Mould on the wall: where it comes from and what lime paint really does

Mould on a wall almost always means excess moisture: most often water vapour condensing on cold parts of the wall, less often a leak or moisture rising from the ground. No paint will remove that cause. Lime paint can, however, genuinely help: the high alkalinity of lime inhibits fungal growth, and the vapour-permeable coating lets the wall release moisture instead of trapping it under an airtight layer. Below we explain where mould comes from, what to do step by step, and what to expect from lime paint and what not to.

Where does mould on the wall come from?

Mould needs three things: moisture, food (dust, remnants of old paint or wallpaper) and to be left alone. Moisture is the key here, and it most often has one of three sources:

• Condensation. Warm, moist air from the kitchen, bathroom, drying laundry or simply the breathing of household members settles on cooler surfaces. That is why mould most often appears in the corners of external walls, behind wardrobes against an external wall and by windows.
• Poor ventilation. New, airtight windows limit the inflow of fresh air. Natural (stack) ventilation only works when air has a way to get into the home.
• Water from outside or from the ground. A leak from the roof or plumbing, a leaking gutter, no damp-proofing of the foundations, flooding.

Mould or damp? How to tell

• Black or greenish dots, often in corners and behind furniture: usually mould from condensation.
• A wet patch with a clear edge, plaster falling off, a white salt deposit near the floor: more likely water from a leak or moisture rising from the ground. This needs building repair, not just painting.
• Steamed-up windows in the morning and a musty smell: the home is simply too humid.

How to get rid of mould on a wall: step by step

1. Find and remove the cause. Without this, mould will come back whatever paint you use. Check the ventilation (whether the vents draw air), window trickle vents, leaks, gutters.
2. Look after daily habits. Air rooms briefly and intensively, use the extractor fan when cooking and bathing, cook with a lid on, do not dry laundry on the radiator, and move furniture a few centimetres away from cold external walls.
3. Remove the growth. Work in gloves and a mask, with the window open. Scrape off the mouldy coating and loose plaster. Apply a fungicide according to the manufacturer's instructions.
4. Dry the wall. Painting a wet wall will only cover up the problem.
5. Restore a breathable coating. This is where lime paint comes in: instead of sealing the wall with an airtight layer, it allows it to "breathe".

If the damp is extensive, keeps coming back despite ventilation or appeared after flooding, it is worth starting with the opinion of a building specialist.

Does lime paint help against mould?

Yes, but in a specific way, and it is worth understanding how.

• High pH. Fresh lime has a pH above 12. Fungi and bacteria grow very poorly in such an alkaline environment. That is why lime has been used for centuries in cellars, cowsheds and pantries.
• Vapour permeability. A lime coating does not hold moisture in the wall. The wall can release it into the air instead of "sweating" under an airtight paint.
• No food source. Lime is a mineral material. Mould has less to feed on than on coatings with a high content of organic ingredients.

What lime paint will not do: it will not remove the cause of the moisture and it will not work indefinitely like a fungicide. Over time, lime reacts with carbon dioxide in the air (carbonation), hardens, and its pH gradually falls. This is a natural process that makes the coating as durable as stone, but it also means that its strongest antifungal effect is in the early period. If a home is constantly too humid, mould will eventually cope with any surface.

Lime paint or an "anti-mould paint" from a DIY store?

Popular anti-fungal paints are most often acrylic or latex paints with added biocides. They work as long as the additive works, and at the same time they form a more airtight coating. Lime paint protects through the very nature of lime and does not trap moisture in the wall. In rooms where the wall needs to breathe (cellars, old houses, walls after flooding), it is usually the better choice. In places that are scrubbed heavily, a paint more resistant to washing will work better.

Which lime paint should you choose against mould?

Muro Sano is our opaque lime paint (min. 35% lime), designed for interiors and exteriors, for dry and damp rooms. It works particularly well on damp walls, in cellars, when renovating old buildings and after flooding. It has drying properties and forms a natural barrier against mould.

Key points when applying: thoroughly dampen the wall with water and do not use primers without consulting us, as they can seal the surface. You will find full instructions in our guide how to paint with lime paint.

Frequently asked questions

Can lime paint be applied over mould?

No. First remove the growth, clean and dry the wall, and only then paint. Lime paint makes it harder for mould to return, but it is not a mould removal product.

Is lime paint suitable for a cellar?

Yes, it is one of its classic uses. Vapour permeability lets the wall release moisture. However, if water is coming in from the ground, you need insulation, not just painting.

Is lime paint suitable for a bathroom?

Yes for a bathroom, as it copes well with humid air. It is not used in areas directly exposed to running water, e.g. in a shower cubicle.

Why does mould appear in corners and behind wardrobes?

Because these are the coldest and least ventilated places. Water vapour from the air condenses there fastest, and furniture additionally blocks circulation.

Is mould dangerous to health?

Mould spores can irritate the respiratory tract and aggravate allergies, so it should not be ignored. If there are small children, elderly people, allergy sufferers or sick people in the home, and there is a lot of mould, consult a doctor and a building specialist.

You can read more about lime paint itself, its advantages and limitations, in our article lime paint (limewash): what it is.

Not sure where to start? Call us: +48 605 642 777. We will help you assess whether lime is a good idea for your wall.