Condensation on windows and other cold surfaces is a natural occurrence when warm air laden with moisture (humidity) drops in temperature and the water vapour in the air is deposited.

Basically air is like a sponge and the warmer the air the more moisture the air will hold, so when the air is cooled below its “dew-point” the sponge effectively shrinks and squeezes out the moisture onto the cold surface. In the winter our windows and doorframes are colder than the inside room temperature and so if air that moisture laden is allowed to dwell there for any time it will wring out the moisture and condensation occurs.Condensation on windows

Although the largest movement of vapour laden air occurs due to airflow, a second, subtler movement is caused by a difference of vapour pressure due to moist air (containing a large mass of water vapour) having a higher vapour pressure than drier air.  Water vapour therefore moves from regions of high vapour pressure to regions of lower vapour pressure.  Unlike heat, which will most readily transfer upwards  (and less rapidly horizontally), water vapour will transfer in any direction – being drawn by a difference in water vapour pressure to the lowest pressure area.

The ability of water vapour, or moisture, to move in this way accounts for condensation on, say, living room windows, occurring as a result of activity in the shower, kitchen, or laundry rooms. By condensing out on the window, water is removed from the air, causing a lower vapour pressure, and inviting higher moisture laden air to migrate towards the window.

It should be noted that water vapour is taken (by evaporation) from everything containing water, including plants, woodwork, and even human or pet skin. Water vapour is also the side effect of many of our day to day activities in the home, where it has been estimated that an average family can generate 7 to 9 litres (12.3 to 15.8 pints) of moisture on an average day – rising to 18 to 23 litres per day on wash days.

Temporary Condensation Problems

Where extensive renovations have been done, the new materials will tend to dry out at the start of the heating season, causing temporary condensation problems. Temporary condensation may also be observed after a sudden drop in temperature, immediately after a thaw, or following heavy rain, when the relative humidity in the home increases and the air is close to its saturation point of 100%.

Condensation on windows

The air in a home will not normally become saturated on its own, because the air is changed through windows, doors, vents and other openings.  This exchange of air from the outside during winter means that with the lower level of absolute humidity of the colder outside air a level of complete saturation is never reached indoors unless, of course, you have your humidifier turned up too high, or add too much moisture to the air.

In fact it is sometimes desirable to increase the humidity in a home, since many medical experts have pointed out there is a relationship between the level of relative humidity in the home and the occupants’ comfort. As soon as the heating season starts people have more colds, coughs and irritation in the nose and throat. The external cold air (with a low absolute humidity) is drawn into the home and heated, increasing its capacity to hold moisture, and if this moisture is not added artificially, using a humidifier, then the dry air will absorb moisture from its surroundings. The normal moisture level in the membranes of the nose and throat will be decreased causing irritation and encouraging other respiratory ailments. Medical authorities indicate that the level of humidity indoors should not be below 15%, and engineers generally acknowledge that the level of humidity in homes with a good vapour barrier should not exceed 40%. A good way to obtain controlled relative humidity is to use humidifiers with built in humidistats.

Low humidity levels will also result in the usual static electricity, shrinkage in woodwork and furniture.  Although the correct humidity level can minimize the static shock it may not eliminate it entirely.  Nylon carpeting and other materials can cause static electricity and so it is not recommended that humidity level be increased over the recommended levels in an effort to cure this problem.

Excessive Level of Humidity

Condensation on windows, or other surfaces, is only one of the problems caused by excessive levels of relative humidity in the home.  Excessive humidity can encourage mould to form on plaster, cupboards to smell musty, rooms to feel damp, and condensation to form on cold water pipes with water dripping on the floor or collecting within a wall/ceiling space. It can also penetrate walls to be trapped under the exterior paint causing it to blister and peel off. Sometimes, however, the most serious damage caused by excessive humidity is not immediately visible.  Moisture laden air can leak into walls with the possibility of causing gypsum board to crumble metals to corrode and excessive moisture movement.

mould on plaster

The room-side glass temperature plays an important role in occupant comfort in the home.  With high room-side glass temperatures there is less likelihood of condensation forming, down-draughts are reduced making sitting nearer window more comfortable, and maximum use can be made of floor space. Poor areas of circulation of warm air promote condensation since the temperature of the inside glass surface is colder than in other parts of the home. By drawing heavy drapes over a window, or patio door, the possibility of condensation is often increased because the flow of warm is restricted over the window surface. It should also be noted that if windows are exposed to prevailing winds, then they will be slightly colder than other windows and may cause condensation to form at humidity levels that are practical for the rest of the home.

For normal day-to-day control of the level of relative humidity in the home, consider the following:

  1. Water vapour is a by-product of cooking. Use the fan when cooking, or open a window.
  2. Showers produce water vapour. Close the bathroom door, and ventilate using the exhaust fan.
  3. Clothes washing, hanging them to on a line indoors, or not using the exhaust fan for dryers will cause moisture build up in the room. Since any impedance to the forced air from the drier exhaust will allow the moisture laden air to penetrate the room, periodic checking to ensure that the lint trap in the exhaust duct is clean is essential. It should also be noted that build up of lint in the trap and the flex hose to the trap becomes a fire hazard and residents should be educated to clean out the lint traps on a regular basis.
  4. Opening a window, or windows, for a brief period to ventilate the suite each day, will allow the drier air from outside to enter and balance out the moist air inside.

For more information about how to address condensation or other building enginnering issues, call us toll free at 1-888-348-8991 or:


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