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Condensation occurs when warm and humid air comes in contact with cold surface. This phenomenon occurs naturally as fog, frost or dew, even clouds. The air always contains some water vapour, and the amount it can hold depends on the temperature. And if the air has more water than it can hold, the excess will be released on the cold surface.  

As we use heaters in winter the air inside has increased capacity to hold water and we add moisture to it by washing, cooking, cleaning, taking showers even breathing. So the air inside the house is usually more humid than outside and when this moisture-packed warm air comes into contact with a chilly surface, like a window, it cools down quickly and releases the water.

Condensation is mostly evident on windows but it can also occur on or inside external walls, especially uninsulated, in corners and other spots due to thermal bridging, or where there is not much air movement like behind furniture or inside closets.  

Condensation becomes a problem when the third factor, drying time combines with the two (moist air and cold surface) causing it. If left for too long the moisture can lead to damp, mould and rot and cause damage to the building shell or health problems when mould spores become airborne. Most common areas are bathrooms, laundries and kitchens and most common signs are mould on walls and ceilings and rotting timber frames. Exhaust fans help removing moisture from those rooms, but by extracting warm air into the roof space the problem is not solved just shifted to another area. Excessive condensation in the roof space could lead to rotting timber structure, plaster crumbling and compromised insulation.

By knowing what's causing it, we can take steps to reduce or eliminate contributing factors, however, condensation is usually a complex issue and needs to be studied and understood before an effective measure can be applied. If the general tips suggested below don't work, you may need to get someone to inspect the house to see if there are reasons for high humidity other than normal day to day use.

Reduce humidity levels 

Ventilate to replace the air inside the house with less humid outside air. Either entire house or rooms like bathrooms, laundry, kitchen, and rooms with condensation. Ventilate bedrooms before turning on the heater and going to bed. Ventilate roof space by installing whirly venst or redirecting exhaust fans to the outside. 

Remove moisture from the air. Use dehumidifier. Some air conditioners have dehumidifying option built in, see the manual if yours does and try it. Wipe off the water from condensation, if you leave it to dry it will be back on the window again. Do not dry washed clothes inside the house.  

Keep internal surfaces warm

Install insulation in the ceiling, walls and floor. Double glaze or secondary glaze the windows. While glazing solutions can be expensive, especially retrofitting the windows, there are other benefits of doing it - making the house more energy efficient and reduce running costs of heating and cooling. 

Prevent warm, humid air from coming in contact with cold surface

Use window coverings like insulating blinds or heavy drapes with pelmets. Tightly fitting blinds or drapes will prevent the room air coming in contact with cold window and reduce the risk of condensation. This may work best in bedrooms which are heated at night when temperatures drop in early morning and if it works, it will be a first step to more permanent solutions 

While all measures here can help reduce the risk of condensation the most important (and free) is ventilation and should be tried first and combined with other measures. Regular ventilation will ensure there is no excess water in the air to condensate. However if the house is located in the area with higher moisture content in the air, like next to a body of water or with frequent fog or morning dew, and the condensation occurring as much on the outside as on the inside, the natural ventilation will not be enough and will have to be aided by other measures.    


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