Every year, more and more homeowners become interested in the subject of window condensation. It may strike you as odd, but if you have problems with window condensation, it’s probably because you live in a “tight” modern home. A home that you can heat for a fraction of the money it took to heat the house your parents lived in—and that’s cleaner and more comfortable as well! Your problems may also result from widespread use of several labor-saving appliances that make life easier than it used to be.
Below is some information that describes the moisture problem of the “tight” home and offers solutions for curing condensation problems in your home and suggestions for preventing excessive moisture and condensation problems in your home.
Humidity (invisible water vapor) is present in almost all air. When this water vapor comes in contact with a surface that is cooler, the vapor can condense into visible droplets of liquid. Condensation frequently occurs on glass surfaces first because they normally have a lower temperature than other interior surfaces in your home. You’ve often seen this happen to bathroom mirrors and walls after a hot shower, or on a glass of iced tea. These glass surfaces do not cause the condensation; they simply reflect the presence of moisture.
It’s natural to believe that your windows are the cause of condensation, but they aren’t. Windows don’t cause condensation; they simply prevent moisture from escaping to the outside and provide a highly visible surface on which to notice it. In fact, the “warm-edge” technology of high-performance replacement windows and doors can actually help reduce typical condensation buildup on glass. Nonetheless, while weather-tight, thermally efficient windows keep cold air outside, they also keep moisture in. Occasional, mild condensation is a normal event and causes no real problems. Even so, when you see excessive condensation on glass surfaces, take it as a warning that you may have excess humidity in your home.
Exterior condensation, which forms on the outside pane of the window, occurs when moist air comes into contact with cool surfaces, such as glass. This type of condensation appears when the dew point in the air is higher than the temperature of the glass. This occurs when a cool night follows a warmer day, most typically during the spring and fall seasons.
As for interior condensation, it forms on the inside pane of glass within your home. Whenever there is excess humidity in a home, it manifests itself in the form of condensation on the coldest area of a wall, which is normally the windows. The warmer the air, the more moisture it will retain, so when air in your home comes in contact with the colder glass surface, it is subsequently cooled and moisture is released in the form of condensation on the glass.
Condensation often forms at the meeting rail and at the bottom of the lower sash on the interior of the glass. This is because when warm air cools, it falls down across the interior surface of the window at the same time the temperature of the air is falling. The air contacts the horizontal surface of the meeting rail, which acts like a dam, slowing the air’s rate of fall and creating the perfect opportunity for the trapped water vapor to escape and form on the meeting rail’s surface. The air then rolls over the edge of the meeting rail and again gains speed until it encounters the lower handle of the sash. At this point, the water vapor again makes its exit and lies at the bottom of the sash.
Low-E glass reduces heat conducted through the glass from the warm interior of the home to the outside glass surface. Heat conduction can be reduced significantly with an efficient Low-E coated glass. This reflected heat energy reduces the outside glass temperature and can result in condensation on the glass. Exterior condensation is actually an indication that the insulating glass in the window is performing as it should
Humidity, water vapor, moisture and steam are all a form of water. This water in the air tries to flow toward drier air and mix with it. This process manifests itself as a force scientists describe as vapor pressure. Often a very powerful force, it can act independently of the flow of air that holds the moisture. Vapor pressure can force moisture easily through most of the materials used in building—wood, plaster, brick and cement. That’s exactly what happens when excess humidity seeks to escape from the air inside your home to the drier winter air outside. If you experience this kind of condensation in your home, you have good reason to be concerned.
Excess humidity and condensation can pose serious threats to your home, from heavy droplets running off windows and staining woodwork to, in serious cases, less visible condensation penetrating and collecting in your walls and ceilings. This can damage wallpaper, paint or plaster and cause rotting wood, buckling floors, insulation deterioration, mildew and moisture spots and even structural damage to your home.
According to the American Architectural Manufacturers Association (AAMA), controlling the amount of water vapor in your home is the most effective action you can take to prevent condensation problems. This begins with monitoring your home’s humidity, using an accurate sling psychometer or a humidistat. This table shows recommended safe Relative Humidity levels to maintain for a 70° F indoor air temperature during the cold, winter season, based on extensive engineering studies at The University of Minnesota Laboratories.
In most cases, reducing your humidity to these levels will cure troublesome condensation. Remember that these levels are for a 70°F indoor air temperature. For higher indoor temperatures, lower humidity levels are required. Likewise, a warmer outside temperature permits higher indoor Relative Humidity.
The best steps you can take for reducing excessive humidity levels and condensation in your home involve controlling sources of moisture and increasing ventilation: