The Three Types of Damp

Three types of damp can affect houses, and the causes and treatment of each are different.

1. Rising Damp

This involves moisture rising up through solid floors in contact with the ground, and up walls (apparently defying gravity) as a result of water being drawn up through the tiny holes in bricks in the same way a sponge will fill when laid on water. The normal visible sign of rising damp in walls is a tidemark on the wall decoration.

It can be prevented in floors if an impervious damp-proof membrane (DPM) is laid in the concrete. If this layer of heavy-duty plastic is damaged or missing, the inevitable result will be ruined floor coverings.

Similarly, a bituminized felt damp-proof course (DPC) is installed low down in exterior house walls to prevent rising damp - you can usually see this as a thin black layer two or three brick courses up the outside wall. A DPC can become 'bridged' (making a path for water to rise) by earth in flower beds or by careless building work (such as allowing render to overlap it on the outside of the wall). However, if neither of these is the cause of rising damp, the normal remedy is to inject a water-repellent chemical into the brickwork affected - definitely a job for the professionals.

2. Penetrating Damp

This is simply damp that gets into the house from outside, through faulty roof coverings, gaps around doors and windows, and through faulty joins between a pitched roof and a chimney, or a flat roof and an adjacent wall. The symptoms of penetrating damp (damp patches on walls and ceilings) are usually worse after rain.

The solution is normally to repair the faulty building component. Look to see whether blocked downpipes or leaking gutters outside are making the problem worse.

3. Condensation

Condensation is caused by excessive moisture that cannot escape from a building rather than damp coming in. It happens as a result of warm moist air meeting a cold surface: when the air is cooled it can hold less moisture, so excess moisture is deposited on the cold surface in the form of water droplets - a familiar sight on single-glazed windows in winter. The first sign that condensation may be a problem is when black pinpricks of mould appear on walls, ceilings and even furnishings. There will be a musty smell present and clothes and shoes in cupboards can become covered in a white or green mould.

Condensation has become more of a problem in recent years as we have:

• improved draughtproofing, thus reducing natural ventilation
• improved insulation, making loft spaces colder and so leading to condensation on roof timbers
• fitted double-glazing, making windows warmer and thus transferring the condensation to nearby, colder, walls.

The solutions are to:

• improve ventilation - for example, fitting extractor fans in kitchens and bathrooms, and ventilators around the loft
• improve wall insulation with polystyrene sheeting
• cut down on the amount of moisture produced in the house, particularly by venting tumble-dryers to the outside and fitting extracting cooker hoods in kitchens.

The airbricks low down on house walls are there to provide ventilation to the timbers supporting ground floors - they should never be blocked.

Black mould on walls is usually a symptom of condensation: the mould needs the pure water of condensation to grow. You can remove the mould, but you need to solve the condensation problem to prevent regrowth.