Winter in Montana brings more than cold and snow, it begins the seasonal cycle of fog and frost appearing on the glass of windows and patio doors. It can cloud your view, freeze on the glass, drip on the floor, and generally cause many folks to reach for the phone to complain about their windows.

So what is this mysterious force that causes such aggravation and leaves people cursing their windows? Condensation.

Condensation occurs when there is excess humidity in a home. The glass in windows and doors merely provides the most visible way of seeing its effects; they are not the culprits. Humidity is nothing more than water vapor or moisture in the air. All air contains a certain amount of humidity or moisture, but indoor humidity is generally caused by such ordinary activities as cooking meals, showers, washing clothes, even the normal breathing of a family.

In fact, the daily living activities of a family of four can add more than 18 gallons of water vapor a week to the air in their home. The more water vapor in the air, the higher the relative indoor humidity. Since outside air usually contains less water vapor, its circulation through a home tends to dilute the inside humidity, but in winter months the normal flow of air in and out of a home tends to be restricted.

You will often find less condensation in older homes because they are usually less insulated than newer, more energy efficient houses. Because of this the movement of air in and out of the house occurs more frequently, reducing the relative humidity in the home.

Relative humidity is a term used to describe the relationship between air temperature and the amount of water vapor it contains. For example, when air at a certain temperature contains all the vapor it can hold, it is said to have a relative humidity of 100%. Cooler air tends to hold less vapor than warmer air, so air at a temperature of 30º F and 100% relative humidity contains less water than air at 70º F and 100% relative humidity.

Condensation, then, occurs from the water vapor in the air when it comes into contact with surfaces cooler than itself. So whether it's on the inside of a window in December or the outside of a glass of iced tea in July, both will show condensation.

Condensation could indicate a greater problem in a home if it is being caused by excessive humidity. Such a condition can damage ceilings and walls by pushing through walls, freezing in the insulation and then melting in the summer. This process is known as vapor pressure.

You can determine if excessive humidity is a problem in your home by measuring the indoor relative humidity with an instrument called a hygrometer. The table below lists the percentage of relative indoor humidity that is acceptable with its corresponding outside temperature range.

If once it's been determined that such a problem exists, there are three methods you can use to reduce humidity.

Control the sources of humidity

By providing more controlled ways for moist inside air in your home to get out, you can help reduce humidity levels. Use kitchen and bathroom fans and ensure dryers and gas burning furnaces are vented.

Provide for winter ventilation

Newer, more efficient homes tend to have elevated moisture levels because they lack the natural flow of air that occurs in older homes. Air out the house for a few minutes each day.

Heat

Heating your home reduces the relative humidity, providing it's dry heat. This heating process will help compensate for much of the moisture that your daily activities contribute to the air inside your home.

There are a few practical steps you can take to help reduce condensation on your windows and doors.

Turn off humidifiers

Use a dehumidifier

See that attic and basement crawl spaces have cross-ventilation 

Run your kitchen and bathroom fans more often

Keep drapes and window shades open as much as possible

Circulate air with ceiling fans

Make certain basements and crawl spaces have moisture  protection

If condensation continues to be a problem you should talk with a heating contractor about an outside air intake for your furnace, possible ventilation for appliances, or installation of ventilating fans. 


Remember, the best way to avoid condensation is to reduce the humidity of the air inside your home. Condensation indicates excessive humidity, so eliminate the excessive humidity and you eliminate the condensation.


We hope this information has been helpful. For a more technical explanation of the concepts discussed contact the National Center for Appropriate Technology, P.O. Box 3838, Butte, Montana 59702 or call 406-494-4572 and ask for the booklet entitled Moisture and Home Energy Conservation.

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