Anti-mould paint: what it does and when painting is not enough

The little black dots on the bathroom ceiling, the grey patch that appears behind the bedroom wardrobe every winter, the kitchen corner that turns black however often it is cleaned. Anyone looking for anti-mould paint almost always has one of these in front of them and wants it gone for good. The paint helps, but not in the way most people expect, and knowing exactly what it does is what separates painting once from painting every year.

Behind what follows are three data sheets for anti-mould paints, the European biocides regulation and the document of the CTE, the Spanish Building Code (Código Técnico de la Edificación), that explains how to check whether a wall is going to suffer condensation.

What is inside an anti-mould paint

An anti-mould paint is an ordinary emulsion with a preservative added. All three data sheets say so in almost the same words: it is a paint “with anti-mould preservative”. One of them even names the substances it contains, and explains what they are there for: “to protect the initial properties of the treated article”.

That way of putting it comes from the European biocides regulation. It has a group of biocides called “film preservatives”: products that control microbial deterioration and algal growth “in order to protect the initial properties of the surface” of materials such as paints. Another of the data sheets, for an additive you mix into the paint, puts it even more plainly: it reinforces “the fungicidal and algicidal protection of the surface film”.

In other words, anti-mould paint protects its own surface: it is a film on which mould finds it hard to grow. It does not dry the wall, it does not warm it and it does not change the air in the room. And it is precisely there, in the wall and in the air, that mould finds what feeds it.

Mould does not wait for the wall to drip

Many people associate mould with visible water: the drop running down the tile, the misted-up window. But the Building Code sets the limit much earlier. To limit the risk of condensation on a wall, its supporting document uses a simple criterion: “a maximum limit of 80% mean monthly relative humidity on the surface”. And it defines the temperature that meets it, which it calls the minimum acceptable, like this: “Minimum interior surface temperature to prevent the formation of mould”.

With the document’s own formulas you can work out where that line falls. With the air in the house at 20 °C and 55%, the wall drips when its surface drops below 10.7 °C, but it goes past 80% below 14.1 °C. Between the two there are more than three degrees in which the wall does not look wet and mould already has a foothold.

And those lines do not stay put: they rise with the humidity of the air. With the same air at 70%, the wall drips below 14.4 °C and the mould limit rises to 17.9 °C. An anti-mould paint moves neither of them, because it changes neither the temperature of the wall nor the water in the air. If you have been offered an “anti-condensation” paint to raise that temperature, what it can actually do is covered in our article on thermal paint.

Why on the coast it comes back to the same corner

55% is the value the document proposes for a home when there is no better data, and the document itself warns of its limit: “for a dwelling in a maritime area the indoor relative humidity may be much higher than the 55% indicated, and condensation may occur very easily”. It gives no figure, but none is needed to see which way it goes: by the sea, both lines in the diagram sit higher.

The figures in its own climate table explain it. In January, Málaga and Madrid have the same mean relative humidity, 71%, but Málaga is at 12.2 °C and Madrid at 6.2 °C. With that difference in temperature, the air in Málaga holds half as much water again. And Málaga’s mean humidity does not drop below 59% in any month of the year.

What does not change is where it appears. Mould comes out on the coldest part of the wall: the corners, where the wall meets the ceiling, around the windows, the north-facing bedroom wall, behind a wardrobe pushed up against it. These are the spots where the surface stays a few degrees below the rest, and that is why the patch comes back to the same corner however much it is painted. Why corners and the edges of floor slabs stay colder is explained on our external wall insulation page, along with what happens when a building is insulated from the outside.

When painting is enough, and when it is not

It is enough, or nearly, when the mould has a cause that can be removed. The bathroom ceiling that stays steamed up after every shower, the kitchen where pans boil without the extractor on, the room that is rarely aired. One of the data sheets defines its scope like this: walls and ceilings “prone to mould formation due to a high degree of humidity or lack of ventilation”. There, anti-mould paint does its job, as long as the water in the air has a way out.

That way out is what the Building Code requires of homes: a ventilation system in which “the air must flow from the dry rooms to the wet rooms”, with air inlets in bedrooms and living room and extracts in kitchens, bathrooms and toilets. If your flat has those vents, do not block them. If it does not, ventilation depends on the windows, and that works in winter too: outside air in Málaga in January, warmed to 20 °C inside the house, comes out at around 43% humidity, below the 55% the Building Code takes by default for a home.

It is not enough when the wall is getting wet from somewhere else. One of the data sheets puts it in writing: its anti-mould paint “is not suitable for painting directly onto surfaces with permanent or intermittent damp coming from leaks or from outside”. A leaking pipe, water getting in through the facade or damp rising from the ground has to be stopped first, and no interior paint does that.

Nor is it quite enough when, even with ventilation, the mould reappears in the same place year after year. That is a wall that is too cold, and what fixes it is raising its temperature: insulating it, from inside or outside. In that case anti-mould paint protects its own surface, but the wall is still inside the mould band.

How to paint a wall that already has mould

First it is cleaned, and not with the paint. The data sheets ask for this expressly: “Surfaces contaminated with mould must be washed and decontaminated first”, says one, with its own cleaner; another says to clean the stains “with bactericide / bleach”. One of them boasts that its paint “also covers existing stains (greenish or blackish)”, and it is true that it covers them, but the same data sheet asks for them to have been cleaned first. Covering mould without removing it is just painting over it.

After that, the wall has to be dry, and so does the air. One of the data sheets forbids applying it with the relative humidity at 80% or more, or below 7 °C: in a bathroom, that means painting with the shower turned off for a good while and the window open, not straight after. Two of the three ask for two coats; the third, one or two.

What really varies between two homes is the work beforehand: whether crumbling plaster has to be made good or a water leak fixed. How the job changes for a wall with mould compared with a sound one is on our page on painting a flat.

Frequently asked questions

Sources

• Documento de Apoyo DA DB-HE/2, «Comprobación de limitación de condensaciones superficiales e intersticiales en los cerramientos», a supporting document of the Spanish Building Code on checking surface and interstitial condensation (October 2013).
• Sección HS 3 «Calidad del aire interior», indoor air quality, in the Building Code’s Basic Document on health and hygiene (DB HS).
• Reglamento (UE) n.º 528/2012, relativo a la comercialización y el uso de los biocidas, the EU Biocidal Products Regulation, Annex V.
• Bruguer, ficha técnica de «Pintura con conservante antimoho» (technical data sheet, version 03/15).
• Titan, ficha técnica de «Pintura con conservante antimoho H22» (technical data sheet, September 2020).
• Revetón, ficha del «Sistema antimoho» (product data sheet).

Quotations from Spanish-language documents are our own translations.