Moisture in your home will appear as condensation on the inside of your windows when warm moist air comes into contact with a windowpane that is colder (i.e., when the outside temperature is sufficiently lower than the inside temperature).

Condensation, Frost & Ice on Windows

The air in your home holds water vapour. The amount of water vapour will change depending on the temperature inside your home. As warm room air comes in contact with a cooler window surface, the air will also become cooler and lose its ability to hold water.

Did you Know?

People often confuse condensation for leaky windows. Condensation occurs when water vapour in the air condenses on a cold interior windowpane. This can occur when people in your home cook, shower, etc. In some cases, ice can form. When this ice melts, the water droplets will collect on the windowsill and can stain or damage the finish overtime.

Air can only hold a limited amount of water vapour at any given temperature. As warm room air comes in contact with a cool window surface, the air cools, and loses the ability to hold water. If the amount of water in the air is high enough or if the surface of the glass is cool enough, the water in the air will appear on the glass surface of your window. This is condensation.

Condensation may appear on your window at different times depending on the humidity (the amount of water vapour in the air) and the temperature inside your home and at the surface of the window. Condensation usually appears on windows before any other surface because windows contain the least amount of insulation, are part of your homeʼs exterior wall, and react quickly to temperature changes inside and outside.

Did you Know?

Some new homes come with a whole house ventilation system. In their simplest for, this ventilation system connects a central exhaust fan and fresh air intake system to the homeʼs heating system. More advanced ventilation systems recapture heat lost in the ventilated air and have dedicated ventilation ducts throughout the home.

You can control condensation by reducing the amount of moisture (or humidity) inside your home when outside temperatures drop and the window surface becomes cooler.

Many everyday activities add moisture to the air in your home, including cooking, showering and doing the laundry. Other household items that add moisture to the air in your home include plants, fish tanks and humidifiers.

Reducing Condensation

There are many ways to reduce condensation and control the moisture content in your home. The following tips can help you reduce the amount of moisture in your home and condensation on your windows:

Useful Tip:

Melt away built up ice on the inside of your window with a small circulating fan. The fan will move warmer air towards the cooler window and slowly melt the ice away. Use towels to dry up the water as the ice melts. A fan can also be used to decrease ice buildup in rooms where the windows are more prone to condensation and icing.

  • Vent moist air outside and bring fresh dry air inside Place exhaust fans near high sources of moisture (i.e., bathroom fans and kitchen range hoods) to remove high concentrations of moisture
  • Install a whole house ventilation system Use a timer or a centrally located on/off switch to activate the ventilation system only when needed.
  • Keep window drapes or blinds open or partially open during cold weather (especially at night) so that indoor air can move along the window to decrease the amount of condensation that collects on the window (closed drapes and blinds restrict air flow near windows).
  • Keep air and heating vents located near windows uncovered. The movement of warm air along the window surface reduces the potential for condensation to buildup.
  • Make sure air and heating vents are deflecting the movement of air towards the window by adjusting the direction of the vent.
  • Run the furnace fan continuously during periods of extreme cold to evenly distribute heat throughout your home.

Condensation Between Sealed Window Panes

Heat moves through dense materials such as glass easily and quickly. But heat does not easily move through stationary or still air space. By separating the pieces of glass (or the windowpanes) in a window frame with air space, the transfer of heat from the inside to the outside of a home can be reduced. In order to be effective, the separation between the windowpanes must be airtight.

Window glass spacers are made from materials such as silicone foam, butyl rubber, metal or a combination of these materials. The spacer is bonded to the glass to form an airtight seal using adhesives, and in some cases, an additional layer of sealant by factories that specialize in manufacturing airtight window units. The spacer contains a small amount of a material called ʻdesiccantʼ (a combination of air and gas that is pumped in between the panes of your window) that absorbs any moisture in the air that is trapped between the windowpanes when the unit is sealed. This seal also keeps insulating gases (i.e., argon) between the windowpanes.

Did you Know?

Manufacturers of wood flooring products may recommend that a certain level of humidity be maintained to prevent warping, cracking and separating of the wood flooring components; however, this need for the floor must be balanced against the condensation that may appear on the window surfaces. The humidity in your home should be low enough not to affect the windows by high enough to prevent wood floors from cracking (i.e., 40 – 50 percent). The correct balance differs from home to home.

As the panes of glass are exposed to changing temperatures and different amounts of sunlight, they will expand and contract. This occurs daily. Your window will experience high temperatures when the sun shines through them during the day and cool temperatures at night. The temperature on the inside and outside of the windowpanes is rarely the same – the continuous changes in temperatures places stress on the adhesive bonds between the edge spacer and the glass panes. Over time, the seals will let go. When this happens, fresh moist air will enter between the windowpanes. The desiccant will not be able to absorb these higher amounts of moisture and as the window surface cools the moisture in the air will condense on one of the glass surfaces above the edge spacer (having the appearance of fog on the glass). When this occurs, the sealed unit is said to have “failed.”

When window seals fail the glass unit in the frame will need to be replaced because the window will continue to show condensation and will have lost its insulating ability. Contact your window manufacturer or a company that specializes in window repair and replacement for advice on the manufacturerʼs warranty – some companies cover seal breakage for up to 10 years.

For more information and tips on reducing condensation in your home, visit the Canada Mortgage & Housing Corporationʼs website at search for “Condensation.”

Moisture Sources Litres Added to the Air (per week)
Occupants (4)
Showers (8/week)
Drying Clothes
Gas Stove
Washing Dishes
Large Plant
Baths (8/week)