Black Mould on Walls in Delhi: Why It Keeps Coming Back

Is it condensation or is it seepage?

Start here, because everything after this depends on the answer. Five questions usually settle it.

• Where exactly is it? Condensation favours cold spots: corners where two outside walls meet, behind furniture pushed flat against a wall, ceiling edges, and the wall behind an almirah in a room that is kept shut. Seepage favours a source: the back of a bathroom wall, the top of a wall in a top-floor flat, a band across the middle of a wall where a pipe runs, or the skirting.
• What shape is it? Condensation mould spreads as a fine speckle over a broad area and is worst in corners. Seepage tends to make a defined patch, often with a darker or browner edge, and the plaster inside it feels different from the wall around it.
• Is the plaster soft or salty? Press it. Seepage softens plaster over time and often leaves a white salty bloom at the edges. Condensation leaves the plaster sound — the growth sits on the paint surface.
• Does it change with the seasons? Condensation in Delhi is worst on cold winter mornings and in the sticky part of the monsoon. Seepage that comes only in July and August points at rain. Damp that is unchanged in May points at a pipe.
• What is on the other side of the wall? Walk round. Somebody else’s bathroom, your own bathroom, the outside face of the building, or a terrace above are the four usual suspects in a flat.

Why it always comes back after you clean it

Because cleaning removes what has grown, not the water that let it grow. As long as that patch of wall spends part of its week damp, something will grow on it again, and it will be the same patch because the reason is a property of the wall, not of the mould.

Three specific things make this worse, and all three are common in Delhi homes:

• Painting straight over it. A fresh coat over a mouldy patch looks solved for a few weeks. The dampness underneath has not changed, so the growth reappears through the new film — and now the paint is failing as well.
• Anti-fungal paint used as the whole solution. These finishes genuinely resist growth on the paint surface, and they are worth using once the wall is dry. Used on a wall that is still getting wet, they slow the return rather than prevent it. The label is not a substitute for finding the leak.
• Sealing the room up. Where the cause is condensation, keeping windows shut and the wardrobe pressed against the wall guarantees a repeat. Moisture from cooking, bathing and drying clothes indoors has to go somewhere, and it lands on the coldest surface in the room.

The pattern to remember: mould is the evidence, damp is the crime. Any plan that does not name a water source is a plan to do this again next year.

The Delhi causes, in the order they turn up

Delhi produces a very specific mix: months of dry heat where nothing grows, then eight to ten weeks of high humidity and heavy rain, then a cold, still winter where warm indoor air meets cold walls. Mould appears mostly in the second and third of those.

• The shared bathroom wall. The single commonest source in flats. Water gets under the tiles on one side — often the neighbour’s side, or the flat above — and travels through the wall. Your bedroom shows a dark patch while their bathroom looks perfectly normal. Nothing you do on your face of the wall fixes it.
• The roof slab, if you are on the top floor. Monsoon water standing on a terrace with aged or cracked waterproofing soaks into the slab, and the first sign inside is dark growth along the ceiling edge and the top of the walls. Our terrace waterproofing guide covers what that involves.
• The weather-facing exterior wall. Days of driven rain on an unsealed or cracked outer wall push moisture right through the masonry, and the inside face stays damp long after the rain stops.
• A concealed pipe. The one that does not follow the seasons. Steady, year-round damp in a fixed band on a wall usually means a supply or drain line behind it.
• Condensation in a closed room. Very common in winter in flats where one room is kept locked, curtains drawn and windows shut for weeks, particularly behind a wardrobe against an outside wall.
• Clothes dried indoors in the monsoon. A whole household’s washing evaporating into a closed flat is a large amount of water, and it settles on the coldest wall in the house.

What actually fixes it

The order matters more than the products. Doing step four before step one is why so many of these jobs get done twice.

• 1. Find and stop the water. Waterproof the terrace, repair the bathroom below or beside it, seal the exterior wall, or fix the pipe. If the source is a neighbour’s bathroom or a common area, this becomes a conversation with them or with the RWA before it becomes a painting job. Our wall seepage guide covers how these are usually traced.
• 2. If it is condensation, change the airflow rather than the wall. Open windows for a while each day even in the monsoon, run the bathroom exhaust during and after a bath rather than for the two minutes you are in there, dry clothes outside or in a ventilated spot where you can, and leave a finger of gap between large furniture and outside walls.
• 3. Let the wall dry completely. This is the step everybody rushes. After a leak is repaired, plaster can hold water for weeks. A dehumidifier or simply fans and open windows over a dry stretch of weather does the job.
• 4. Clean the surface, gently and with the room ventilated. Wear gloves, open the windows, and do not dry-brush or sand a mouldy patch, which just throws spores around the room. Wipe rather than scrub so you do not drive it into the plaster.
• 5. Then treat and repaint properly. Scrape back anything loose, use a primer suited to the wall, and use an anti-fungal interior finish in the rooms that are prone to it. On a dry wall these finishes earn their place; on a wet one they do not.

Season matters here too. Repainting in the middle of the monsoon on a wall that has not dried out is one of the most reliable ways to be back at the same patch next year — see our guide on the best time to paint and waterproof in Delhi.

When to stop trying things yourself

Cleaning a small patch and improving ventilation is reasonable to do yourself. Call somebody in when any of these are true:

• The plaster is soft, hollow or crumbling. That is water damage in the wall itself, not on it, and it needs opening up and repairing.
• It has come back twice or more in the same place. You have proved the cause is still there. More cleaning is not going to find it.
• It runs along a ceiling edge in a top-floor flat. That is a roof or slab issue and it will get worse each monsoon until the terrace is dealt with.
• The damp does not change with the weather. Steady year-round damp usually means a pipe, and that is a plumbing job before it is a painting one.
• It is spreading across more than one wall or into a neighbour’s flat. Shared-wall and common-area seepage often needs the building to act, not just you.

When you do get quotes, ask each professional to tell you where they think the water is coming from and how they know. A quote that is only for repainting the affected wall, with no answer to that question, is a quote to repeat the problem. Our monsoon waterproofing tips and wall and paint care tips cover the upkeep that keeps it from starting again.