Condensation On Your Windows or Doors

Condensation, indoor humidity, and exterior condensation.

Condensation is moisture that suddenly appears in cold weather on the interior or exterior of window 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 or doors. Interior condensation is really an indication of excess humidity in the home.

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 condensation. It may also be causing problems elsewhere in your home.

Some other signs of excessive humidity:

  • There is a “Musty smell” in the home
  • Mold or mildew on surfaces
  • It “Feels Damp” in the home
  • Staining on interior surfaces
  • Warped wood surfaces
  • Cracking or chipping of interior paint

Questions & Answers

Can relative humidity affect my health?

Most experts agree that relative humidity can affect your health. They suggest maintaining indoor humidity levels between 30% and 50%. Levels higher than 65% can encourage upper respiratory illness and might occur in people suffering from asthma and allergies. Lower moisture levels (below 20%) may encourage itching or dry skin.

Where is condensation most prevalent?

Exterior condensation occurs mostly where the average temperature is 35°F or colder.

What will high humidity do to my home?

Excess humidity contributes to the deterioration of any home. It can pass through walls and freeze in the insulation. In spring it melts, damaging your sheetrock, paint, and studs. High humidity can also force its way out through siding to cause blistersing under exterior surfaces. Excessive humidity can also lead to higher levels of unwanted mold and mildew growth in homes.

Moisture passage through wall surfaces?

Moisture in wet air tries to flow toward dry air. This is due to “vapor pressure.” The flow acts independently of air currents. In winter, inside air is much more humid than colder outside air. So the vapor pressure, or equalization process, can actually force inside moisture through cement, wood, plaster and brick. Some varnishes and paints block the flow of the moisture, so condensation can occur between the inside and outside walls, or under exterior paint surfaces. This can cause rot in a home’s wood frame, blistering in paint, and deterioration of other building materials.

Does condensation occur only in winter?

Condensation is most 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 saturated and produces dew). For example, use a glass filled with Ice water to watch the accumulation of moisture on the surface of the glass. In rare instances, during the spring and fall (and occasionally, during hot, humid summer days), exterior condensation can also form on windows. This is usually a good indicator of the presence of energy efficient windows.

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 or infiltration as the older homes so it is very important to watch the humidity levels in the house.

In winter, is it a good idea to use a humidifier in my home?

While some people may find it easier to breathe humidified air, humidification can sometimes have negative side effects. Humidifiers need to be cleaned regularly. If not, molds and bacteria can live in them. Also, if the air is humidified excessively, condensation and other excess humidity-related problems can occur.

On the plus side, humidified air can help to reduce static electricity in carpets, shrinkage in wood furniture, and wall cracks sometimes caused by over-drying. You must carefully weigh the advantages and disadvantages of humidification. Remember, too much humidity can cause condensation and other moisture related problems.

Comfort and Moisture

The relationship between humidity and comfort

Comfort in a room is dependent on many factors, including the temperature of the air, the relative humidity, the flow of air, the temperature of all of surfaces in the room, and the presence of sunlight heat through the windows or doors. Since indoor humidity is one critical component of comfort, you should carefully consider the indoor humidity conditions in both summer and winter.

Humidity and Comfort

Higher humidity levels give more comfort in the winter. Some people find it easier to breathe humidified air. Soft tissues such as the linings of your nose and throat don’t dry out as easily, and, in some cases, the dry winter air might even need to be humidified to help achieve good thermal comfort in winter.

Most people will be comfortable in the winter if the indoor relative humidity is between 25% and 60% and the indoor temperature is between 65°F and 70°F. Humidity in the summer is reliant of air conditioning to remove excessive moisture in the air.. Again, many elements contribute to summertime comfort — air temperature, surface temperatures, relative humidity, air movement, and direct solar heat. Most people will feel comfortable in the summer if the indoor relative humidity is between 25% and 60% and the indoor temperature is between 72° and 82° F.

How does indoor humidity affect window 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.

Controlling Indoor Humidity FAQ

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 o 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, then reduce the humidity level when condensation occurs. In many homes this simply means turning off your humidifiers in the winter.

Does the amount of condensation depend on the window type?

Bay or bow windows usually experience more condensation than other window styles. This is because inside air circulation around these window styles is usually more restricted. And, since they hang away from the insulated house wall, bays and bows are usually a few degrees cooler in temperature. To help control excessive condensation, it’s smart to insulate between the window head and platform. In extremely cold climates, additional insulation above and below the window platform may be needed. As a secondary measure, placing a common electric fan near the window helps promote air circulation and can reduce window condensation.

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.

What causes condensation on the inner surface of storm windows?

All operating windows leak some air between the window frame and sash. So when warm household air seeps in around the sash and becomes trapped by the colder storm window, condensation forms on the inside surface. Providing outside ventilation to the combination storm window can usually reduce condensation buildup. What causes moisture to form on the outside of the windows? It’s dew, the same condensation you see on windshields, lawns and streets on many mornings.

Condensation like this happens only when the exterior surface temperature of the glass falls below the dew point of the air. When humidity levels are higher, this kind of condensation is more likely to form. Most of the time, exterior window condensation takes place in the Spring and Fall, when cool nights follow warm days.

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.

Is exterior condensation anything to worry about?

Dew on windows is a natural atmospheric phenomenon, and it doesn’t mean your windows 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.

Are there any cases where window condensation is only temporary?

There are primarily three causes for temporary window condensation.

  1. 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.
  2. 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.
  3. Preceding Temperature Shifts: Sharp, quick drops in temperature can also create temporary condensation problems during the heating season.


In order to provide accurate information in these pages, we used the following sources

ASHRAE Handbook of Fundamentals 1999
[American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.]

1791 Tullie Circle, NE
Atlanta, GA 30329

Builders Guide
Energy & Environmental Bldg. Assoc. [EEBA]

6520 Edenvale Boulevard
Suite 112
Eden Prairie, MN 55346

Building Science Consulting
70 Main Street
Westford, MA 01886
Cold Climate Housing Center
University of Minnesota

203 Kaufert Laboratory
2004 Folwell Avenue
St. Paul, MN 55108

EWC [Efficient Windows Collaborative]
Alliance to Save Energy

1850 M Street NW, Suite 600
Washington, DC 20036
Energy Star® Programs
United States DOE & EPA

NFRC [National Fenestration Rating Council]

8484 Georgia Avenue, Suite 20
Silver Spring, MD 20910

Window & Door Manufacturers Association (WDMA)

1400 East Touhy Ave., Ste 470
Des Plaines, IL 60018