How to stop condensation on windows

Condensation on windows is a common phenomenon that occurs in many homes. Many think it’s caused by the quality of windows, but this is not the only factor that explains the problem.

How Does Condensation Occur

In order to comprehend the phenomenon of condensation, the first thing that must be understood is how air behaves. Air is a gas whose density varies according to the temperature; it is less dense at higher temperatures, which means warm air can hold more water vapour than cold air. Condensation happens when warm air full of water vapour comes into contact with a cold surface, making the air temperature drop suddenly and causing water vapour to condense on the surface. Since window surfaces are the least insulated areas of a home, condensation happens first of all in this location.

Condensation on windows is linked to the following factors:

  • Exterior temperature;
  • Interior temperature;
  • Interior relative humidity;

Out of those factors, the only ones that can be controlled by the occupant are the interior temperature and the interior relative humidity. Paying attention to controlling the relative humidity will produce the most results. It is essential for occupants to properly control the relative humidity level in their home.

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The Right Relative Humidity Level

In the literature, there are several organizations that recommend different relative humidity ranges within homes. Here are some of the most important ones:

  • The Canada Mortgage and Housing Corporation (CMHC) recommends a relative humidity of between 30% and 50% in the winter. When the temperature drops below
    -10 °C, maintaining a relative humidity of around 30% is recommended.
  • Health Canada recommends keeping the humidity level close to 50% during the summer and 30% in the winter.
  • According to National Bank Insurance, the ideal relative humidity in a healthy house is around 50% in the summer and 30% in the winter.
  • According to Équiterre, a non-profit organization, the humidity level should be between 30% and 50% in the winter: closer to 30 % on colder days, and 50% on milder days.
  • According to the Québec magazine La Maison du 21ieme siècle, the relative humidity in the winter should never exceed 45% in order to prevent the grown of bacteria, viruses and mould, and on very cold days, around 30% is the proper level.

All in all, a relative humidity of 30% should ideally be maintained in the home during the winter months in order to avoid health problems as well as condensation on windows and doors. For example, with a condensation index of 60 and a relative humidity of 30%, the exterior temperature can drop as low as -25°C. That’s well below the average temperature in the month of January, which is around -13 °C, according to Environment Canada. But during deep freezes (below -25°C), the relative humidity should be lowered to 20% to 25% to avoid the risk of condensation.

How to Maintain the Correct Relative Humidity

As mentioned above, of the four factors that influence the occurrence of condensation on windows, the one that is of most concern to the occupant is the relative humidity. A hydrometer can be used to monitor it. They are inexpensive and easy to find in stores. Certain lifestyle habits can be adopted in order to reduce the amount of water vapour released into the air, lowering the relative humidity.

  • Ensure good air evacuation for cooking and showers;
  • Take shorter showers in the winter;
  • Keep fewer plants in the home;
  • Avoid storing wood in the basement;
  • Don’t close the blinds or curtains unless you need to;
  • Keep interior doors open as much as possible to ensure good air circulation;
  • Avoid large variations in room temperature.

It is also possible to reduce the relative humidity of a building by ensuring that its mechanical functions are optimized. Here are a few simple, inexpensive examples of ways to control the interior environment of a building:

  • Install heat sources directly below windows;
  • Ensure good air circulation near windows and in closed rooms by using a ceiling fan or regular stand fan;
  • Install a dehumidifier to control relative humidity;
  • Install an air exchanger to improve ventilation and bring in fresh air;
  • Make sure the dryer vent is properly installed and leak-proof so that humidity will be directed outside the home.

In short, the phenomenon of condensation is a seasonal problem that arises when the weather gets very cold, and it can be easily controlled by the occupant through the adoption of certain habits and the optimization of building mechanicals.