How to stop condensation on double glazed windows in winter

How to stop condensation on double glazed windows in winter


How to stop condensation on double glazed windows in winter? Water can take many forms from ice, snow, rain, steam and mist, but in each of these forms they are visible to the naked eye. For the householder damp can cause mould growth, fungal attack, rotten timberwork, infestation of bugs or mites. Moist air also requires a far greater amount of heat input to achieve the desired temperature than that of dry air.

Statistics show that the average adult breaths out between ¾ and 1 pint of water in a typical sleep cycle, so in a double bedroom with no ventilation this would amount to an extra 1½ to 2 pints of water added to the room, which is over 1½ gallons per week!

In this type of environment bed bugs and mites will thrive. Research is starting to show links between these bugs and Asthma in young people. The dryer the air the easier it will be to heat and the less we will see the problems associated with damp.

Condensation is when moisture laden air hits a cooler surface and drops the temperature of the air and moisture mix, the cooler air can no longer hold the moisture and water droplets are formed, initially a fine film of mist appears, but eventually larger water droplets form and water flows down the surface in lines, often pools of water are formed on window cills.

Unless this water is collected and removed the cycle will continue. The more absorbent materials in a room the more water is retained, until saturation point will leave permanent damp patches on the coolest surfaces.

So what can be done to control condensation?

The simplest way and the cheapest is through ventilation, this is done by opening windows. Once out of bed each morning, the covers should be thrown back, the door kept closed and the windows opened. Prior to leaving the house for the morning the windows can then be closed leaving the room drier and fresher. At weekends a through ventilation of the whole house will drop the moisture level even further.

How do I know if I have moisture problems?

There are sophisticated moisture meters called humidi stats on the market, but a very simple test is to put a small mirror in the fridge until cold and then take the cold mirror into the room you want to check, the misting on the mirror surface will be an indication of the amount of moisture present in the air.

What is an acceptable level?

40% to 60% is ideal, so 50% is an easy to remember figure. Too dry and the air will leave you with a sore throat and will be difficult to maintain.

On wet days what can I do?

Opening windows on a very damp day will not significantly drop the moisture content of a room’s air. At this point the only way to remove moisture and dry the air down to 50% relative humidity is with mechanical means. Extractor fans will remove steam from bathrooms and kitchens but as this air has to be replaced from somewhere else then this air would still be too damp if coming from the outside on a rainy day.

An electric de-humidifier is the best way to go and with de-humidifiers you get what you pay for. The cheaper ones have no control on the moisture content you want to achieve and will run continuously until you switch them off, where as a built in humidistat can be set to a desired level and the motor will only switch on when the moisture level in the air rises above the set level saving power and money.


An electric de-humidifier is basically a fridge element, a bit like a car radiator, that a fan blows the air from the room through. On hitting the cold surface the air cools and deposits, the laden moisture, as the moisture droplets clump together they run down the surface to be collected into a storage tank or in more permanent installations away to a drain avoiding the need to empty the tank. In most machines the storage tank may hold about a gallon of water, and there is a cut off floatation switch once it is full. When you first put a de-humidifier in a room you suspect may be damp you would expect to fill the tank within 24 hours and extending to longer periods with regular use.

Over the last 100 years our houses have become more insulated, draught proof, warmer and have much more steam generated from showers, cooking and even drying washing. Ventilation and dehumidification is crucial to avoid damp, mould, dust mites, bed bugs and even illness.