Moisture vapour in the air forms condensation when it comes into contact with cold surfaces, because cold air can hold less water vapour than warm air; as the air cools it reaches a dew point temperature where the water vapour it contains can no longer remain as a gas, so it condenses. This is frequently seen on windows, and in some cases the condensation forms on walls, leading to mould growth.
The presence of water vapour in the air depends on what you are doing in the house, so one way of reducing condensation is to change your habits. For example, if you are in the habit of boiling nappies on the stove, or if you frequently wash down the floor with a lot of water you are adding to the water vapour in the air. Some activities are unavoidable, but others can be managed to reduce the vapour content of the air. The vapour emitted around the house is shown in the table below, measured in kilogrammes per day.
- Two adults asleep for eight hours 0.6
- gas cooking: breakfast 0.4
- lunch 0.5
- dinner 1.2
- washing up: breakfast 0.1
- lunch 0.1
- dinner 0.3
- floor mopping 1.1
- washing clothes 2.0
- drying clothes 12.0
- bathing 0.05
- showering 0.23
- house plant 0.84
- paraffin stove 0.35/hour
- portable gas heater 0.13/kWh
You can see from the table that it is a good idea not to hang damp washing round the house to dry it; try to hang it outside or use a spin drier before you put it on an indoor line. If you must hang washing indoors because it is raining or there is nowhere else to hang it, put it in the bathroom and open the window to let the extra ventilation remove some of the moisture vapour.
Another way to reduce the effects of condensation is to use natural materials and furnishings. Materials such as wood, cotton, linen and wool absorb moisture; provided that the periods of excess moisture are limited these materials can absorb the excess and give it out later. Try to use cotton or linen curtains, wooden furniture with linen upholstery, cotton sheets, sisal or wool carpets and other natural materials in preference to man-made fibres and you may reduce the condensation. However, these materials will not solve the more serious cases which will require more drastic remedies.
The usual ways to deal with condensation are ventilation, insulation and heating. If you seal your house to reduce its ventilation heat loss rate you will prevent air from circulating and removing water vapour, and the result may be an increase in condensation. Condensation mainly occurs in kitchens, bathrooms and, to a lesser extent, bedrooms. One simple solution is to open a window if condensation starts to form; but be sure to shut the door of the room so that the draught does not blow through the whole house. This is an advantage of old fashioned houses with separate rooms, but if you have an open-plan house you will just have to put up with the draught.
If the building is well insulated the walls and windows will be warmer and there will be less condensation. In properly insulated houses you can heat all the rooms constantly fairly cheaply and doing so will help to keep surfaces above the dew point temperature.
If a combination of changed habits, insulation and heating do not reduce condensation to manageable levels that can be controlled by occasionally opening a window, you can also try using an extractor fan. To prevent excessive ventilation, wire the fan to a dewstat, a device that turns the fan on when there is a risk of condensation and turns it off when the risk is passed. This ensures that the fan operates for the least possible time to remove the water vapour.
If you use insulation in a building be sure that you follow the instructions concerning vapour barriers and ventilation of air spaces, because condensation in insulation can cause a lot of damage. On walls and windows, it is a nuisance and it may stain paint or wallpaper; but within insulated walls and roofs it can lead to dry rot and to very expensive repairs, if not the complete deterioration of the house.