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 (vapor) 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 vapor 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, vapor 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 vapor, 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 vapor 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.
  • Below Chart at 20°c
Relative Humidity Dew Point (Celsius)
20% -4°
50% 10°

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.


Activity (For a Family of 4) Dew Point (Celsius)
Cooking (3 meals a day for 1 week) 6.3
Dishwashing (3 times a day for 1 week) 3.2
Bathing (0.2 litres per shower) 2.4
Clothes washing 1.8
Clothes drying indoors, or using an unvented dryer 10
Floor mopping (per 9.3m) 1.3
Occupants 38
Total Moisture Production per Week 63
  • In addition to the above sources of humidity, such things as gas appliances, dryers vented inside, plants (which put out almost as much water as they receive), pets, humidifiers, damp basements, etc., all increase the humidity level in a home.
  • *Note: A new home has significant quantities of moisture because of the water used in various areas of construction (700 gallons of water are used in plastering alone for a six room house!) This will be eliminated after a period of time.


Celsius Single Glazed Double Glazed Triple Glazed
32% 60% 70%
-5° 23% 53% 67%
-10° 18% 47% 62%
-15° 9% 40% 59%
-20° - 37% 54%
-25° - 31% 49%
-30° - 29% 43%
-35° - 24% 41%

Reducing Humidity Levels with a Forced-Air Furnace

  • Unless indoor humidity is kept below 10%, it is impossible to avoid some condensation.
  • Lower outside temperatures require lower inside humidity levels.
  • 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.


  • Ontario Ministry of Municipal Affairs and Housing
  • Canadian General Standards Board
  • National Research Council (Division of Building Research) Rolscreen Co.