Condensation on windows is a common occurrence ….. here are some thoughts that hopefully explain why and how this can be corrected in most cases.

The first thing is to understand the physical principals of the environment and the behavior of the air within the home … with relation to heating and cooling.

House air contains a significant amount of moisture in the form of dissolved water vapor. In an average home things like : cooking , showers , cleaning , washing clothes , etc. can generate up to 70 pints of this dissolved water vapor a day .
This moisture is typically measured as Relative Humidity (RH) and is a useful measurement method to explain condensation on windows within the home.

The reason the humidity is “relative” is the fact that air can hold a finite amount of water vapor at any given temperature. The amount of vapor will increase significantly with air temperature.
In other words … warm air can hold more moisture than cold air.

If your home were to have 50% RH at a room temperature of 70 degrees … this may increase to 75% RH or higher if the temperature was dropped several degrees without adding any extra moisture. If the temperature is reduced dramatically the RH will rise to near the saturation point of the air … or 100% RH. If this threshold … called “dew point” … is exceeded then the moisture in the air will be released in the form of condensation.
An example … in the Earth’s atmosphere this process leads to the formation of clouds and precipitation.

Inside the house this precipitation can form on any surface that is significantly colder than the heated air in the home… that surface is the cool glass of the windows.
In normal conditions warm moist air inside the home will surround the window. On very cold days the heated air will deposit some of the moisture on the cool glass …. which is colder than the surrounding wall and window frames.

Modern sealed window units that may be filled with inert gas and with a specialized coating are still subject to temperature fluctuations.
On older windows that may not be modern glazed or simply leaky allowing cold drafts will create greater temperature fluctuations. The area where the glass is inserted into the frame will have a seal to prevent air leakage , but it’s difficult to completely seal these areas and prevent transfer of air and heat through these transition areas. Typically the coldest area of the glass is at the corners where most air and heat loss occurs. On very cold and windy days the condensation may even freeze forming ice … typical with drafty windows.

Based on what has been mentioned so far there are two basic methods to stopping the condensation and ice forming on windows.

The first approach would be to prevent the drastic temperature drop at the windows, which causes the warm air to condense. To some degree caulking or sealing the windows better may accomplish this. Also improving heated air flow toward the windows which may warm the window surface enough to prevent excessive sweating. The improved airflow will also aid in evaporation of any condensation that does form. While this may help to some extent it may not solve the problem on the coldest days.

The second and important approach is to reduce the moisture in the air. As previously mentioned dropping the air temperature will raise the relative humidity making the likelihood of condensation much greater. If you can’t do much about the temperature drop at the windows on really cold days then you must concentrate your efforts on the RH of the room air. If the RH of 40%-50% at normal room temperature … which is not uncommon …. It doesn’t take that dramatic of a temperature drop to reach the saturation point of 100%. If the RH at room temperature was reduced to 30% or lower it will require much more of a temperature change to reach saturation.
In other words ….The less moisture in the warm air the less the chance of condensation on the cool windows.

The approach of managing the moisture (RH) within the home may be much more effective to prevent condensation forming on your windows.
Using things like bathroom exhaust fans longer and regularly as well as kitchen exhaust fans that are vented to the exterior. A Heat Recovery Ventilator (HRV) is very effective for controlling the humidity levels in homes. Simply by exhausting the moist stale air from the home and replacing it with relatively dry exterior air. These can be controlled by a humidistat located next to the thermostat. Running a furnace fan continuously will also help in improving airflow over the windows.

Generally speaking the colder the outside temperature the less the RH should be inside the home.

The only draw back to this second approach is that the air in the home may feel dry. Fortunately it will be healthier for the components of the home in maintaining a lower RH and prevent rot and mold growth … which will be healthier for you and your family.

Hopefully this makes sense and helps.