Fungus on Walls: Why It Grows, How to Clean It, How to Stop It

Almost all fungus on walls in Goa comes from one of three sources, and each needs a different fix. Seepage is liquid water coming through the wall or roof from outside: driven rain entering through cracks, failed joints around windows, a leaking terrace or gutter, or a downpipe discharging against the wall. It produces damp patches that grow after heavy rain, often with a tide-mark edge, blistering paint and white, powdery salt deposits.

Condensation is water from the air inside the house, forming on surfaces cooler than the air. It is typical on external walls, in corners, behind furniture, on ceilings under a roof slab, and in bathrooms. It tends to produce diffuse black speckling across a surface rather than a stain with a defined edge, and it is worst in still, closed rooms. When indoor humidity follows the monsoon's 83–91%, condensation on cool masonry is routine.

Rising damp is ground moisture drawn up through masonry without an effective damp barrier at its base. It shows as a band of damp, staining and salts from the floor up to a fairly consistent height, and it changes little with the weather. It is most common in older houses built directly on the ground; our guide to laterite and old walls explains why.

Mould releases spores and fragments when disturbed, and some people react badly to them. The EPA recommends an N-95 respirator, gloves that reach the middle of the forearm, and goggles without ventilation holes. Keep anyone with asthma or allergies out of the room and open the windows.

Scale decides whether this is a household job. The EPA's guideline is that an area smaller than about 10 square feet, roughly a patch one metre square, can in most cases be cleaned by the occupant. Larger areas, mould inside wall cavities or false ceilings, growth that follows a significant leak, and mould that returns within weeks of cleaning are for a specialist, and all of them need the moisture source fixed first.

One rule is absolute: never mix bleach with ammonia, or with any cleaner that contains it. The combination gives off toxic fumes, and many glass cleaners and multipurpose sprays contain ammonia. The EPA's own method for hard surfaces is detergent and water, which is what the steps below use.

Seepage is a building repair, and a job for a builder or waterproofing contractor rather than a paint shop: sealing cracks, re-making joints around window frames, repairing terrace waterproofing, clearing gutters and redirecting downpipes, adding drips under sills. Timing matters. External repairs need dry weather to cure and must be finished before the pre-monsoon storms, which in practice means between November and mid-May.

Condensation is fixed by lowering indoor humidity and raising air movement: ventilating at the right time of day, running a dehumidifier or air conditioner in closed rooms, fitting exhaust fans in bathrooms and kitchens, and pulling furniture off external walls. Our guide to humidity in Goa covers each of these, including what a dehumidifier costs to run.

Rising damp is the most specialised of the three. In old lime and laterite houses, the instinct to seal the base of the wall with cement plaster and plastic paint usually makes things worse, because it traps moisture that previously evaporated through the face and pushes it higher up the wall. Take advice from someone experienced with traditional construction; our piece on laterite houses is a useful starting point.

The EPA is explicit that mouldy surfaces should not simply be painted or caulked over: clean the mould, dry the surface, then paint. Painting over growth seals it in, and it comes through the new film, often within one season. Anti-fungal primers and paints contain biocides that slow regrowth on the paint film, and they are worth using on walls that have had mould, particularly in bathrooms and on external walls prone to condensation. They are not a cure. A biocide on the surface does nothing about water in the wall, and if the source is still active the mould returns, sometimes behind the new paint.

On older lime-plastered and laterite walls, choose breathable finishes such as limewash or mineral paints over plastic emulsions, so moisture in the wall can still leave through the face. A vapour-tight paint on a damp wall moves the problem to the back of the film, where it shows as blistering and flaking.

Wallpaper is where fungus on walls becomes expensive. A wall that grows mould under paint will grow it under paper faster: the paste is food, the paper holds moisture against the plaster, and the damage is hidden until it stains through. Vinyl-faced papers, which let no moisture out, are the least forgiving.

So wallpaper follows the same order as paint, with a stricter test: the wall must be dry, the source fixed, and ideally the wall should have come through a monsoon without the mould returning. Which papers and constructions suit Goan houses, and how walls are prepared for them, is covered in our piece on wallpaper for humid homes; for the papers we hang, see home wallpaper.

If mould has already come through an existing wallpaper, the paper generally has to come off. Paper, lining and paste are porous materials of the kind the EPA says may need to be discarded once mouldy. Clean and treat the wall beneath as described above, let it dry out through the dry season, and rehang then.