is window condensation bad?

October 14, 2022

Does condensation on my window glass mean they are not energy efficient or have failed? Not necessarily, and here’s why:

1. Condensation on the exterior surface of the glass is quite normal and is a result of the outside environmental conditions.

Condensation on the external side of the glass is normal and is caused by outside environmental conditions, similar to dew on your lawn or on the hood of your car. Most windows today are double (or even triple) paned, filled with argon or Krypton, and have coatings to control heat transfer through radiation.

As homeowners, we all strive to limit heat loss in the winter and heat gain in the summer. Builders and renovators attempt to make our homes as airtight as possible, which can present new challenges as a healthy home needs fresh air; how do you constantly bring in fresh air while not being too chilly or too hot? Keep reading…

2. Humidity is the most common cause of condensation on the glass surfaces in your home.

Where does this increased humidity come from? It can come from:

  • The air we exhale. Each person can add 1.7 Litres of moisture per day to a home.
  • Improper use of bathroom fans and stove top fans. Every shower can contribute almost half a litre of moisture, as can steam from cooking. These fans are crucial for keeping your humidity levels low.
  • Believe it or not, having too many household plants placed in one area can increase the humidity in that room due to the amount of watering.

New home builders are integrating Energy Recovery Ventilators to combat the constant increase of humidity (ERV). ​​These units pull in fresh air from outside, first passing it through your furnace to warm it as it enters. The fresh air has a lower humidity (below 30%) but has been warmed to the temperature of your home.

Humidity is crucial, as it can cause condensation to form on window panes due to a phenomenon known as the Dew Point. At a certain humidity, this is the temperature at which air vapour turns to water. For example, if a home has a temperature of 21 degrees Celsius and a humidity level of 40%, the dew point is 7 degrees Celsius. By increasing the humidity level to 55%, the dew point rises to 12 degrees Celsius.

In other words, if the vapour in the air reaches or touches a surface with a temperature of 12 degrees Celsius, it will condense. Health Canada recommends keeping humidity levels between 30% and 55%. During the cold months, 30-40% is ideal.

You might be thinking, if my thermostat is set at 20 degrees Celsius, I should never notice condensation, right? Unfortunately, 20 degrees C is the average in the room recorded by the thermostat. The temperature that comes from the heat registers can be 10 to 15 degrees higher. In contrast, the temperature of your outside walls and windows will be lower.

The reason for this is that heat transfer occurs continuously even through your walls. When the outside and inside temperatures differ, heat always moves, either to the outside in the winter or to the inside in the summer. The warm interior air is constantly warming these surfaces, a constant battle that will never be won.

3. Raising the thermostat temperature in the winter

We all want to keep our homes warm in cold weather climates like Canada, and higher humidity levels are more comfortable. Unfortunately, when the outside temperature is low, the humidity in the house cannot be the same as it is during the warmer months.

“During colder periods, indoor RH (Relative Humidity) higher than 35% will cause significant condensation on windows,” according to the Ontario Building Code (2021 Building Code Compendium A-

Some people may find this too dry for their skin and will need to increase the amount. That is fine, but you may notice some condensation on your windows, which you will need to wipe off from time to time.

Lack of Proper Airflow

For those who require complete darkness when sleeping, heavy curtains or blinds are sometimes required. The disadvantage is that this not only filters out light but also restricts airflow over windowpanes, as it is the movement of this warm air that warms the window surface and keeps it above the dew point.

If we restrict this flow too much, the surface temperature of the glass will naturally drop on those chilly days, and if the humidity is high enough, it may hit the dew point and begin condensing.

Heat registers are installed beneath windows for a reason. This is not to say you should close your drapes, but condensation may form under certain situations even with energy-efficient windows.

Some people prefer to maintain their homes at greater humidity levels in the winter, and that’s fine; just remember to wipe the sill on those particularly chilly days.

Condensation is only bad…

If you see condensation between your glass panes, this indicates that your window’s glass seal has failed and it must be replaced.

As a result, you should hire qualified personnel who follow the highest standards to ensure effective operation and a tight seal.

If you have any other questions about condensation on your window glass, please contact us and speak with a Fieldstone expert.

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