How to Treat Damp on an Interior Wall: Causes and Solutions

A growing damp patch, a musty smell, paint that blisters or white saltpetre rising at the bottom of the walls: these signs do not lie, your wall is damp. But watch out, treating damp without understanding its origin is like putting a plaster on a wound. The first step is always the diagnosis, because the causes and therefore the solutions are very different.

Identifying the cause of the damp

Damp in a wall can come from three main families of problems: rising damp (ground water rises up the wall by capillary action, especially in old houses without a damp-proof course), infiltration (roof, gutter, window or facade crack that lets rain in), and condensation (moisture in the air settles on a cold wall, particularly in poorly ventilated rooms). Their position and behaviour give valuable clues.

• Rising damp: saltpetre and rings at the bottom of the walls, usually on facade or party walls, up to about 1 m in height.
• Infiltration: a localised patch that worsens after rain, often near a window, a roof corner or a facade crack.
• Condensation: cold walls, black mould in the corners, poorly heated or ventilated rooms like a bathroom or a bedroom.
• Pipes: a patch that appears unrelated to rain, sometimes accompanied by an increase in the water bill.

Acting on ventilation and heating

For all damp problems, improving the ventilation is a basic measure that is always useful. Air each room for at least ten minutes a day, open the windows after the shower and the cooking, and maintain a minimum temperature throughout the house to avoid cold spots. In a very damp room, an electric dehumidifier (allow 100 to 300 € depending on the capacity) can help temporarily, but it does not replace good ventilation.

Treating an infiltration

1. Remove the source: repair the gutter, the tile, the window seal or the facade crack letting the water in.
2. Let the wall dry completely: several weeks, sometimes several months, depending on the thickness of the wall.
3. Scrape off the damaged render and brush the saltpetre areas until you get a sound substrate.
4. Apply a water-repellent treatment or a suitable repair render, then redo the finish with a paint permeable to water vapour, never a waterproof one, to let the wall breathe.

Special case: rising damp

Rising damp is the trickiest case. Treating only the surface is not enough: you must stop the water rising. The techniques exist (injection of a water-repellent resin into the wall, physical barrier, external drainage), but they demand a precise diagnosis and specialist equipment. It is the type of intervention where it is strongly advised to call in a renovation professional or a surveyor, especially in an old house.

Do not forget the exterior side

When the infiltration comes from the facade, the treatment is not limited to the inside of the wall: you must also repair the cracks with a suitable mortar, redo the degraded pointing and check the flashings and waterproofing upstands at the junction with the roof. Drainage around the foundations can help in ground where water pools. These exterior works sometimes involve scaffolding and particular expertise: do not hesitate to ask a building professional for advice before starting, especially if the crack is wide or evolving.

The mistakes to avoid

• Repainting with a waterproof paint or an oil-based paint on a damp wall: the water will stay trapped behind and the paint will blister.
• Applying an anti-damp product without having identified and removed the source: the problem will come back for sure.
• Blocking up a facade crack with a simple filler without checking the watertightness and the evolution of the crack.
• Covering saltpetre without scraping it: it will come back through the new paint.
• Ignoring a growing patch: an untreated infiltration can damage the roof structure and encourage health problems.

When to call in a professional?

If the damp persists despite your treatments, if the patch comes back quickly, if you suspect rising damp or if you smell a persistent musty odour, do not wait. A professional will carry out a diagnosis with a moisture meter to measure the damp level of the wall precisely and point you towards the right solution. The cost of a diagnosis is generally between 80 and 200 €, often less than the price of the work you would have done blindly.

Treating damp on an interior wall is above all a matter of diagnosis: each cause calls for a different response. Air the room, ventilate, remove the source of water, let it dry, then only treat and repaint with suitable products. By proceeding methodically, you will solve the problem durably and protect both your wall and the air quality of your interior.