Condensation — the word itself is enough to strike fear in the most stoic of home owners. Let’s face it: no one wants to contend with leaky windows, or failed glass, or even mop up an endless trickle of water from our window panes.

What exactly is condensation, and are your windows to blame for all that moisture?

Condensation on a window


Let’s look at ten common questions or concerns about this phenomenon:

What is condensation?

Simply put, condensation is the process by which water vapor (gas) in the air turns into a liquid state (water). The water droplets that you may see on your window panes in cold weather — that’s condensation.

What is exterior condensation?

Condensation on the outside of windows generally occurs in the summer months. It is caused by three main conditions: high outdoor humidity, little or no wind, and a clear night sky. It forms in the same way as interior condensation when the temperature of the glass falls below the dew point of the outside air (as opposed to inside air in interior condensation).

What about interior condensation?

Fog, water droplets, or frost on the interior of your windows forms on surfaces when the humidity in the air is too high and surface temperatures are too low. Think of how “steamed up” your bathroom mirror is after a hot shower, or how water beads form on a drinking glass filled with iced tea on a hot summer day.

What causes condensation?

There are three important elements that combine to cause condensation:

The level of moisture in the air
The temperature of the air in your home
The surface temperature of the windows

When the heated air from the interior of your home comes into contact with the cooler surface of the glass in your windows, condensation forms. The change in temperature of the air causes the moisture to condense onto the pane.

My windows are foggy and wet! Are my windows causing this condensation?

Windows do not cause condensation — moisture on the interior pane of your windows is caused by excessive humidity in your home. Because the surface of your window panes typically have the lowest temperature in your home, you will usually notice condensation there first. This doesn’t mean that your windows are defective or leaking, they are just a cool surface on which the moisture in the heated air of your house will condensate on.

Are my new windows defective? My old windows didn’t have condensation, but my new windows do.

Condensation can be an annoying problem. While it may seem natural to blame the windows and/or doors, interior condensation is really an indication of excess humidity in the home. It is sign that your high performance, energy-efficient windows are doing their job.

Modern homes – and high performance windows – are better sealed against air leakage and natural ventilation to the outdoors is reduced. Energy efficient building designs, techniques and products keep cold air outside and, also, keep warm, moist air inside. High performance windows and good insulation provide a barrier to the air exchange of your home, and along with additional water vapor from breathing, cooking, showering, etc., homes now have a high relative indoor humidity level, which can result in interior condensation. Older homes were less energy-efficient and allowed the excess moisture to escape through cracks and leaks, resulting in little or no condensation but much greater heat loss.

What is raising the humidity level in my home?

Did you know that every day up to 50 litres of moisture is being released into the air in your home? There are many things that generate indoor moisture: humidifiers, heating systems, washers and dryers, aquariums, etc. Everyday activities — such as cooking meals, washing dishes or taking showers — also contribute to the humidity level in your home. Pets, and even plants, add moisture into the air.

What can I do to reduce the humidity in my home?

Controlling the humidity in your home will help to reduce or eliminate condensation on your windows. Try these eight tips to help reduce humidity levels:

  1. Open a window or door daily to air out your house.
  2. Turn off all humidifying devices and make sure that your clothes dryer is vented to the outside.
  3. Install and use a dehumidifier.
  4. Use exhaust fans and proper venting in rooms with high moisture levels such as the kitchen, laundry room and bathroom. Exhaust fans should be used during every shower or bath, and run for at least 15 minutes after.
  5. Keep drapes and blinds open as much as possible to allow air circulation against the windows.
  6. Plants can contribute to condensation — they add moisture to the air. Consider moving plants away from your windows.
  7. Do not hang wet clothes to dry inside.
  8. Opt for better insulated windows — the higher the R-value of a window, the better it can handle humidity.  For example, condensation is less likely to form on the inside of triple glaze windows than double glaze.

Can condensation be temporary?

There are three occurrences where your home may experience temporary condensation:

Condensation can occur in newly constructed houses. Wood, plaster, cement and other building materials used in new construction and renovations produce a lot of moisture. When the house is heated during the first cold season, this moisture will gradually flow out into the air in the home, thereby raising the relative humidity of the interior. The moisture from these materials usually dissipates during the first year and is not a cause for further concern.

Your house can absorb moisture during humid summers. This will result in condensation when you start heating your house again when the weather turns cooler. This condensation should clear up in the first few weeks of turning up the heat.

Also, during the colder months, sharp, quick, and sudden drops in temperature can create temporary condensation problems in your home.

Is it normal for condensation to form between the panes of window glass?

No, condensation between the panes in an insulated glass unit can indicate that the airtight seal may have failed and require replacement. Contact your window company or builder if you notice this.

Want more information about condensation?

Natural Resources of Canada

Canada Mortgage and Housing

Moisture and Air: A Guide for Understanding and Fixing Interior Moisture Problems in Housing (pdf file)

Understanding Indoor Condensation in Your Home

Avoiding Condensation – Homeowner Protection Office  (pdf file)