Causes of Condensation

Condensation occurs when warmer air (the air inside your house for example) comes into contact with a cold surface such as a window. Warm air can carry much more water vapour than cold air, so when warm air cools due to proximity with a cold surface, the water vapour is released and condenses into water. Breath on a cold window and you can see the problem in action (albeit on a small scale). Lack of air circulation can add to the problem, meaning that condensation will almost always be worse in the winter when the outside air is colder and doors and windows are kept shut.

This means that there are a couple of general ways to stop or reduce the problem of condensation:

1. Increase the warmth of the air in the house.
2. Reduce the amount of cold surfaces in the house.
3. Increase air circulation in the house.

Doing any one of these things will be unlikely to completely stop condensation forming (although it should help) and even doing all three may not completely cure the problem.

Main Causes of Condensation

Lack of Heating

As mentioned above, the warmer the air, the better it holds the naturally occurring water vapour. Increasing the heating in a room will very often stop general (all over the room, not just in one area) condensation.
Solution – Install a heater. Avoid using an oil heater in rooms prone to condensation as oil releases water vapour as it burns, potentially making the problem worse rather than better.

Uninsulated Walls and Ceilings

If a wall is uninsulated (or badly insulated) the inside of it will be cold and allow condensation to form. Solid walls (walls without a cavity) are most prone to this. If condensation and mould growth is mainly on the exterior walls of your house, or in patches between the ceiling joists, your insulation is not up to scratch.
Solution – Install loft insulation and consider dry lining the exterior wall, particularly if it is single skin. Dry lining creates the air cavity that a solid wall is missing.

Unlagged Pipes

Cold water pipes running through a warmer room can attract condensation. Often you will see a line of damp or mould growth along a ceiling, or an isolated spot of mould where water has dripped from a low point in the pipe run.
Solution – Have a look at the pipes above the effected area to see if there is water collecting on the underside. If so, lag the pipes with foam or mineral-fibre tubes (available from most DIY stores).

Cold Bridge

Walls which have a cavity can have “bridges” in them where the two leaves are connected, usually where there is a lintel above a window or doorway. These bridges make that part of the wall colder than the rest and therefore can attract condensation. This problem is usually signified by condensation or mould around the windows and doors only.
Solution – Line the window or door reveal with thin polystyrene sheets.

Cold Windows

A single sheet of glass is a very poor insulator and in cold weather will be the coldest part of any room. Condensation on windows is not a problem in itself, but as the water runs and collects at the bottom of the window, damage can occur on wooden sills.
Solution – Get your windows double glazed. If that is not an option, fit secondary double glazing. The aim is to create an insulating air gap between the sheets of glass. If you already have secondary glazing and condensation is forming between the sheets of glass, place a couple of sachets of silica gel crystals between them to help absorb the moisture.

Incorrectly fitted Loft Insulation

If you loft insulation material is fitted too close to the eaves, it may be stopping air circulating and can cause condensation to occur. Almost all houses are built with a gap between the eaves and the walls for better air circulation.
Solution – Make sure your loft insulation stops at least 100mm back from the gap in the eaves. If possible, fit some ventilation grilles in the soffit board.