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Home Energy Efficiency and Sustainability​ Assessors

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insulating blinds or thick curtains with pelmetsCondensation 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, warm air can hold more water than cold. 

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 external walls, especially uninsulated and where there is not much air movement like behind furniture, in the corners or inside closets. If left for too long it can lead to problems like damp, mould and rot and cause damage to the building shell or health problems when mould spores become airborne.

The temperature at which condensation occurs depends on moisture content and its called a dew point. The dew point doesn't have to be very low, for example, when the air at 20 degrees is 80% saturated with water vapour, the dew point is at 17 degrees. At this point the saturation is 100% so any further drop in temperature the water will continue to be released on the cold surface. This could be a lot as overnight temperatures in winter often drop to close to 0 degrees. Note this doesn't mean the temperature in the room, just local surface temperature of the wall or window. 

So the condensation is caused by excess moisture/humidity of the air inside the house, and now that we know this, what can be done to avoid it?

By reducing internal humidity levels either by ventilation or by removing the moisture from the air. 

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.

Use dehumidifier. Some air conditioners have dehumidifying option built in, see the manual if yours has and try it. Keep bathroom and laundry doors closed to slow down the humid air mixing into other rooms in the house. 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.    

By keeping internal surface temperatures warm (above dew point)

Install insulation in ceiling, walls and floor. Double glaze the windows. While this 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.  

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

Install 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. 

Condensation is usually a complex issue, and there may be a combination of reasons, so whatever measure you try, be systematic about it. Note the condensation prior and after. Was it just one area or all windows? Which rooms? Did the measure have any effect? How much water was there on the window? Keep in mind there will be more condensation on colder nights. You can note the temperature at night, most news services will provide daily high and nightly low, and there are also weather apps you can use. The more information you collect the easier it will be to find the cause and deal with it. 

If the measures suggested here 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.