Mould grows where humid air meets a cold wall.

What you see

Black dots turning into patches, in ceiling corners, behind furniture pushed against the wall, around window frames, in the bathroom. Misted windows in the morning. A stale smell. Often in a north-facing bedroom, or a flat where laundry dries indoors.

Mould is not water ingress: it grows on a wall that is damp at the surface, not wet in depth. A reading tells the difference in ten minutes.

Where it comes from

Too much vapour, not enough air. New, very airtight windows with no trickle vents, a recirculating cooker hood, a tumble dryer in the living room. The moisture in the air has no way out.

A cold spot. An outside wall corner, a concrete lintel, a party wall unheated on the other side: vapour condenses there first.

Irregular heating. A room heated then switched off sees its walls drop below the dew point every night.

What we do

We measure: surface moisture, wall temperature, and we check there is no ingress or rising damp behind the mould.

We get the air out: trickle vents in the windows, an extractor in bathroom and kitchen, mechanical ventilation when the home calls for it.

We insulate the cold spot: insulating render or lining on the corner or lintel that condenses.

We treat and repaint: fungicide, removal of affected plaster if needed, anti-condensation or breathable paint. The painting is done by the same group.

Health

Mould spores worsen asthma and allergies, especially in children. In Brussels the Region runs an indoor air analysis service on medical referral, the CRIPI, through Bruxelles Environnement. We mention it when the situation calls for it.