The key to preventing condensation is to control humidity and temperature. The ideal humidity level is between 40-50% when air temperature is 20ºC. If condensation is starting to become a problem, try the following:
Increase ventilation:
It is necessary to refresh warm moist air with dry air, so air rooms for five to 10 minutes, several times a day. Make sure to air the house evenly – don’t just open kitchen or bathroom windows, as this will simply move the air with the highest humidity into (rather than out of) the house. Air your house even if it is raining - the warm internal air will still hold more moisture than the cold air outside.
Trickle ventilation:
If your windows have trickle ventilators – narrow slits placed in the frame – then keep these open all day (it is possible to retro-fit trickle ventilators into Rationel windows if necessary). Make sure other air vents – such as air bricks – are large enough, and are
not blocked.
Ventilate well when cooking and bathing:
If you have an extractor hood in your kitchen, or fan in your bathroom, then turn these on as soon as you start cooking or bathing and keep them running for 10 to 15 minutes after you have stopped. If you don’t have such extractor fans, then these rooms have to be aired more frequently.
Dry clothes outside:
Whenever possible, avoid drying clothes inside the house. If you use a tumble drier, make sure the warm air is vented outside.
Managing other less obvious sources of humidity
People, pets and houseplants:
Every living thing within a building adds to the overall humidity; for example, a single adult will produce around two litres of water vapour a day. Ventilation should take account of the number of people, pets and houseplants living together.
Lower night time temperatures and unheated rooms:
Cold air can’t hold as much moisture as warm air, so if temperatures drop – when heating is turned off – then the cooling air will deposit moisture, which is why morning condensation is common.
An empty house:
If your house has been unoccupied for any length of time (while you are on holiday, for example), then walls, furniture, carpets and so on will have absorbed moisture while the house was unheated, especially if the weather was damp when you were away. For up to 10 days after you return, try to keep windows open as much as possible while raising the internal temperature by 4-5ºC.