Rising damp, condensation or penetrating damp?
Three causes of damp walls, how to tell them apart, and why the fix for one will not solve the others.
When a damp patch appears, the first question is: where is the water coming from? There are three common causes in homes, and each needs a different solution.
Rising damp: water from the ground
The wall soaks up moisture from the ground like a sponge. You can recognise it because it affects the lower part of the wall, often comes with salts and is present all year round.
What can be done: stop or reduce the movement of moisture up the wall. Common options include injected chemical damp-proof courses, physical damp-proof courses, electronic systems such as BiXos and, in some cases, improving drainage or lowering external ground levels.
Condensation: water from the air
The moisture we produce by cooking, showering, drying clothes indoors and breathing turns back into water when it meets cold surfaces. It shows up in corners, on ceilings, behind wardrobes and around windows, often with black mould. In the UK it is the most common cause of damp in homes.
What can be done: improve ventilation, reduce indoor humidity and deal with cold spots. A rising damp treatment will not fix condensation.
Penetrating damp: water from outside
Rain gets in through cracks, a leaking roof, failed pointing, a window seal or a broken gutter or downpipe. The patch appears or grows after rain and is usually near where the water is getting in.
What can be done: find and repair the point of entry.
Why telling them apart matters
It is common to spend money on dehumidifiers or anti-damp paint when the problem is coming from the ground, or on a damp-proof course when the real issue is condensation. An assessment with readings saves both time and money.