What is Condensation?

Condensation is the passing of a substance from a lighter to a denser physical state. In this instance, water passes from a gaseous (vapour) state to the denser liquid one. Because it is caused by too much humidity, condensation is also the symptom of a more serious problem – excessive moisture at work in your home’s atmosphere.

In seeking cooler, drier outside air, water vapour exerts pressure and forces its way through most building materials. Wood, brick, and drywall are too porous to stop moisture from migrating to the outside. But because window glass is denser and its surface colder than the surrounding walls, vapour is stopped by glass and condenses on it. While glass – the coldest, least porous (and most visible) material in a building – may show condensation first, it may also be noticed on exposed nail heads and other metal surfaces.

Why Does Condensation Occur?

Problems arise because air can hold only a limited amount of water vapour, varying with the temperature. Condensation is related to two conditions always present in the atmosphere inside your home. It is likely to form whenever there is an improper balance between:

Relative Humidity

A ratio between the water vapour in the air compared to the maximum amount that air can hold at a given temperature. For example, 50% means the air is carrying 1/2 of the total water vapour it is capable of holding at that temperature.


A particular temperature where the relative humidity becomes 100%, called the “dew point” (see chart). At the dew point, the air is saturated with moisture and begins to lose it in the form of condensation.

Where Does Condensation Occur?

In practice, condensation will occur first over the lower part of the window because glass surface temperatures are not uniform, being lower at the bottom than at the top. Windows with a metal sash usually have more severe problems at the base and sides. Occasionally, it may occur on cold spots, such as nail heads and in corners of outside walls and closets where insulation value is reduced.

Sources of Moisture

The humidity level in a house during the winter will depend on both the moisture added to the air through family living habits (see chart) and the rate at which this moisture is removed by ventilation or condensation.

Solutions and Facts About Humidity


  • Turn off all humidifiers, particularly in homes  with forced-air heating.
  • Move plants away from windows.
  • Vent the clothes dryer and gas appliances outside.
  • Do not dry firewood indoors.
  • Ensure kitchens and bathrooms are well ventilated by windows or exhaust fans.
  • Ensure that basement walls are kept as dry as possible.
  • A de-humidifier may have to be installed
    (Note: a de-humidifier alone cannot eliminate the problem).
  • Do not cover windows with heavy curtains, this will restrict air flow over glass.
  • Produce less moisture inside the house.


  • Unless indoor humidities are kept below 10%,  it is impossible to avoid some condensation.
  • Lower outside temperatures require lower inside humidity levels (see chart).
  • Electrically heated homes are difficult to rectify because there is very little movement of air.
  • Ventilation is generally the most effective means available to reduce humidity and can be done by opening windows, operating exhaust fans and/or installing a ventilator from outside into the cold  air return plenum of a forced air heating system