Damp, condensation and mould: causes and cures

Three main causes

Condensation. Warm, moist air meets a cold surface and turns to water. It is the most common cause of damp and mould in homes. It appears on windows, in corners, behind furniture and on cold walls.
Penetrating damp. Water gets in from outside through a defect: a leaking roof, gutter or pipe, cracked render, a bridged cavity, failed pointing or a poorly sealed window.
Rising damp. Moisture from the ground rises up a wall through the masonry. It is much less common than people think, and often wrongly diagnosed. It normally needs a failed or missing damp-proof course.

Other causes include leaking plumbing, flooding, and water trapped in new construction that is still drying out.

How to tell them apart

Condensation usually appears in cold weather, on cold surfaces, with water droplets and black mould, often in bathrooms, kitchens and bedrooms.
Penetrating damp usually shows after rain, in a patch near the defect, and may get worse in wet spells.
Rising damp usually shows as a tide-mark low on the wall, often with salts, and does not usually extend high.

A professional uses a moisture meter, observation and sometimes monitoring to decide. A meter reading alone can mislead, because salts and other materials can give false readings.

Warm, moist air meets a cold surface and turns to water.
Damp, condensation and mould: causes and cures

What helps

For condensation: reduce moisture and improve ventilation and heating. Use extractor fans in kitchens and bathrooms, dry washing outside, and keep the home reasonably warm. In new or refurbished buildings, plan ventilation properly. See Airtightness and ventilation.
For cold surfaces: improve insulation and reduce thermal bridges. See Thermal bridging.
For penetrating damp: find and fix the source, such as a blocked gutter, a cracked pipe, damaged pointing or a failed flashing.
For rising damp: check the ground levels, drainage and damp-proof course first. Do not assume chemical injection is needed. Get independent advice.

What to avoid

Painting over the problem. Special paints do not cure damp.
Sealing a wall that needs to breathe. In older buildings, modern waterproof finishes can trap moisture. Compatible materials, such as lime plasters, may be better.
Buying treatment first and a survey second. A free quote from a firm that sells treatments may not be independent.
Ignoring it. Persistent damp can harm health, damage finishes and structure, and affect the value and insurance of a building.

In new buildings

Good design and construction prevent most damp: a damp-proof course and membrane that connect, a cavity or rain-screen that drains, careful flashings, continuous insulation, ventilated roofs and floors, and a proper ventilation strategy. See Cavity walls explained.

In short

Most damp is condensation, but it can also be penetrating or rising damp. Find the cause before you choose the cure, and take independent professional advice.

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