temperature, especially during the heating season, can create temporary condensation.
  • During new construction or remodeling? Building materials contain a great deal of moisture. When the heat is turned on, this moisture will flow into the air inside the home. It usually will disappear after the first heating season.

Problem Condensation

Excess moisture in your home may eventually cause problems. It may be time to take action if you notice the following signs in your house:

  • Condensation remains on windows/skylights and doors throughout the day, even when the outside temperature has warmed up.
  • Condensation is forming and running down the walls. It may also be causing discoloration, staining, peeling wallpaper and blistering paint.
  • The air smells musty– this could indicate mold, mildew, or in the worst cases, rot– or odors from everyday household activities that linger too long. Odors increase in intensity with high relative humidity.
  • Mold, mildew, rot and/or decay are visible. Mold and mildew thrive in most areas and can cause health and house damage.

​Interior Condensation– Winter Prone

Interior condensation forms on the inside pane of the glass within your home. This is the type of condensation most homeowners notice and become concerned about. It usually forms in the winter, as the outside temperature drops, the inside surface will also get cooler; therefore, condensation will form at lower relative humidity on cold days.

The colder the air outside, the more likely condensation is to occur.

​Exterior Condensation– Summer Prone

Conversely, exterior condensation, which forms on the outside pane of the window, typically occurs in the summer. This type of condensation can occur for several reasons: the glass temperature drops below the dew point temperature of the outside air, the air is still, there is a high relative humidity, there is a clear night sky, or there are plants located near your window.

While unsightly, exterior condensation should not concern you since it usually evaporates as the day wears on and will not affect the interior of your home. Since you cannot control the relative humidity outside your home, the only step you can take to combat exterior condensation is to warm the inside surface of the window, as this is a way to warm the outside surface. Seeing exterior condensation on those rare days should be reassurance that your windows are doing their job: keeping your heating and cooling in your home where it belongs and saving you money.

The facts about window, skylight and door condensation and what can be done about it:

  • During winter months, homeowners often encounter condensation on the windows/skylights in their homes. The goal of this letter is to define the problem, the causes, and the solutions to this bothersome and ever present problem.
  • Of course, windows/skylights do not manufacture water. If you were out in the desert, you would want a canteen with you, not a window. But people seem to believe that windows do manufacture water. They call up window contractors and say, "My windows/skylights are all wet, and it must be the fault of the window/skylight." 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.
  • If you are troubled during the fall and winter by condensation on the windows of your home, you aren't alone. It is a common problem in cold climates. The typical family produces a lot of moisture indoors (washing, showering, cooking, just breathing, etc.) and it can be difficult to reduce it.
  • Understanding the causes of the problem is the first step in solving it. Condensation and ice form on windows/skylights because the window/skylight glass surface is below the dew point for the air near the window/skylight glass, so some of the moisture in the air condenses on the glass. The higher the relative humidity of the air near the window, the higher the temperature of the dew point.

Answers to your questions about condensation, indoor humidity and exterior condensation:

  • Condensation is moisture that suddenly appears in cold weather on the interior or exterior of windows and patio door glass, drips on the floor or freezes on the glass.
  • Seasonally, it can be an annoying problem. It may seem natural to blame the windows/skylights and/or doors. Interior condensation is really an indication of excess humidity in the home.
  • Windows serve as visible areas for moisture to condense, warning you that there is too much moisture inside your home. Windows/Skylights & Doors do not cause condensation.
  • Exterior condensation, on the other hand is a form of dew-the glass simply provides a surface on which the moisture can condense.
  • The important thing to realize is excessive humidity is causing window/skylight & door condensation.

Questions & Answers

  1. Does Condensation occur in winter? Condensation is mostly common in winter, but it can occur whenever water vapor in the air comes in contact with a surface temperature lower than the dew point (the temperature at which air becomes saturates and produced dew). In rare instances, during spring & fall (and occasionally, during hot, humid summer days), exterior condensation can also form on windows/skylights. This is usually a good indicator of the presence of energy efficient windows.
  2. Is this a sign of poor quality windows or construction of my house? No, it is a sign of higher quality construction of windows, doors and your home construction. The newer home designs do not allow air and moisture escape of infiltration as the older homes, so it is very important to watch the humidity levels in the house.
  3. How does indoor humidity affect window/skylight condensation?​ Excessive humidity is the cause of most window condensation. As the outside temperature drops, the window glass temperature also drops. When moist air comes in contact with the cold glass pane, the moisture condenses and forms water droplets. Determining when the condensation will occur and preventing depends on the energy efficiency of the window, the relative indoor humidity of the home, and the exterior and interior temperature.
  4. Can excess condensation damage windows? Excess window condensation can cause paint to peel from the sash of wood windows. Excess moisture can also damage the wood window frame on a wood window. Normally it does not affect vinyl or aluminum windows.
  5. Is exterior condensation anything to worry about? Dew on windows is a natural atmospheric phenomenon, and it does not mean your windows/skylights are leaking air or malfunctioning in any way. In fact, exterior condensation is a sign of energy efficiency, since it means the outside pane is thoroughly insulated from the heat indoors. Depending on where you live, it may occur just a handful of times per season.
  6. Are there any cases where window/skylight condensation is only temporary? New Construction: Wood, plaster, cement and other building materials used in new construction and remodeling produce a great deal of moisture. When the heating season starts, this moisture will gradually flow out into the air in the home. It will usually disappear during the first heating season and not cause any further trouble. Heating Season: At the beginning of the heating season, there may be a certain amount of temporary condensation. During the humid summer months, your house can absorb some moisture. After the first few weeks of heating, this moisture should dissipate. Preceding Temperature Shifts: Sharp, quick drops in temperature can also create temporary condensation problems during the heating season.
  7. How else can I reduce indoor humidity? Vent all gas appliances, clothes dryers and exhaust fans to the outside. Your attic and crawl space should also be ventilated. Cover the earth in the crawl space with a good vapor barrier. When cooking, make sure to run the exhaust fans in the kitchen. When you bathe or shower, run the fans in the bathroom until your mirror is clear. Avoid storing firewood in your house. If your home is extremely "tight" it may be helpful to install an air-to-air heat exchanger. As the outside air temperature drops, you should also decrease the humidity level within your home. The bottom line: Maintain as high a relative humidity level as you can for comfort, and then reduce the humidity level when condensation occurs. In many homes this simply means turning off your humidifiers in the winter.
  8. Does the amount of condensation depend on the window type? Sometimes. Recessed windows like bow and bay windows usually experience more condensation than other window styles. This is because air circulated around those window types is usually more restricted, and since they hang away from the insulated house wall, bays and bows could be a few degrees cooler in temperature. Placing a common electric fan near the window to produce air circulation may also be helpful.
  9. Do drapes and shades affect window condensation? Drapes and other window coverings can contribute to a condensation problem by restricting the flow of warm room air over the glass surface. Therefore, indoor condensation is more likely to occur when the drapes are closed or the shades are pulled down.
  10. Why wasn't it always there? Old drafty windows/skylights allow moisture to escape through inefficient seals and cracks. Today's technology produced more energy efficient, "tighter" homes. This is great for keeping your home more comfortable, quieter, and clean, 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.
  11. When should I be concerned?​ If you find condensation between the two or three layers of glass in an insulated window, the airtight seal has probably been broken and the glass will need to be replaced.

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