Damp walls:
what to do?

What you see: a darker tide mark, paint that blisters or flakes, wallpaper peeling from the bottom, render that turns to powder under your finger, swelling skirting boards, black or greenish spots, sometimes a crystallised white fringe. What you smell: a cellar smell that persists after airing, a cold wall that feels slightly tacky to the touch. What you measure: a hygrometer placed in the room shows the humidity of the air. The public website sante.fr recommends keeping it between 40 and 60% in a room heated to between 18 and 22 °C.

A dry, old mark that has not moved for years does not necessarily call for works. The real problem begins when the stain comes back after being cleaned or repainted, spreads, the hygrometer stays durably above 60%, or mould appears. Mould is not just a visual nuisance: according to sante.fr, breathing it in can cause allergies and irritation, and make asthma worse. Before anything else, trace the outline of the stain in pencil and date it: in a fortnight, you will know whether it is progressing.

All of them produce stains that look alike from a distance. Up close, each has its own signature: the height at which it sits, its shape, the season in which it shows and how it reacts to rain.

1. Condensation: the water comes from the air in the house

Cooking, showering, drying laundry or simply breathing loads the air with water vapour. When this air touches a surface colder than itself, the vapour is deposited as liquid water, like on a bottle taken out of the fridge. The places affected are always the coldest ones: room corners, the wall-ceiling junction, window reveals, the back of wardrobes pushed against an exterior wall, poorly insulated lintels and slab ends. Condensation dominates in winter, when we heat and air little, and mostly produces mould in black spots. Misted windows on waking are its first sign: we explain it in our article on condensation on windows, and the cleaning of stains in the one on mould on walls.

2. Rising damp: the water comes from the ground

Masonry is full of very fine pores. Standing on damp ground, it draws up water through these pores, just as a sugar lump dipped in coffee darkens from the bottom up. The water rises until evaporation from the faces of the wall balances what it draws from the ground. The old walls of our villages, in stone, rammed earth or rubble, have no barrier at their base and have always lived with this phenomenon: they stay sound as long as they are allowed to dry.

The signs: a stain that starts at the floor, with an undulating, roughly horizontal upper edge, present all year round with little variation, on an exterior wall as well as on an internal wall. A whitish fringe often underlines this edge: these are the salts that the water leaves behind as it evaporates, the famous saltpetre (efflorescence), which we deal with in our article on saltpetre on walls. The problem almost always worsens after works that have sealed the wall: cement render, waterproof paint, tiles or lining stuck on the inside face, a concrete or tarmac pavement poured against the facade. As the water can no longer evaporate at the bottom, it rises higher.

3. Infiltration: the water comes from rain or from the ground

Here, the water enters through the envelope: a cracked facade render, a window sill without a drip, a worn joinery seal, a gutter overflowing against the wall, a damaged roof covering, or a below-ground wall receiving water from the bank behind it. The telltale sign: the stain appears or grows after a spell of rain, often on the facade most exposed to the weather, around an opening or at the top of the wall under the roof. It dries, partially, when fine weather returns. Finding where the water gets in and the repairs are covered in our article on water infiltration in a wall.

4. A leaking pipe: the water comes from the plumbing

A concealed supply pipe that weeps, a badly fitted waste pipe, a heating circuit, a shower or bath joint that lets water through at each use. The stain is localised, often near a bathroom, a kitchen, a WC or under a wet room upstairs, and it spreads whatever the weather, summer as well as winter. When the bathroom is involved, the line between this and condensation is thin: our article on mould in the bathroom helps to sort the two out.

The square of waterproof film. Tape a square of plastic film or aluminium foil about twenty centimetres across over the damp area, sealing all four sides tightly. Leave it for a few days. If droplets form on the side facing the room, the water comes from the air: it is condensation. If it is the side stuck to the wall that is wet, the water comes from the wall itself: rising damp, infiltration or a leak.

The record over time. Photograph the stain, date its outline in pencil, and look at it after heavy rain, after a week of showers and cooking with the windows closed, then in the height of summer. A stain that follows the weather points to an infiltration; a stain that follows winter and occupancy, to condensation; a stain that stays still at the base of the wall, to the ground; a stain that grows with no seasonal logic, to a leak.

The water meter. In the evening, turn off all the taps, run neither the washing machine nor the dishwasher, and note the meter reading. Note it again in the morning before drawing any water. If it has moved, there is a leak somewhere in the installation, even if you cannot see it.

This table summarises the most common situations. It gives guidance; it does not replace observation on site: in an old house, two causes often add up, typically a wall base that pumps up ground water and a poorly ventilated room that condenses on top.

Painting over it. A paint, even one sold as “anti-damp”, does not remove the water supply; if it is waterproof, it stops the water escaping into the room and pushes it elsewhere, higher up or to the side. On mould, sante.fr is explicit: do not cover it with paint or a new covering.

Relying on a dehumidifier alone. The appliance removes water from the air. It relieves a room that condenses, while the ventilation is sorted out. It does nothing about water rising from the ground, coming in through the facade or escaping from a pipe: it empties its tank, and the wall stays wet.

