Windows & Condensation

What is Condensation?

Condensation is the fog that suddenly appears in cold weather on the glass of windows and sliding doors. It can block out the view, drip on the floor, freeze on glass… it’s annoying. It’s natural to blame the windows… but you shouldn’t.

What causes window condensation?

Window condensation is the result of excess humidity in your home. The glass only provides a cold surface on which humidity can visibly condense. The fog on your windows is a form of condensation; so is the water that forms on the outside of a glass of iced tea in the summer and on the bathroom mirrors and walls after someone takes a hot shower. Condensation usually occurs first on windows because glass surfaces have the lowest temperature of any of the interior surfaces in the house. When the warm moist air comes into contact with the cooler glass surfaces, the moisture condenses.

The important thing is your foggy windows and sliding doors are trying to tell you to reduce indoor humidity before it causes hidden, costly problems elsewhere in your home. Problems like peeling paint, rotting wood, buckling floors, insulation deterioration, mildew, even moisture spots on the ceiling and walls.

FOGGY WINDOWS AND SLIDING DOORS ARE THE INDICATORS, THE WARNING SIGNS, THAT HUMIDITY COULD BE DAMAGING YOUR HOME.

How can my home have indoor humidity?

Humidity is water vapor, or moisture, in the air. All air contain a certain amount of moisture, even indoors.

Where does the moisture come from?

There are many things that generate indoor moisture; humidifiers will, heating systems, even plants. Cooking three meals a day adds four or five pints of water to the air. Each shower contributes 1/2 pint. In fact, every activity that uses water (like dishwashing, mopping floors, doing laundry) adds moisture to the air.

MORE WATER VAPOR IN THE AIR MEANS A HIGHER INDOOR HUMIDITY; HIGH INDOOR HUMIDITY MEANS CONDENSATION.

How much indoor humidity is too much?

The householder can use the windows as a guide to the proper humidity level within the house. IF OBJECTIONABLE CONDENSATION OCCURS ON THE INSIDE SURFACE OF THE WINDOWS, THE HUMIDITY LEVEL IS TOO HIGH.

To avoid excessive condensation, the following winter humidity is recommended in the house:

Outside Temperature	Indoor Relative Humidity
-290 C	15 to 20%
-230 C	15 to 20%
-170 C	20 to 25%
-120 C	25 to 30%
-70 C	30 to 35%

Will reducing the humidity in my home during winter help control condensation?

Eliminate the excessive humidity, and you will eliminate most… possibly all… the condensation.

How can I reduce the humidity in my home?

Control the sources of moisture and increase ventilation.

As a temporary solution to an acute problem, open a window in each room for just a few minutes. Opening windows allows the stale, humid air to escape, and fresh dry air to enter. After a shower, for example, open the bathroom window, or turn on the exhaust fan, so steam can go outside instead of remaining in the home.

Vent all gas burners, clothes dryers, etc. to the outdoors. Install kitchen and bathroom exhaust fans. If there are a large number of plants in the house during winter, concentrate them in one sunny room and avoid over-watering.

Opening the windows slightly throughout the house for a brief time each day will go far toward allowing humid air to escape and drier air to enter. The heat loss will be minimal.

Does condensation occur more often in particular climates or types of homes?

Absolutely! Condensation is more apt to occur in climates where the average January temperature is 2° C or colder because there will be greater extremes between indoor and outdoor temperatures affecting the glass surfaces in the home.

DURING THE SUMMER AND FALL, ALL PARTS OF A HOUSE PICK UP MOISTURE FROM DAMP AIR IN THE FALL. WHEN THE WINDOWS ARE CLOSED AND HEAT IS TURNED ON, THIS MOISTURE WILL PASS INTO THE AIR OF THE HOUSE AND FOR A WEEK OR TWO, THERE IS LIKELY TO BE CONDENSATION.

New materials and techniques in weather-stripping, insulation, vapor barriers, etc., which are intended to keep cold air out, also lock moisture inside. As a result, moisture created by bathroom, kitchens, laundries and occupants, no longer flow to the outside, unless mechanically ventilated.

Are there any cases where window condensation is only temporary?

Yes, there are primarily three:

• New construction or remodeling
• The beginning of each heating season
• Quick changes in temperature