Rendering an old wall in cement. This is the classic mistake on old buildings. Cement is barely permeable to vapour: the water the stone used to evaporate stays trapped behind it, the render ends up coming off in sheets and the stain rises above it. On this type of wall, we go back to lime, as we explain in connection with the stone walls of Chautagne.

Lining or insulating a wall that is not dry. A board stuck on or insulation fitted against a damp wall hides the problem for a few months, then the insulation gets wet and mould develops behind the board, where you can no longer see it. The rule is the same in our guide to internal wall insulation: the damp is dealt with first.

Blocking the air vents to feel less cold. The grilles above the windows and the mechanical ventilation (VMC) vents are there to extract the vapour produced by occupancy. Blocking them means manufacturing condensation.

If it is condensation

Start with the habits, which are free. Sante.fr advises opening wide for at least 10 minutes, twice a day, airing during cooking and after washing, drying laundry outside or with a window open, and regularly cleaning the ventilation systems. Move furniture a few centimetres away from exterior walls and heat rooms steadily rather than in bursts. If condensation persists, the next step is mechanical ventilation that works: vents cleaned and unblocked, extraction added in a wet room that has none, or a full mechanical ventilation (VMC) system. Cold walls that condense despite good ventilation are then a matter for insulation: window reveals, north walls, thermal bridges.

If the water rises from the ground

There is no miracle product, but several levers, to be chosen after diagnosis and often combined:

• Give the wall back its ability to dry. Remove whatever seals it: cement render, waterproof paint, stuck-on tiles or lining, plastic covering. Clear the base of the facade of the soil or tarmac covering it.
• A lime render, permeable to vapour, in place of the cement render, outside as well as inside. It lets the water evaporate over the whole height instead of concentrating it at one point.
• Drainage around the house when the ground sends water towards the wall: house below the level of the ground, clay soil, roof water that ends up at the foot of the facade.
• Ventilation of the rooms concerned and, where there is one, of the crawl space, to carry away the damp that evaporates from the wall.
• Injection of a water-repellent product at the base of the wall, which aims to create a barrier against rising damp. The technique exists and has its uses; it does not remove the need to let the wall dry afterwards and to remove waterproof coverings.

In new construction, the question arises differently: on our extension quotes, our masons lay a water-repellent screed under the first course of blocks precisely to cut off capillary rise.

If the water gets in through the envelope

The only real solution is to find where the water gets in and close it off: repair of cracks and facade render, window sill, zinc flashing and guttering, roof covering, protection and drainage of a below-ground wall. The interior wall is only treated afterwards, once dry. The points to check, from the facade to the roof, are detailed in our article on infiltrations.

If it is a leak

Turn off the water at the meter or at the valve for the area, then tell your insurer. When the source is not obvious, service-public.fr states that a leak search must be carried out with the help of a professional, and that insurance may cover the cost: as a general rule, you pay first, then claim reimbursement. Once the pipe is repaired, let the masonry dry before redoing plaster, tiles or paint; this is the delay most often cut short, and regretted afterwards.

A damp wall almost always involves several trades: the mason for the base of the wall, the render or the drain, the roofer for the roof and the zinc work, the plumber-heating engineer for the ventilation and the pipes, the plasterboard fitter and the painter for the finishes once the wall is dry. With us, these are our salaried craftsmen, each in their own speciality: the same company follows the trail from the stain back to the cause and prices everything in a single quote.

Prices taken from our 2024-2026 quotes in Savoie and Haute-Savoie, supplied and fitted, rounded:

The VMC ranges are those in our VMC price guide, drawn from the same quotes. The other lines are rarer: lime render and drains on two projects each, protection of below-ground walls on nine, most often as part of an extension or a basement. When scaffolding or earthworks are needed, they appear as separate lines. Amounts exclude VAT: in a home completed more than two years ago, a humidity-controlled VMC is in principle subject to VAT at 5.5% and the other items to 10%, as detailed in our guide to VAT on works.

We would rather say what our quotes do not contain: no line for injection against rising damp, for a leak search or for a dehumidifier. We therefore cannot give you a price for them.

You are a tenant: report the problem to your landlord in writing, preferably by registered letter with acknowledgement of receipt, backed by dated photos. Service-public.fr reminds us that a rented home must protect against water infiltration, against runoff and rising water, and have ventilation in good condition that removes damp. Routine upkeep, airing and cleaning the grilles without blocking them, remains your responsibility.

You notice water damage: report it to your insurer within the period set by your contract, at minimum 5 working days according to service-public.fr. The agreed incident report is not compulsory, but it speeds up processing.

You are an owner and want to treat the wall: call in a company that begins by establishing the cause. In the quote, check that it includes:

• the identified cause, written in black and white, and what led to that conclusion;
• lines that treat that cause, separate from those that redo the finishes;
• materials compatible with your wall, permeable to vapour on old masonry;
• a drying time allowed for before renders, linings and paint, as we allow for when we redo walls and ceilings;
• quantities in m² or linear metres and unit prices, so that you can compare.

Be wary of a quote that proposes a single treatment without having looked at the facade, the base of the wall, the roof and the ventilation: it may be curing a cause that you do not have.